Section 3 d) Summary

Waves can be reflected, refracted and diffracted, whether they're transverse or longitudinal.

Sound waves can be measured using a device called an oscilloscope and a microphone. The microphone is connected into the oscilloscope, a sound made, and from the tracing made the frequency, amplitude, time period and wavelength can be calculated.
Louder sounds have a higher amplitude, and more high-pitched sounds have a higher frequency.

Reflection
The law of reflection says that the angle of incidence = the angle of reflection

i = r

This principle can be applied to a phenomenon known as Pepper's ghost, where lights and reflections are used to project a virtual image.

One light shines onto the background, lighting it up so the viewer can see it through the glass. The other light shines onto an image, hidden from view. The light is reflected off of this image, and travels to the glass, where it is reflected and the viewer sees it. The reflection merges with the background, so appears to be there and not just a reflection. This is called a virtual image.
This concept can also be demonstrated in a more simple manner:

The glass is equidistance from the candles, so when the light from the front candle is reflected it appears to be on the unlighted candle.

Sound experiment:

1. Measure 100 metres from a wall, and stand there and clap with clapping blocks. Every time you hear the echo of the previous clap, clap again.
2. Once you have a steady rhythm, time how long it takes for the time periods of 10 claps (count 11 claps) 
3. Divide this time by 10 to find the time for 1 clap, then divide double the distance (200 m) by this number (you must double it because the wave travels the distance to the wall and back).

This will give you the speed of sound in air, but is also affected by human reaction times so will not give a totally accurate result.

Total internal reflection (finding the critical angle):

1. Shine a ray into a semi circular prism (this is ideal, because you can shine at the mid-point of the flat side without being refracted the first time due to the shape having normals at all angles)
2. Shine it at the mid-point from a range of different angles, starting with a smaller angle of incidence, and gradually moving it around, until the ray is in line with the flat edge. This is the critical angle: the last point at which the light is reflected. 
3. Once the angle is greater than the critical angle, it will become totally internally reflected. 

Critical angle can be calculated with this formula: sin c = 1 / n 

Total internal reflection is useful in optic cables among other things. It allows the information to be transferred long distances without being lost, all of the light is reflected. This concept is also used in jewellery and cutting jewels such as diamonds. By creating total internal reflection, they reflect light more and are more 'sparkly'. 

Refraction

Refraction is when the direction of a wave changes as it changes from one medium to another.
You can see this when you put a straw in water, it appears bent or broken.

We can see how different objects and media refract objects through investigations.

Finding the refractive index of glass
1. Using a rectangular glass block, and trace its outline onto a piece of paper. Create a series of lines to create different angles about a normal.
2. Shine a line/ray of light from a ray box along each line, and mark the emergent line with two x's, and connect the line to the normal, and then to the angle of incidence.
3. Measure each of the angles of refraction in comparison to the angle of incidence, and use
n = sin i / sin r
for each, and find the average of n to try to erase inaccuracies.

This same experiment can be done with different shaped blocks to see how this affects it, or different media (e.g. plastic)


Diffraction

Diffraction is the spread of waves beyond a barrier. This can be seen with waves at the beach, after passing through a gap

This effect happens in the same manner with sound and light waves. For example, when a door is opened into a dark room, we see the light spread.
 

Diffraction is increased when the ratio of gap size to wavelength is balanced so the gap size is smaller and the wavelength is larger.

Diffraction doesn't just happen through gaps, though, it also happens when passing an edge.

Signals: Analogue vs. Digital

Signals can be digital or analogue, the difference being that analogue has continuous variables and digital having two fixed settings, on and off or 0 and 1.

Advantages and disadvantages:

• Digital signals have a wider bandwidth and can carry more information
• Digital can be more easily restored if distorted
• Analogue signals are near impossible to restore if badly distorted. 
• Analogue signals have an infinite range of data
• Analogue signals are easy to process
• Analogue signals are easily distorted
• Digital signals travel faster
• Digital signals are more complicated to process. 

Section 3 d) Key Words

Amplitude: The distance between the crest of a wave and the neutral point 0. Larger amplitudes mean louder sounds (in sound waves)

Analogue signal: A signal that continuously varies. It is easily distorted and difficult to restore.

Angle of incidence: The angle at which a wave changes from one medium to another, compared to the normal.

Angle of refraction: The angle at which a wave is refracted from the perpendicular surface.

Bandwidth: The frequency of a wave. Digital signals have wider bandwidths, meaning they can carry more information at once.

Critical angle: The angle at which the wave is no longer refracted, but changes to becoming reflected.

Diffraction: When a wave passes an object and spreads beyond it. Its effect is affected by the wavelength in proportion to the size of the gap/blockage. If the gap is smaller in proportion to a larger wavelength, the effects of diffraction are greater.

Digital signal: A signal with only two settings, 0 and 1. It is easily restored and has a larger bandwidth than an analogue signal, so it is able to carry more information.

Frequency: How many times a wave is completed in one second.

Longitudinal waves: Waves that oscillate in the direction of travel. Sound waves are longitudinal.

Oscillation: The vibration. One wave.

Oscilloscope: A machine that measures the vibrations of a wave and creates a trace.

Pitch: How high or low a sound is. A flute makes a high-pitched noise, whereas a double bass makes a low-pitched noise.

Plane mirror: A completely flat mirror.

Reflection: When waves bounce off a surface they are reflected.

Refraction: The change in direction of waves after passing from one medium to another.

Refractive index: The refractive index of a medium tells us how much it will refract light waves. It can be found by this formula: n = sin i / sin r

Total internal reflection: When all waves are reflected inside a medium, none are refracted.

Transverse waves: Waves that oscillate perpendicular to the direction of travel. Light waves are transverse.

Virtual image: An image that is not real, it is a projection or reflection.

Section 3 d) Specification

3.14 understand that light waves are transverse waves which can be reflected,
refracted and diffracted

Light waves (electromagnetic waves) are transverse, meaning they oscillate perpendicular to the direction of travel. They are able to be reflected off of shiny surfaces at the same angle that they hit the surface with, refracted by the angles when changing from one medium to another (e.g. glass and air) slowing different parts of the light and causing it to bend, and diffracted by spreading out after passing a barrier.

3.15 use the law of reflection (the angle of incidence equals the angle of
reflection)

When you look into a plane mirror, the image is not distorted, it shows it as it is. This is because the angle does not change when light is reflected. It hits the mirror at one angle, and it will reflect at the same angle, about the normal.

This is easily remembered: i = r

3.16 construct ray diagrams to illustrate the formation of a virtual image in a
plane mirror

A virtual image can be created using a phenomenon called Pepper's Ghost. The ray diagram below provides explanation.

Another example of virtual imagery is placing two candles  equidistance from glass, so they are symmetrical, then lighting the one in front of the glass. The light will be reflected by the glass, and the candle behind will also appear to be lit.

3.17 describe experiments to investigate the refraction of light, using rectangular
blocks, semicircular blocks and triangular prisms

1. Place a glass or clear plastic rectangular prism on a sheet of paper and outline with pencil.
2. Decide the point at which you will shine the light in, and draw a perpendicular line - the normal. Draw other lines at different angles (ever 10 degrees) using a protractor.
3. Shine a light into the block at the normal, and mark two Xs on the emergent line. Repeat this with all of the angles you marked.
4. Connect the Xs, then connect them up to the original to create the light paths. Do the same thing for different types of blocks (semicircular prisms, triangular prisms, etc. )

Using a coloured light, maybe red or blue, would be easier than using white light because white light is made up of all different coloured light and so may split into a rainbow and be difficult to measure.

3.18 know and use the relationship between refractive index, angle of incidence
and angle of refraction:
n= sin i / sin r

This relationship is important to know, and will likely be asked for you to recall.
refractive index = sin (angle of incidence) / sin (angle of refraction)

3.19 describe an experiment to determine the refractive index of glass, using a
glass block

Shine a light through a glass block at different angles, with an interval of 10 degrees. Mark the emergent line, then connect back to the original line. Measure the angle of incidence, and use sin(i) / sin(r) to find the refractive index of each of the angles you measured, then find an average of them to get a more accurate result.

3.20 describe the role of total internal reflection in transmitting information along
optical fibres and in prisms

Total internal reflection allows light to be reflected inside a medium, so the information can be transmitted long distances, it is essentially trapped in there, because it is constantly reflected at an angle larger than its critical angle.

3.21 explain the meaning of critical angle c

The critical angle is the angle at which the light is neither reflected or refracted; any bigger and it will be totally reflected, any smaller and it will be refracted.

3.22 know and use the relationship between critical angle and refractive index:
sin c = 1 / n

This is another really important formula to know, and again will probably come up in the exam.
sin (critical angle) = 1 / refractive index

3.23 understand the difference between analogue and digital signals

Analogue signals are continuous, they have constantly varying frequencies and amplitudes.

Digital signals have two settings only: 1 and 0

3.24 describe the advantages of using digital signals rather than analogue signals

Analogue signals are easily distorted or altered, and are difficult or impossible to restore to their original state. Digital signals are more easily restored if distorted because they only have two settings.

3.25 describe how digital signals can carry more information

Digital signals have a larger bandwidth than analogue signals, meaning more information can be carried at once.

3.26 understand that sound waves are longitudinal waves and how they can be reflected, refracted and diffracted

Sound waves are longitudinal, they oscillate parallel to the direction of travel. Similarly to transverse electromagnetic waves, they can be reflected, refracted and diffracted.

3.27 understand that the frequency range for human hearing is 20 Hz – 20,000 Hz

Humans can only hear a small range of frequencies, between 20 and 20 thousand hertz .

3.28 describe an experiment to measure the speed of sound in air

1. Measure the distance between where you're standing and a large flat wall. 
2. Using clapping blocks, make a noise, then make another every time you hear an echo.
3. Once you are clapping at a steady rhythm, use a stopwatch to time how long 10 time intervals are (count 11 claps)
4. Use this to calculate how long it is for 1 clap (divide by 10), then divide the distance by the time for one clap. (if the distance from you to the wall is 100 m, use 200 m because the vibration needs to travel there and back)
5. The result should be around 343 m/s

3.29 understand how an oscilloscope and microphone can be used to display a sound wave

A microphone can be connected to a cathode ray oscilloscope, and used to detect the sound. The oscilloscope will begin tracing the pattern of frequency and amplitude and display it as a wave or a straight line.

3.30 describe an experiment using an oscilloscope to determine the frequency of a sound wave

Make a noise into a microphone connected to an oscilloscope. It will make a trace, which you can count the number of divisions for one oscillation. You can also adjust the time base to get more workable numbers. For example, if one division is 5 milliseconds, and one oscillation is 4 divisions, we know that it is 20 milliseconds per oscillation, or 0.02 seconds.
f = 1 / t
so f = 1 / 0.02
f = 50 Hz

3.31 relate the pitch of a sound to the frequency of vibration of the source

High pitched sounds have a high frequency, low pitched sounds have a low frequency.

3.32 relate the loudness of a sound to the amplitude of vibration.

The larger the amplitude, the louder the sound will be.

Section 3 c) Summary

The electromagnetic spectrum is a spectrum of electromagnetic waves/radiation, split into 7 main parts, each with its own uses and hazards.

The first section is radio waves. These waves have the longest wavelength, and lowest frequency. They are used for satellite transmissions: communicating and broadcasting. Very large doses can cause cancers (such as leukaemia), and other disorders. Luckily, most people aren't ever in contact with large doses, so this isn't much of a cause for concern.

Microwaves have a slightly higher frequency and lower wavelength than radio waves, and are obviously used for cooking in microwave ovens (the energy causes particles to vibrate in food), but they are also used in satellite transmissions. These waves however, can cause damage to body tissues as it heats them up. This can cause enzymes to denature and irreparable tissue damage. Microwaves are contained, though. In microwave ovens, they are contained within the appliance and only emitted when sealed and on. People who work on aircraft carriers and are exposed to high doses of microwaves wear clothing that reflects the radiation.

Infrared radiation is used for short range communications, such as in tv remote controls, and also for heating (toasters, heat lamps, etc.) as well as night vision equipment and thermal imaging. All objects emit IR to some degree. Its only danger is overheating, which can be avoided by attempting to cool yourself down. Have a cold drink maybe?

Visible light is the light and colours detectable by our unaided vision. It is used in photography, optic cables, screens, printers, DVDs, and anything we need to see! Dangers are simply that too much light can damage the retina in the eye and cause temporary or permanent vision loss. This can be remedied by simply not looking at bright things, like the sun (our body's natural reflexes tend to help us avoid this anyway)

Ultraviolet, or U.V. light, is emitted by the sun. It's used for tanning beds, fluorescent lights, detecting forged bank notes, hardening dental fillings, reading invisible ink, and sterilising water and food. It has many dangers, however. For example, sun burn, skin cancer and vision damage or loss. By wearing U.V.-blocking sunglasses, sun cream, and covering up with hats, clothing and shade, the effects of U.V. can be reduced.

X-rays are useful for seeing inside things, such as the bones in your body, or looking in suitcases in airports without having to open and check through it. In seeing bones, x-rays are sent through the patient, and areas such as bones absorb the radiation, leaving a white patch, while the soft tissue is easily travelled through by the rays. X-rays can cause cell damage and cancer, so precautions people who work with them daily, such as doctors or airport security include standing behind a lead shield, or containing it within a lead box. Patients receiving an x-ray near to vital organs may have lead sheets placed over their heads or chests.

Gamma rays are able to kill living cells, so are used to target cancer cells and kill them. This is very precise and avoids damaging other cells or tissue. This is called radiotherapy. It is also used in radioactive tracers, which can be put inside a patient's body to see what part of it isn't working correctly. They are also used to sterilise food and medical equipment. However gamma rays can be very dangerous. They cause mutations, especially in rapidly growing tissue (in unborn babies this is especially dangerous). It can cause cell damage, and a variety of cancers. Gamma rays can be trapped by a few feet of lead, water or concrete. Thick, dense shielding is necessary for protection.

Useful resource: http://www.darvill.clara.net/emag/index.htm

Section 3 c) Key Words

Electromagnetic spectrum: A spectrum of frequencies or wavelengths of electromagnetic radiation divided into 7 sections.

Electromagnetic waves: A wave produced by the acceleration of electric charge. It is transverse, so it oscillates perpendicular to the direction of energy travel.

Gamma rays: The highest frequency, and shortest wavelength on the electromagnetic spectrum.

Infrared: Waves with a lower frequency than visible light, but higher than microwaves.

Microwaves: Waves with a lower frequency than infrared, but higher than radio waves.

Radio waves: Waves with the lowest frequency, and longest wavelength found on the electromagnetic spectrum.

Ultraviolet (U.V.): Waves with a higher frequency than visible light, but lower than x-rays.

Visible light: Found in the middle of the electromagnetic spectrum, visible light is the light and colours we can see, split into 7 colours: red, orange, yellow, green, blue, indigo, violet.

X-rays: Waves with a higher frequency than U.V., but lower than gamma rays.

Section 3 c) Specification

3.10 understand that light is part of a continuous electromagnetic spectrum which
includes radio, microwave, infrared, visible, ultraviolet, x-ray and gamma
ray radiations and that all these waves travel at the same speed in free
space

Visible light is found in the middle of the electromagnetic spectrum, a range of frequencies of waves:

3.11 identify the order of the electromagnetic spectrum in terms of decreasing
wavelength and increasing frequency, including the colours of the visible
spectrum

In order of increasing frequency and decreasing wavelength, the electromagnetic spectrum is:

Radio Waves

Microwaves

Infrared radiation

Visible light

• Red
• Orange
• Yellow
• Green
• Blue
• Indigo
• Violet

Ultraviolet light

X-rays

Gamma rays

3.12 explain some of the uses of electromagnetic radiations, including:

• radio waves: broadcasting and communications
• microwaves: cooking and satellite transmissions
• infrared: heaters and night vision equipment
• visible light: optical fibres and photography
• ultraviolet: fluorescent lamps
• x-rays: observing the internal structure of objects and materials and medical applications
• gamma rays: sterilising food and medical equipment

Radio waves are used for broadcasting and communications, satellite transmissions, etc.

Microwaves are used for cooking and satellite transmissions

Infrared radiation is used for heaters, cooking (like in toasters) and night vision equipment, short range communications (e.g. tv remotes).

Visible light is used for optical fibres, photography, and everyday screens, lighting, etc.

Ultraviolet light is used for fluorescent lights, tanning beds, disinfecting water, detecting forged bank notes, etc.

X-rays allow us to see inside objects, for example bones inside a body or security screening through suitcases at airports.

Gamma rays are used for killing cancer cells, sterilising food and medical equipment, and detecting cancer.

3.13 understand the detrimental effects of excessive exposure of the human body
to electromagnetic waves, including:

• microwaves: internal heating of body tissue
• infrared: skin burns
• ultraviolet: damage to surface cells and blindness
• gamma rays: cancer, mutation

and describe simple protective measures against the risks.

Even with their many uses, electromagnetic waves have dangerous side effects. The most common hazards are:

Microwaves cause internal heating of body tissues, which is dangerous because it can cause enzymes to denature and badly damage the body. Contact with microwaves can be avoided by ensuring that when they are used, it is in low doses and higher doses are contained (in microwave ovens, they are only emitted when it is switched on and the door is closed)

Infrared radiation can cause burns to the skin, it is felt as heat therefore its hazards are heat related. It can be reduced by shiny silvery surfaces.

Ultraviolet rays, emitted by the sun, kill and cause mutations in skin cells, leading to skin cancer. It can also permanently damage sensitive parts of the eye and cause blindness, as well as sun burn, which is why it is important to wear sun cream, sun glasses, and hats or clothes that cover and protect from the sun.

Gamma rays can kill cancer cells, but in untargeted or incorrect exposure, it can cause cells to mutate and therefore cause cancer. This can be avoided by simply avoiding environments where you could come into contact with gamma rays, but it can be reduced and contained by concrete or lead.

Section 3 b) Summary

Waves can be transverse or longitudinal. These two types of waves have similarities and differences:

You can see transverse waves by jerking a rope up and down, it will make an 'S' shape.
If you stretch out a slinky spring, you can push it forwards and backwards to create waves of compression, like longitudinal waves.
You can also see longitudinal waves in a ripple/wave tank.

You can use a variety of formula triangles to calculate the different aspects of waves, such as speed, frequency, wavelength, and more.
These can be used in different contexts and are important to learn and know.



Waves, when passing an edge or a gap, can be diffracted. This is when waves are bent as they pass the edge of an object, and then spread. The effect of diffraction varies depending on the ratio of the wavelength to the size of the gap.

If the gap is proportionally smaller, the diffraction effect will be more. The reverse is also true.

Section 3 b) Key Words

Amplitude: The height or size of a wave. Measured in metres.

Crest: The highest point of the wave.

Diffraction: When a wave is bent and spread as it passes around an edge or through a gap.

Frequency: The number of waves in a certain time period.

Longitudinal: A wave that oscillates parallel to the direction of travel (e.g. sound waves)

Transverse: A wave that oscillates perpendicular to the direction of travel (e.g. waves on the electromagnetic spectrum)

Trough: The lowest point of the wave.

Wavelength: The distance between the crests of two waves, the length of one wave.

Wave period: The length of time it takes for one wave to be completed.

Section 3 b) Specification

3.2 understand the difference between longitudinal and transverse waves and describe experiments to show longitudinal and transverse waves in, for example, ropes, springs and water

Transverse waves oscillate perpendicular to the direction of travel, and look like this:

Longitudinal waves oscillate parallel to the direction of travel, and look like this:

Transverse waves are able to travel in a vacuum, longitudinal waves require particles to be able to transfer energy.
You can see transverse waves by holding a rope and jerking it up and down. It will form the 'S' shape that is recognisable as a transverse wave.
You can see longitudinal waves by stretching out a spring, then moving your hand backwards and forwards to create compressions in the spring.

You can use a wave tank to see longitudinal oscillations; a vibrating bar is placed at one end of the tank, creating waves as it moves. By shining a light through, the waves can be projected onto paper underneath, or onto the ceiling.

3.3 define amplitude, frequency, wavelength and period of a wave

Amplitude: How high a wave goes, how big it is. The distance from 0 to the crest of the wave.
Frequency: How many waves pass in a certain period of time
Wavelength: The distance between the same points of two consecutive waves (e.g. two crests or two troughs)
Period: How long it takes for one wavelength to travel past a certain point

3.4 understand that waves transfer energy and information without transferring matter

We know that waves are able to transfer energy and information without transferring matter because we can get the information through matter. Light is able to travel through solid glass, and we can hear people talking through walls and doors. These objects do not move, but we still receive the energy and information through waves.

3.5 know and use the relationship between the speed, frequency and wavelength of a wave:
wave speed = frequency × wavelength
v = f × λ

This relationship is important, as it helps us to calculate a variety of different quanities that are necessary in this topic.

3.6 use the relationship between frequency and time period:
frequency = 1 / time period
f = 1 / t

3.7 use the above relationships in different contexts including sound waves and electromagnetic waves

Using the triangle method, you can manipulate different questions to answer it correctly.

3.8 understand that waves can be diffracted when they pass an edge

When waves hit an edge, they can be diffracted to spread around the space as shown in the diagram below

3.9 understand that waves can be diffracted through gaps, and that the extent of diffraction depends on the wavelength and the physical dimension of the gap.

When a wave passes through a gap, it is forced through, and then once it is able to reach the other side it can spread out:

How much the wave is diffracted depends on the width of the gap, and the wavelength. The smaller the gap in proportion to the wavelength, the bigger the diffraction effect.

Section 3 a) Key Words

Angle: The space between two intersecting lines close to or at the point they meet.

Degrees: A unit of measurement for angles.

Frequency: The number of crests of a wave that travel past a point in a given unit of time.

Hertz: A measurement of frequency, the number of waves that pass in one second.

Metres: A measurement of distance

Metres/second: A measurement of speed or velocity. How many metres travelled in one second.

Velocity: How fast something is moving in a particular direction; a vector quantity.

Section 3 a) Specification

3.1 use the following units: degree (°), hertz (Hz), metre (m), metre/second (m/s), second (s).

Degrees are the unit of measurement for angles. There are 360 degrees in a full turn/circle.
Hertz are the unit of measurement for frequency.
Metres are a unit of measurement for distance
Metres per second are the unit of measurement for velocity.
Seconds are a measurement of time

Section 3 Specification

Section 3: Waves

a) Units

3.1 use the following units: degree (°), hertz (Hz), metre (m), metre/second (m/s), second (s).

b) Properties of waves

3.2 understand the difference between longitudinal and transverse waves and describe experiments to show longitudinal and transverse waves in, for example, ropes, springs and water

3.3 define amplitude, frequency, wavelength and period of a wave

3.4 understand that waves transfer energy and information without transferring matter

3.5 know and use the relationship between the speed, frequency and wavelength of a wave:
wave speed = frequency × wavelength
v = f × λ

3.6 use the relationship between frequency and time period:
frequency = 1 / time period
f = 1 / t

3.7 use the above relationships in different contexts including sound waves and electromagnetic waves

3.8 understand that waves can be diffracted when they pass an edge

3.9 understand that waves can be diffracted through gaps, and that the extent of diffraction depends on the wavelength and the physical dimension of the gap.


c) The electromagnetic spectrum

3.10 understand that light is part of a continuous electromagnetic spectrum which
includes radio, microwave, infrared, visible, ultraviolet, x-ray and gamma
ray radiations and that all these waves travel at the same speed in free
space

3.11 identify the order of the electromagnetic spectrum in terms of decreasing
wavelength and increasing frequency, including the colours of the visible
spectrum

3.12 explain some of the uses of electromagnetic radiations, including:

• radio waves: broadcasting and communications
• microwaves: cooking and satellite transmissions
• infrared: heaters and night vision equipment
• visible light: optical fibres and photography
• ultraviolet: fluorescent lamps
• x-rays: observing the internal structure of objects and materials and medical applications
• gamma rays: sterilising food and medical equipment

3.13 understand the detrimental effects of excessive exposure of the human body
to electromagnetic waves, including:

• microwaves: internal heating of body tissue
• infrared: skin burns
• ultraviolet: damage to surface cells and blindness
• gamma rays: cancer, mutation

and describe simple protective measures against the risks.


d) Light and sound

3.14 understand that light waves are transverse waves which can be reflected,
refracted and diffracted

3.15 use the law of reflection (the angle of incidence equals the angle of
reflection)

3.16 construct ray diagrams to illustrate the formation of a virtual image in a
plane mirror

3.17 describe experiments to investigate the refraction of light, using rectangular
blocks, semicircular blocks and triangular prisms

3.18 know and use the relationship between refractive index, angle of incidence
and angle of refraction:
n= sin i / sin r

3.19 describe an experiment to determine the refractive index of glass, using a
glass block

3.20 describe the role of total internal reflection in transmitting information along
optical fibres and in prisms

3.21 explain the meaning of critical angle c

3.22 know and use the relationship between critical angle and refractive index:
sin c = 1 / n

3.23 understand the difference between analogue and digital signals

3.24 describe the advantages of using digital signals rather than analogue signals

3.25 describe how digital signals can carry more information

3.26 understand that sound waves are longitudinal waves and how they can be reflected, refracted and diffracted

3.27 understand that the frequency range for human hearing is 20 Hz – 20,000 Hz

3.28 describe an experiment to measure the speed of sound in air

3.29 understand how an oscilloscope and microphone can be used to display a sound wave

3.30 describe an experiment using an oscilloscope to determine the frequency of a sound wave

3.31 relate the pitch of a sound to the frequency of vibration of the source

3.32 relate the loudness of a sound to the amplitude of vibration.

Section 2 d) Summary

Electrostatic charge is unmoving electric charge, caused by the transfer of electrons when two insulators are rubbed together. Usually, electric charge is allowed to flow in a current, through a conductor. Conductors have free charged particles that allow the current to carry the charge, but insulators do not, so the charge is static; it does not move.

Common conductors include metals, molten or aqueous ionic compounds, and carbon.
Common insulators include plastics such as rubber or acrylic, as well as air, paper and glass.

Like charges repel, and opposites attract. These forces of attraction and repulsion get weaker as distance is increased. This principle allows us to harness charges and use them for our benefit

Applications of electrostatic charge:

Inkjet printers
1. Tiny inkdrops become electrically charged as they are forced out of a small nozzle.
2. They travel past two plates, one positive and one negative. The drop is repelled by the like charge and attracted to the opposite charge, directing its landing spot on the paper.
3. The voltage across the plates is changed to direct each drop to form the image.

Photocopiers
1. A projection of your image is shone onto a positively charged image plate.
2. The light causes charge to leak away from some places.
3. Negatively charged powder (toner) is attracted to the positively charged parts of the plate, then transferred onto positively charged paper.
4. The paper is heated to seal the toner.

Problems and Dangers of electrostatic charge:

Problems

• Screens attract dust as they get charged
• Clothes cling to one another, and the body
• Brushing hair can cause it to become statically charged and stand up

Dangers

• Can build up in clouds and cause lightening, which can be dangerous
• Buildup when fueling machinery could cause a spark, and an explosion
• Touching an object with a large charge can give you a burn, or even kill you

Investigations into electrostatic charge:

Rod and cloth
1. Hold a polythene rod next to running water. Nothing will happen.
2. Rub the rod with a cloth, so friction causes electrons to transfer from the cloth onto the rod, causing both to become charged equally but oppositely.
3. Hold the rod next to the running water again. The stream will be bent, as like charges repel and opposite charges attract.

Gold leaf electroscope
1. Hold the object near the metal disc of a gold-leaf electroscope.
2. If the object is charged, this will induce a charge and cause the gold leaf to rise as it is repelled from the similarly-charged metal plate.

Suspended rods
1. Suspend a rod with a known charge, then hold your object near it.
2. If nothing happens, the object is not charged
3. If there is repulsion, the object has the same charge as the suspended rod.
4. If there is attraction, they have opposite charges.

Section 2 d) Key Words

Conductor: A material that allows current to flow through it due to free charged particles.

Earthed: When an object allows its charge to flow to earth and becomes neutral.

Earthing strap: A flexible metal strap that provides a path of least resistance for charge to travel down to earth.

Electrostatic charge: An electric charge that is not in motion; it is not flowing. Basically electricity without the current.

Insulator: A material that does not allow current to flow through it as it has no free charged particles.

Static Electricity: Unmoving electricity, electrostatic charge.

Section 2 d) Specification

2.19 identify common materials which are electrical conductors or insulators, including metals and plastics

Conductors:

• Copper 
• Lead
• Molten or aqueous ionic compounds
• Silver
• Gold
• Carbon

Insulators:

• Rubber
• Air 
• Acrylic
• Glass
• Polyester
• Paper

2.20 describe experiments to investigate how insulating materials can be charged by friction

Polythene and acetate rods experiment

1. Rip up some pieces of paper and wave each of the rods near them. Nothing will happen.
2. Rub the polythene rod with a duster. The electrons will transfer from the duster to the rod through friction, giving the rod a negative charge and the duster an equal positive charge. 
3. Rub the acetate rod with a duster. The electrons will transfer from the acetate rod to the duster, giving it a negative charge and the rod an equal positive charge.
4. Hold the rods near the paper individually. The pieces will be attracted to the rod. 

This can also be done by rubbing a balloon on a woolly jumper. The balloon can then attract small light objects, like ripped up paper or hair. 

2.21 explain that positive and negative electrostatic charges are produced on materials by the loss and gain of electrons

When rubbed together, friction is created between two objects. This force causes electrons to be transferred from on object to another, giving the objects equal but opposite charges.
These charges are electrostatic, meaning they do not flow.

2.22 understand that there are forces of attraction between unlike charges and forces of repulsion between like charges

Like charges repel, while opposite charges attract. These forces of attraction and repulsion weaken with distance.

2.23 explain electrostatic phenomena in terms of the movement of electrons

Electrostatic phenomena is the transfer of electrons from one object to another through friction. Positive particles never move; positive electrostatic charge is simply due to absence of negative particles.

2.24 explain the potential dangers of electrostatic charges, eg when fuelling aircraft and tankers

Electrostatic charges can cause sparks (this is what lightening is - buildup of electrostatic charge caused by friction between particles within the clouds that cause the bottom of the cloud to have a negative charge and the top to be negatively charged). These sparks can be very dangerous, for example when fueling a car or aircraft, a spark could cause an explosion. Because of this, electrostatic buildup in the tank is earthed by the metal fuel nozzle and earthing straps.

2.25 explain some uses of electrostatic charges, eg in photocopiers and inkjet printers.

Electrostatic charges can also be used to our advantage, for example in photocopiers and inkjet printers.

How an inkjet printer works:

1. Tiny ink drops are forced out of a fine nozzle, the friction making them electrically charged.
2. The charged drops are deflected as they pass through two oppositely charged metal plates.
3. The drops are attracted to the plate with an opposite charge, and repelled from the similarly charged plate.
4. The size and direction of the voltage across each plate is changed as each drop falls, causing them to land on different parts of the paper

How a photocopier works:

1. An image of what you want to copy is projected onto a positively-charged image plate.
2. Lighter parts of the image cause the charge to leak away in some parts of the paper.
3. The positively charged parts of the image plate attract negatively charged black powder, which is then transferred onto positively charged paper.
4. The paper is heated, causing the powder to stick.

Section 2 c) Summary

Series and parallel circuits

The current is the rate of flow of charge: I = Q/t
The voltage is the energy transferred per unit of charge: V = E/Q

In a series circuit, all of the components are connected in a line. This means the voltage is split between each of the components, but the current stays constant throughout, which is why ammeters are connected in series with components.

In a series circuit, the components are not individually controllable: they are either all on, or all off (e.g. Christmas lights). This can be disadvantageous as it means that if one of the components is broken, the whole circuit is disconnected and it can be difficult to find out which is faulty.


In a parallel circuit, the components are connected in 'branches'. Current is split between the 'branches', but each 'branch' receives the same voltage, which is why voltmeters are connected in parallel with components.

In a parallel circuit, the branches are individually controllable. A switch can be attached to each 'branch' and the components can be turned on and off as desired (e.g. different lights in a house). If one component breaks, the circuit will still work and the faulty component is easily located. 

Resistance and Resistors

Each component in a circuit has resistance; this can be thought of as like drag or friction opposing motion.
A component has greater resistance if it:

• Emits heat (e.g. toaster)
• Has a larger cross sectional area (of where the current flows)(e.g. thicker wires)
• Total length of wires
• Is made out of a less conductive material (e.g. copper has less resistance than carbon)

If there is more resistance in a circuit, less current will flow (or it will require more voltage to reach the same current). 

Variable resistors, fixed resistors, LDRs and thermistors are all components used to increase or decrease resistance in a circuit. 

Variable resistors allow the user to choose how much resistance they want, this is useful in dimmer switches for example. 

Fixed resistors are used to reduce the flow of electrons in a circuit, some appliances need a lower current to be able to work correctly. 

LDRs, or light dependent resistors, decrease in resistance when exposed to more intense light, and increase in dimmer light. These are used in burglar alarms and light intensity meters.

Thermistors are temperature dependent resistors, they increase in resistance when exposed to cooler temperatures. They are used in many appliances to maintain temperature, e.g. microwaves.

The resistance of a component can be determined by measuring the voltage and current across it in a circuit, and using this formula:

Voltage = Current x Resistance

The results can be graphed on an I-V graph (current on the y-axis, voltage on the x-axis), where the gradient is 1/R. These are the most important I-V graphs to know:

^ This is the I-V graph of a wire, or a resistor at a constant temperature. A steeper gradient means a lower resistance, and a shallower gradient means it has a higher resistance. This is according to Ohm's law: Electrical current is proportional to voltage and inversely proportional to resistance.

^ This is the I-V graph of a filament lamp. It is curved, because as the temperature of the metal filament increases so does the resistance.

^ This is the I-V graph of a diode. Diodes only allow current to flow in one direction; the resistance is very high in the opposite direction. The point at which the current increases dramatically is around 0.6 V.

To create a graph like this yourself, you must set up a circuit as shown:

Then, using the variable resistor to adjust the voltage, record the current across the component. Plot these points in an I-V graph, and draw a line of best fit.


Electrical symbols

Below is a guide on the basic components in an electrical circuit. There shouldn't be any other components that come up in the exam and most of these won't either.
The 10 most important are:

• Cell
• Battery
• Filament lamp
• Variable resistor, LDR, Thermistor
• Power supply
• Ammeter
• Voltmeter
• Diode
• Switches
• Fuse

Section 2 c) Key Words

Cell: Chemical power source, provides a circuit with electrical energy (basically a battery)

Current: The rate of flow of charge in a circuit. The movement of electrons from negative to positive.

Filament bulb: A light bulb containing a thin wire - a filament - that emits light when current passes through it.

LDR: Light-Dependent Resistor. Resistance decreases as light intensity increases.

LED: Light Emitting Diode. A component that emits light when current flows through in the forward direction. Used in traffic lights, digital clocks, remote controls, etc. No filament that can burn out.

Parallel circuit: A circuit connected with "branches". Voltage is constant over the different branches, but current is split. Allows individual control over components in the circuit.

Resistor: A component that has the sole purpose of opposing and decreasing the current flowing through a circuit.

Series circuit: A circuit where all of the components are connected in one line, and controlled together. Voltage is split across components, but current stays constant.

Thermistor: A resistor dependent on temperature. More heat = Less resistance, Less heat = More resistance. These are useful for temperature detection.

Guide to the most important electrical symbols plus some extra ones:

Section 2 c) Specification

2.8 explain why a series or parallel circuit is more appropriate for particular applications, including domestic lighting

Series circuits split voltage (potential difference) across the appliances, and maintain the same current (which is why we connect ammeters in series). Components in series cannot be controlled individually; they are all on or all off, no variation. This makes series circuits ideal for circuits such as that used in appliances such as Christmas lights, lamps, chargers, etc.

Parallel circuits split current, but maintain the same voltage across each component (which is why we connect voltmeters in parallel). Components in parallel circuits can each have individual switches, meaning they can be controlled individually. If one component breaks, the others all still work. This makes parallel circuits ideal for powering a room, because if this were in parallel it would require a component to be plugged into each socket for the lights to turn on. Power boards work in a similar way, you can plug as few or as many appliances in as you like and it will work the same.

2.9 understand that the current in a series circuit depends on the applied voltage and the number and nature of other components

The current of a circuit can be calculated by knowing voltage and resistance:
Current = Voltage / Resistance , or I = V/R
The voltage is applied by the power source, and the resistance is created by the components in the circuit. Each component has a different resistance, larger components, components requiring heating, and resistors generally have more resistance than a smaller component, for example a filament bulb.
The current can be calculated by adding up the resistance of all the components in the circuit, and dividing the applied voltage by it.

2.10 describe how current varies with voltage in wires, resistors, metal filament lamps and diodes, and how this can be investigated experimentally

Different components have different resistances, so by connecting the component in series with an ammeter, and changing the applied voltage, you can measure how the current varies. This can be recorded on an I-V graph, the gradient of the line being the 1/R.

2.11 describe the qualitative effect of changing resistance on the current in a circuit

When resistance is increased, less current will flow (or more voltage will be required to achieve the same current). This is because resistance directly opposes the flow of electrons.
Likewise, when resistance is decreased, more current will flow (or less voltage will be required to achieve the same current).
More components = more resistance, therefore more components = less current and less components = more current.

2.12 describe the qualitative variation of resistance of LDRs with illumination and of thermistors with temperature

LDRs, or light-dependent resistors, detect light and vary their resistance based on the light intensity. This is useful for devices such as burglar detectors. In bright light, the LDR has a very low resistance, but in darkness the resistance can be very high.

Thermistors work similarly to LDRs, but instead of detecting changes in light levels, they detect temperature changes. In hot conditions, the thermistor has a low resistance, whereas in cool conditions the resistance increases. This is useful for detecting temperature, for example in a car engine.

2.13 know that lamps and LEDs can be used to indicate the presence of a current in a circuit

You can test the connection of a circuit using a lamp or an LED. Just connect the bulb into the circuit, if it lights up, there is a current flowing.

2.14 know and use the relationship between voltage, current and resistance:
voltage = current × resistance
V = I × R

Voltage (in volts) = Current (in amperes) x Resistance (in ohms)
This relationship can be used to calculate current, resistance or voltage with simple rearrangement.

2.15 understand that current is the rate of flow of charge

I = Q / t
Meaning that current is the number of coulombs per second, the rate of flow of charge.

2.16 know and use the relationship between charge, current and time:
charge = current × time
Q = I × t

This relationship can be used to find the current, charge or time depending on how it is rearranged and what pieces of information you are given.
Q = Charge in Coulombs
I = Current in Amperes
t = Time in seconds

2.17 know that electric current in solid metallic conductors is a flow of negatively charged electrons

Electric current is the flow of charged particles, from negative to positive. These particles can be charged free-flowing ions, or electrons.
In a metallic structure, there is a sea of free electrons surrounding the positive ions which are fixed in place. This means that electric current is able to pass through; when the energy is applied the electrons flow from negative to positive in an electric current.

2.18 understand that:

• voltage is the energy transferred per unit charge passed 
• the volt is a joule per coulomb.

Voltage, or potential difference, is a very important part of learning about electricity and circuits, but what exactly is it? 

Voltage is defined as a measurement of the energy transferred per unit of charge passed, essentially the amount of energy in each unit of charge. 

A volt is the most common unit of measurement for potential difference, it is 1 joule of energy per coulomb of charge passed. 

E = V I t 

V = E / (I t)