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)
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