Section 2 b) Summary

Mains electricity is the electricity supplied to each home. In the UK it is a.c. and 230V. A.c. means alternating current: it is constantly changing. By contrast, d.c. means direct current and does not change direction. This can be found in cell or battery powered circuits.

Current flowing though a circuit causes resistance. This can be calculated with the following formula:

Voltage = Current x Resistance
V= I R
(Remember this by calling it VIR pronounced veer)

Resistance causes a heating effect due to energy transfer. This increase in temperature also increases the resistance. This is used to our advantage in a variety of domestic situations

• Fuses melt when the current is too high because it causes resistance and heating
• Toasters toast bread with coils of high resistance wires that heat up
• Kettles can heat water with a high resistance coil in the bottom of the appliance. 

Fuses and circuit breakers are chosen based on the current that should flow through the appliance with normal use. Fuses generally come in 3A, 5A, 7A and 13A. An appliance that would usually have a current of 2.3A flowing would use a 3A fuse, for example. 

Appliances are usually rated on power and voltage, so to calculate the necessary fuse we need to find out the current through this formula:

Power = Voltage x Current
P = I V
(Remember it by calling it PIV like pivot)

So an appliance with a rating of 1000W and 240V would be
1000 / 240 = 4.17 A
So we should use a 5 A fuse. 

Another important formula to know is this:

Energy transferred = Voltage x Current x Time
E = V I T
(Remember it by calling it EVIT like eviter, to avoid, in French)


Electrical circuits come with hazards that can cause injury or damage:

But there are often safety features put in place to ensure we are not hurt:

Section 2 b) Key Words

Alternating current: Current is constantly changing direction

Circuit breaker: An electrical safety device that will trip and break a circuit if too much current is flowing through. Use electromagnets to trip the switch and can be reset.

Conductor: A material that can easily transmit electrical energy, so aqueous/molten ionic compounds and metals, due to the free charged particles allowing current to pass through.

Double insulation: Two layers of insulation of a product, e.g. a plastic coated kettle. Greatly reduces chances of electric shock in event of fault.

Earth wire: Provides a path of least resistance to earth for high voltage currents in event of a fault.

Fuse: A thin wire that will melt when too much current flows through due to heating effect. Electrical safety device, requires replacement.

Insulator: A material that does not transmit electrical energy easily.

Mains electricity: A.c., 230V : the electricity that enters each home.

Resistance: Slows the flow in a circuit down. Causes a heating effect.

Section 2 b) Specification

2.2 understand and identify the hazards of electricity including frayed cables, long cables, damaged plugs, water around sockets, and pushing metal objects into sockets

2.3 understand the uses of insulation, double insulation, earthing, fuses and circuit breakers in a range of domestic appliances

2.4 understand that a current in a resistor results in the electrical transfer of energy and an increase in temperature, and how this can be used in a variety of domestic contexts

When there is an electrical current flowing through a wire, there is energy transfer which causes a heating effect. Higher currents cause more heat to be emitted, which means there is more resistance, causing less current to flow at the same voltage.
This effect can be used to our advantage, the principle is used in fuses: they melt when too much current is flowing because they heat up.
It can also be used in other appliances. Toasters and kettles often contain high resistance wires that heat up and boil our water or cook our toast. This was also used in old-style light bulbs, but since then more efficient ways have been developed.

2.5 know and use the relationship:
power = current × voltage
P = I × V
and apply the relationship to the selection of appropriate fuses

If an appliance has a power rating of 1000 W and 230 V, we can use this relationship to determine that the current is 1000 / 230, which equals 4.3 A. So in selecting a fuse, we would choose one that is close, but just slightly higher to normal operating current. In this case, a 5A fuse would be selected.

2.6 use the relationship between energy transferred, current, voltage and time:
energy transferred = current × voltage × time
E = I × V × t

If we know the current in an appliance is 2A and it is connected to a 4.5V battery, what is the energy transferred over 5 minutes?
2 x 4.5 x 5 x 60 = 2700 joules

2.7 understand the difference between mains electricity being alternating current (a.c.) and direct current (d.c.) being supplied by a cell or battery.

Alternating current means the current is constantly changing direction. Mains electricity is alternating current because it is easy to transport long distances, and easily converted using a transformer (advantageous because it allows current to be reduced and therefore less resistance will happen in power lines and therefore less energy lost)

^a.c. reading on an oscilloscope

Direct current means the direction of current does not change. This means it will provide a constant voltage.

^d.c. reading on an oscilloscope