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Energy in Circuits

Energy in Circuits

As an electric current flows through a circuit, energy is transferred from the battery or power supply to the components in the circuit. For example, electrical energy is transferred to light and heat by bulbs.

An electric current is a flow of charge. The charge is carried by electrons flowing around the circuit.

When charge flows through any resistor, some electrical energy is transferred as heat.

Power

Power is the amount of energy an appliance transfers every second. The more powerful it is, the more energy it transfers per second.

Power is measured in joules per second, or watts (W).

So, 1 J/s = 1 W.

A kilowatt (kW) is 1 000 watts.

A kettle transfers 2 300 joules of energy every second. It has a power rating of 2 300 W, or 2,3 kW.

A radio only transfers about 12 joules per second. It has a power rating of 12 W.

Electrical power can be calculated using the following equation:

power	=	potential difference	x	current
P	=	V		I
(watt, W)		(volt, V)	(ampere, A)

For example, if a 230 V fan heater takes a current of 5 A:

P = V I
P = 230 x 5
P = 1150 W

This equation can be used to calculate the size of a fuse that should be fitted in a plug.

The fuse should have a higher value, but as close as possible to, the current through the appliance when it is working normally.

For example, a 230 V kettle has a power rating of 2 300 W.

P = V I can be rearranged to:	I =	P
		V

	I =	2 300
		230

	I =	10 A

The closest fuse above this is 13 A.

Charge

Electrical charge is measured in coulombs (C).

A higher voltage gives more power, because each electron carries more energy. More energy is transferred for a given amount of charge which flows. This is shown in the equation:

energy transferred	=	potential difference	x	charge
E	=	V		C
(joule, J)		(volt, V)		(coulomb, C)

If a current of 1 ampere flows for 1 second then 1 coulomb of charge is passing.
This is shown in the equation:

charge	=	current	x	time
Q	=	I		t
(coulomb, C)		(ampere, A)		(second, s)

These two equations can be put together. If I x t is substituted for Q into the first equation we get:

energy transferred =	potential difference	x	current	x	time
E =	V		I		t
(joule, J)	(volt, V)		(ampere, A)		(coulomb, C)

For example, if a 250 V fan takes 2 A of current, how much energy will it transfer in 10 minutes?

E = V I t
Time needs to be in seconds so,
t = 10 x 60
= 600 s
E = 250 x 2 x 600
E = 300 000 J