Download Electrical power (N4/5)

REVISION NOTES
ELECTRICAL POWER
NATIONAL 4/5
Electrical power (N4/5)
Electrical appliances convert electrical energy into other types of energy.
Fan: Electrical  Kinetic
Lamp: Electrical  Light + Heat
Electrical power is the rate at which an appliance uses energy – the number of
joules of energy per second.
P = E/t
P = power in watts (W)
E = energy in joules (J)
t = time in seconds (s)
A 60 watt light-bulb uses 60 joules every second.
The cost of electricity depends on:
- the power rating of the appliance
- the time it is switched on for
Electricity use is measured in kilowatt-hours (kWh).
If a 1 kW (1000 W) heater is switched on for 1 hour, it uses 1 kWh.
If a 500 W (0.5 kW) TV is left on for 6 hours, it uses 0.5 kW x 6 hours = 3 kWh.
A kWh usually costs about 15 pence.
Efficiency (N4)
Efficiency measures how much useful energy we get out compared to how much we
put in. Nothing in real life is ever 100% efficient.
% efficiency =
useful energy out x 100%
energy put in
If a bulb uses 500 J of electrical energy and gives out 350 joules of light energy:
% efficiency = (350/500) x 100% = 70% efficient
We can also use calculate efficiency based on power.
% efficiency =
useful power out x 100%
power in
Power, current and voltage (N4/5)
If you increase the current in a bulb, it gets brighter.
If you increase the voltage across a bulb, it gets brighter.
So the power of the bulb depends on the current and the voltage.
P=IxV
P = power in watts (W)
I = current in amperes (A)
V = voltage in volts (V)
Example
A 60 W light bulb is attached to mains voltage (230 V). Calculate the current in the
bulb.
I = P/V
= 60/230
= 0.26 A
Fuses (N4/5)
As mains voltage is fixed (230 V), this means that the higher the power rating of an
appliance, the higher the current it uses.
This is why different appliances need different values of fuses in their plugs.
Appliances up to 690 W use a 3 A fuse.
(I = P/V = 690/230 = 3 A)
Appliances up to 3000 W use a 13 A fuse.
(I = P/V = 3000/230 = 13 A)
Alternative power formulas (N5)
We know that:
P=IxV
We also know Ohm’s Law:
V=IxR
So:
P = I x (I x R)
giving
P = I2R
Also
P = (V/R) x V
giving
P = V2/R
Combining power formulas (N5)
A 12 V motor draws a current of 3 A. How much energy does it use in 30 seconds?
Step 1:
P = I x V = 3 x 12 = 36 W
Step 2:
E = P x t = 36 x 30 = 1080 J