# physics

HideShow resource information

## the formula for power

power (watt, W) = voltage (volt, V) × current (ampere, A)

1 of 8

## units

power is the rate at which energy is transferred or used

the unit of power is joule (J)

current is shown as I and measured in amperes (amps)

potential difference is V, with the units measured in volts (V)

2 of 8

## current

current is measured in amps, this ranges from 3a, 5a, 8a and 13a

you know which fuse you need because you choose the one that is bigger, for example if something was 2.5 watts i would buy a 3a fuse

3 of 8

## energy

The amount of electrical energy transferred to an appliance depends on its power and the length of time it is switched on. the elecrtricity is transferred in killowatt per hour (kWh)

E is the energy transferred in kilowatt-hours, kWh

P i is the power in kilowatts, kW

T is the time in hours, h

4 of 8

## question 1

Double-glazing might cost £2,500 and save £100 a year. What is the payback time?

payback time = cost of energy-saving measure ÷ money saved each year

= 2,500 ÷ 100

= 25 years

5 of 8

## cost of electricity

The more units of electricity used, the greater the cost. The cost of the electricity used is calculated using this equation:

total cost = number of units × cost per unit

For example, if 5 units of electricity are used at a cost of 8p per unit, the total cost will be 5 × 8 = 40p.

total cost = power (kW) × time (h) × cost per unit

6 of 8

## watts to killowatts

power is measured in kilowatts here instead of watts. To convert from W to kW you must divide by 1,000.

For example, 2,000 W = 2,000 ÷ 1,000 = 2 kW.

7 of 8

## converting time

time is measured in hours here, instead of the more usual seconds. To convert from seconds to hours you must divide by 3,600.

For example, 7,200 s = 7,200 ÷ 3,600 = 2 h.

8 of 8