Energy from batteries and the mains
When workEnergy transferred by a force. Work done = force Ă distance moved in the direction of the force. is done on an object, energy is transferred. The ratePer unit time or âper secondâ. For example, if 2,000 J are transferred over a period of 10 s, then the rate of transfer is 200 J/s or 200 W. This value is the power rating. at which this energy is transferred is called powerThe energy transferred each second, measured in watts (W). Power = work done Ă· time taken.. So the more powerful a device is, the more energy it will transfer each second. This is important when considering how much an electricity bill might cost.
Calculating electrical power
Power can be calculated using:
\(power = \frac{work~done}{time}\)
This is when:
- power is measured in watts (W)
- work done is measured in joules (J)
- time is measured in seconds (s)
The power output of a device increases if:
- more energy is transferred in a given time
- the same amount of energy is transferred in a shorter time
The energy transferred by an electrical device can be investigated using a joulemeter. The power output of a device can be calculated if the length of time the device is switched on is also measured.
Example
An electric lamp transfers 1,200 J in 2 minutes. Calculate its power.
2 minutes = 2 Ă 60 = 120 s
power = 1,200 Ă· 120
= 10 W
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Calculating energy transferred
Joules and seconds
The energy transferred can be calculated using:
energy transferred = power Ă time
This is when:
- energy transferred is measured in joules (J)
- power is measured in watts (W)
- time is measured in seconds (s)
Example
A 250 W computer is used for 10 minutes. Calculate the energy transferred.
10 minutes = 10 Ă 60 = 600 s
energy transferred = 250 Ă 600
= 150,000 J (150 kJ)
Kilowatt-hours and hours
The electricity bills from an energy supplier show the energy used in kWh, rather than in J or KJ.
The energy transferred is still calculated using:
energy transferred = power Ă time
But:
- energy transferred is measured in kilowatt-hours (kWh)
- power is measured in kilowatts (kW)
- time is measured in hours (h)
Example
A 10 kW shower is used for 12 minutes. Calculate the energy transferred in kWh.
12 minutes = 12 Ă· 60 = 0.2 h
energy transferred = power Ă time
= 10 Ă 0.2
2 kWh