Energy transfer
When workEnergy transferred by a force. Work done = force 脳 distance moved in the direction of the force. is done on an object, energy is transferred. The ratePer unit time or 鈥榩er second鈥. 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 梅 time taken.. So the more powerful a device is, the more energy it will transfer each second. This is important when considering the total cost of electricity used as shown on an electricity bill.
Learn more on energy transfers in everyday appliances in this podcast
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Calculating energy transferred
Joules and seconds
The energy transferred can be calculated using:
energy transferred = power 脳 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 脳 60 = 600 s
energy transferred = 250 脳 600
energy transferred = 150,000 J (150 kJ)
Calculating electrical power
Power can be calculated using:
\(power = \frac{energy~transferred~(work~done)}{time}\)
This is when:
- power is measured in watts (W)
- energy transferred (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 脳 60 = 120 s
power = 1,200 梅 120
power = 10 W
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 脳 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 梅 60 = 0.2 h
energy transferred = power 脳 time
energy transferred = 10 脳 0.2
energy transferred = 2 kWh