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.
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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