# Physics - Energy efficiency

a light bulb has an efficiency of 0.3. what does this mean?
30% of the energy transferred in the light bulb is useful as it's transferred as light energy while the rest is wasted
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what happens to the wasted energy?
it disspotates into surroundings as thermal (heat) energy
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How do you calculate energy efficiency?
Useful energy output/ total energy input
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if an object is 70% efficient, how much energy is wasted / transferred usefully?
70% is transferred usefully. 30% is wasted, dissipating into surroundings
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A hob is heating water in pan. 15KJ of thermal energy is transferred by a hob. 12KJ passes into water. Calculate efficiency
12/15 =0.8 = 80%
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How could you increase efficiency?
Add lid so less energy is transferred
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Electric current deliverers 60 watts to bulb. 3 watts are transferred as light. Calculate efficiency
3/60 = 0.05 = 5%
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is it also true that efficiency = useful power output / total power input ?
yes
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## Other cards in this set

### Card 2

#### Front

what happens to the wasted energy?

#### Back

it disspotates into surroundings as thermal (heat) energy

### Card 3

#### Front

How do you calculate energy efficiency?

### Card 4

#### Front

if an object is 70% efficient, how much energy is wasted / transferred usefully?

### Card 5

#### Front

A hob is heating water in pan. 15KJ of thermal energy is transferred by a hob. 12KJ passes into water. Calculate efficiency