# Physics 1A - Energy: Efficiency of Machines & The Cost of Electricity

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What is the formula for efficiency?
Efficiency = Useful Energy Out / Total Energy In
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How do you figure out efficiency if you don't know the energy in and out of the device?
Efficiency = Useful Power Out / Total Power In
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Can a device be 100% efficient?
No device is 100% efficient and the wasted energy is usually spread out as heat.
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What is the formula for energy transferred?
Power x Time.
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1 Kilowatt-Hour is...
...the amount of electrical energy used by a 1kW appliance left on for 1 hour.
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What is the formula for cost of energy?
Cost = Energy Transferred (kWh) x Price per kWh.
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## Other cards in this set

### Card 2

#### Front

How do you figure out efficiency if you don't know the energy in and out of the device?

#### Back

Efficiency = Useful Power Out / Total Power In

### Card 3

#### Front

Can a device be 100% efficient?

### Card 4

#### Front

What is the formula for energy transferred?

### Card 5

#### Front

1 Kilowatt-Hour is...