# PHY C6: Power

Here’s what you need to know:

• Power in terms of energy
• Power in terms of force
• Kilowatt-Hours

Let’s get moving!

What is Power?
Rate of energy usage.

Two objects may use the same amount of energy, but the one that requires LESS time to convert the energy to a useful output is MORE powerful.

Power can be expressed as the formula:
P = Work done/t
Where Work = Fd

Since work done = change in energy,
P = ΔE/t

TOTAL Power INPUT is equal to the USEFUL output + WASTEFUL output.
P =

Power is measured in Watts (W),
1W = 1Js-1

Power can also be expressed in terms of Force:
Power = Force x velocity

From P = W/t,
Since W = Fd,
P = Fd/t

Since d/t = velocity,
P = Fv

*Remember, just like Work,
Power is POSITIVE when force, displacement & velocity are in the same direction.
Power is NEGATIVE when force is opposing the displacement & velocity 9such as friction).

What is a Kilowatt-Hour?
Energy expended when work is done at a rate of 1 kW for 1 hour.
kWh is another method to measure ENERGY.
It is equivalent to the unit of JOULES.

1 kWh = 1000 W x 1 hour
1 kWh = 1000 W x 3600 s
1 kWh = 3.6 x 106 J

Electricity bills for houses are commonly measured in kWh, since the electrical energy spent in a common household would be too large to express easily in Joules.

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