Energy

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Calculating Power

Calculating Power

Power can be defined using an equation which we will use here to calculate its value, expressed as a unit.

Imagine you lifted one end of a sofa because your friend dropped something that rolled under it. You lifted the sofa for 10 seconds. It required 500 Joules for you to do that.

What was the energy transferred?

True or false? "Energy transferred" and "power" are the same thing.

True or false? "Energy transferred" and "work done" are the same thing.

How much energy was transferred per second when you lifted the sofa?

The power is essentially the amount of work done per second

In this case that was 50 Joules per second.

So power is amount of work done per second. That means that power is...

Now, we don't actually use "Joules/second" as the unit for power. What do we use?

To recap!

Power is work done ( or "energy transferred") per second

So if the work done over 10 seconds is 500 Joules, the power is 50.

But we don't use the unit "Joules/second" for power

We use the unit Watts, which essentially means the same as "Joules/second".

So power is a measure of how much energy is transferred per second

That means that power is the rate of energy transfer.

You spend 50 seconds moving an object 10 metres. The energy transfer is 50 Joules. What was the power you used?

1 Watt is essentially the same as...

The formula for power

If the work done over 10 seconds was 500 Joules, then the power was 50 Watts. How do we calculate power?

We can also write the formula for power shorter, like this $P=\frac{W}{t}$ . What does $W$ stand for?

Be careful not to confuse the $W$ in $P=\frac{W}{t}$ with the W you use to abbreviate Watts!

If you write out the power formula with units, it looks like this $Watts = \frac{Joules}{second}$

What is the unit you should use for time when calculating power?

What is the unit for energy transferred or work done?

What is the unit for power?

A car uses 300 Joules of energy from its engine to travel for 3 seconds. What is the power of the car?

Which one of these require more power?

Summary!

Power is the rate of energy transfer (work done) per second

You calculate it like this $P = \frac{W}{t}$ where $W$ is work done.

The unit for power is Watts

Watts essentially means "Joules/second", but we use the term "Watts".