Dr Pusey

www.puseyscience.com

Syllabus points

• Power is the rate at which energy is transformed by a

circuit component; power enables quantitative analysis of

energy transformations in the circuit

This includes applying the relationship

= = W

P V I t

Learning goals

Define:

Power – Watts – Joules/second

Recall that the kilowatt hour (kW h) is an alternative unit for energy and is equal to 3.6 MJ

Be able to use and rearrange the equations for

power 𝑃 =𝑊

𝑡= 𝑉𝐼

Give the SI unit for: power

Power Work = transfer of energy

Power = rate of doing work, or rate of energy transfer/transformation (same as in motion topic)

Units Watts, W

1 W = 1 J/s

𝑃 =𝑊

𝑡

Power Work = transfer of energy

Power = rate of doing work or rate of energy transfer/transformation (same as in motion topic)

Units Watts, W

𝑃 =𝑊

𝑡Power (W)

Work (J)

Time (s)

𝑃 =𝑊

𝑡

𝑃 =𝑞𝑉

𝑡

𝑃 = 𝑉𝐼

We can substitute in

𝑊 = 𝑞𝑉

To get

𝐼 =𝑞

𝑡

So we can replace the q/t with I to get

Remember how

1.

2.

3.

𝑃 =𝑊

𝑡= 𝑉𝐼

POWER IS SIMPLY THE MULTIPLICATION OF CURRENT AND VOLTAGE

Example 1Revisiting:

A current of 2.0 A flowing through a heater for one hour, converts 1.7 MJ of electric energy into heat energy. Calculate:

a) The power rating of the heater

b) The energy used by the heater in 2 hours

Example 1Revisiting:

A current of 2.0 A flowing through a heater for one hour, converts 1.7 MJ of electric energy into heat energy. Calculate:

a) The power rating of the heater

Example 1Revisiting:

A current of 2.0 A flowing through a heater for one hour, converts 1.7 MJ of electric energy into heat energy. Calculate:

b) The energy used by the heater in 2 hours

From: http://www.originenergy.com.au/4218/How-to-read-your-electricity-bill

This chart is wrong:

Energy is measured in Joules

Power is measured in Watts

However:Energy = Power x Time

So kWh (Kilowatt Hours) can be a unit of Energy, although not technically SI!

If a typical Australian home uses 17 kWh per day, what is the typical POWER drawn at any given moment?

From: http://www.originenergy.com.au/4218/How-to-read-your-electricity-bill

This chart is wrong:

Energy is measured in Joules

Power is measured in Watts

However:Energy = Power x Time

So kWh (Kilowatt Hours) can be a unit of Energy, although not technically SI!

If a typical Australian home uses 17 kWh per day, what is the typical POWER drawn at any given moment?

17kWh / 24h = 708 Watts

Power (con) E.g.

A 50 W light bulb will use 50 J in one second

A 100 W light bulb will use 100 J in one second

In Australia the kilowatt hour (kWh) is used to measure domestic energy use

1 kWh is the energy used by a 1 kWh appliance in one hour

E(kWh) = P(kW) * t(h) = 103 W * (60 min * 60 s)

1 kWh = 3.6 * 106 J

Example 2A TV needs 250 W. It is switched on for 30 minutes. If each kWh costs 20 cents, how much does it cost to run the TV?

Example 2A TV needs 250 W. It is switched on for 30 minutes. If each kWh costs 20 cents, how much does it cost to run the TV?

ResourcesFurther Reading

Origin Energy - How to read your energy bill