Electricity Watts

• Watt is the unit of power!

• What is you unit of power?

The Watt

• The watt can be a little confusing. It is the unit of power; not all power is electrical!

• Sometime petrol motors outputs are stated in watts; this tells you how much power the motor is capable of giving out.

• We will be concentrating on the electrical definition of the watt; but must be aware that it has other uses.

Power (W) • A watt is the capacity to do work.• One watt is the rate at which work is done

when an object's velocity is held constant at one meter per second against constant opposing force of one newton.

W = Nm = Kg M² S S³

Power (W) electrical• In terms of electromagnetism, one watt is the

rate at which work is done when one ampere (A) of current flows through an electrical potential difference of one volt (V).

W = VA

• This formula can be transposed to fit the valve we want to find and values we have been given.

Watts

• To help remember the relationship between Watts(W), Current (I) and Volts (V) we can put the variables into a triangle.

W

VI

Watts

• If you want to find the value of V you need to know the values of W and I.

W

VI

=

Watts

WI =V

WV =I

V x I=W

W

VI

Watts

• Example;– An immersion heater has a rating of 3kW

(3000W), the voltage is 230V how many amps does the heater use?

– First extract the relevant information;• V = 230V• W = 3000W

Watts

• First draw the Watts triangle.• Cover the value you want to find.• Write the formula down.• Substitute the known values into the

formula.• Calculate the formula, give answer

remember to use the correct units.

First draw the Watts triangle.

Cover the value you want to find.

Write the formula down.

WV =I

Substitute the known values into the formula.

3000230 =I

Calculate the formula, give answer remember to use the correct units.

• Therefore the heating element will use; 13.04A (Amps).

3000230 =13.04

Questions • Calculate the following;

– 1. A bulb is powered with 230v and draws 1A how many Watts is it using.

– 2. A motor is rated at 95W it is running on 230v how many amps is it drawing.

– 3. An immersion heater is rated at 3Kw and is drawing 27.27A how many volts are powering the heater.