Electrical Appliances and Power

Learning Objectives

To be able to calculate power, and cost of electricity

To be able to apply these calculations to help explain the efficiency of devices

To be able to apply this knowledge to complete exam style questions

Bell Work

Appliance Useful Energy Energy Wasted

Light Bulb

Electric Heater

Electric Toaster

Electric Kettle

Hairdryer

Electric Motor

Fill in your table with the useful and wasted types of energy

SUCCESS CRITERIA•A completed table showing the type of energy which is

useful and the type of energy which is wasted•A comment about where the energy is used or wasted

Appliance Useful Energy Energy Wasted

Light Bulb Light from glowing filament Energy transfer from filament heating the surroundings

Electric Heater Energy heating the surroundings

Light from the glowing filament

Electric Toaster Energy heating the bread Energy heating the toaster case and air around it

Electric Kettle Energy heating water Energy heating kettle itself

Hairdryer Kinetic energy of air driven by the fan, energy heating airflow past the filament

Sound of motor, energy heating the hairdryer itself

Electric Motor Kinetic energy of object driven by motor, Potential energy of object lifted

Energy heating the motor, sound energy of the motor

Electrical power is the rate at which an electrical appliance uses electrical energy. All appliances have a power rating.

What is electrical power?

Power is measured in watts (W). 1000 watts = 1 kilowatt (kW). 1 watt of power means that 1 joule of energy is used every second.

Appliances that need to create heat, such as washing machines, cookers, hair dryers and kettles, usually use the most power.

TVs, radios and computers usually use the least amount of power.

Power is measured in watts (W) or kilowatts (kW).

Current is measured in amps (A).

Voltage is measured in volts (V).

What are the units of power, current and voltage?

What is the formula for electrical power?

power = current x voltage

P = I x V

In electrical devices, power can be calculated using the formula:

A formula triangle helps you to rearrange a formula. The formula triangle for P = IV is shown below.

Cover up whatever quantity you are trying to find to leave the calculation required.

So to find current (I), cover up I…

…which gives the formula…

I =P V

Can I use a formula triangle?

x

A filament bulb has a potential difference of 200 V across it and a current of 0.2 A running through it.

How is power calculated?

At what power is the filament bulb operating?

= 40 W

= 0.2 A x 200 V

P = IV

Mini Review

Power is measured in kilowatts (kW) and the time is measured in hours (h), so what are the units of electricity measured in?

1 unit of electricity = 1 unit of electrical energy

Example:

How many units of electricity is 17.6 kWh? 17.6 units

The amount of electrical energy (i.e. the amount of electricity) used by an appliance depends on its power and how long the electricity is used for.

electrical energy = power x time

Calculating the units of electricity

= 1 kilowatt hour (kWh)

Electricity costs money, which is why every home has an electricity meter.

The units of electrical energy are called kilowatt hours (kWh).

How is electricity paid for?

The cost of an electricity bill is calculated from the number of units used.

The meter records how much electricity is used in a house in units of electrical energy.

The cost of electricity is the number of units of electrical energy multiplied by the cost per unit.

Example:

How much would 10 units of electricity cost at a price of 9p per unit?

cost = 10 units x 9 p/unit

cost = number of units x cost per unit

How much does electricity cost?

= 90 p

A kettle uses 45.2 kWh of energy.If electricity costs 10 p per unit, how much does it cost to use the kettle?

Number of units:

Cost of electricity:

Buying electricity – example

= 45.2 units

number of units of electricity = number of kilowatt hours

= 452 p or £4.52

= 45.2 units x 10 p / unit

cost = number of units x cost per unit

An iron that operates at a power of 3 kW for 4 hours uses electricity that costs 8p per unit. How much does it cost for the electricity used by the iron in that time?

Number of units:

Cost of electricity:

Buying electricity – example

= 12 units

= 12 kWh

= 3 kW x 4 h

number of units of electricity = number of kilowatt hours

= 96p

= 12 units x 8 p / unit

cost = number of units x cost per unit

Mini Review

Construct your Knowledge – Power of a Motor

You are going to be watching a simulation of an electric motor. Firstly you need to calculate the power of the motor when lifting 100gm.

What values do you need to write down from the simulation to calculate this?

Next we will be recapping efficiency in order to

calculate the efficiency of a motor!

The energy efficiency of a device can be calculated using this formula:

Energy efficiency does not have any units.

It is a number between 0 and 1 which can be converted into a percentage by multiplying by 100.

Useful energy is measured in joules (J).

Total energy is measured in joules (J).

energy efficiency = useful output energy

total input energy

How is energy efficiency calculated?

Now calculating efficiency

Mass of load (g)

Time/s Average Time (s)

Total energy in (J) (IxVxt)

Useful work (J)

(mgh)

Efficiency (%)

150 3.1 3.0 2.1 5.95 2.25 38

200 3.5 3.5 3.5 I=0.64

V=33.00

250 4.1 4.1 5.6 I=0.813

V=33.75

300 4.5 4.4 4.3 I=1.063

V = 34.5

350 4.9 4.8 5.0 I=1.303

V = 35.25

Copy this Table (but not the bits in red!)

Mini Review – Think Pair Share

• How does the efficiency of the motor change with different amounts of weight?

Try and explain this in terms of energy

Apply your knowledge

Complete exam style questions

Review

To be able to calculate power, and cost of electricity

To be able to apply these calculations to help explain the efficiency of devices

To be able to apply this knowledge to complete exam style questions

Review

Create your own question with answers

(make it as difficult as you can but remember you have to give the correct answers with the correct calculations)

These will be used for revision next week so make them good!