Write down the energy transfers in the following:

Hydroelectric power station

Fan

Speakers Wind turbines

EfficiencyLearning objectives

• To be able to collect and use measurements of V and I to calculate power.

• To be able to calculate energy used from measurements of power and time.

• To be able to state what energy is ‘wasted’

This means that energy never just ‘disappears’.

The total amount of energy always stays the same,i.e. total input energy = total output energy.

In most energy transfers, the energy is transferred to several different forms, which may or may not be useful.

Energy that is transferred to unwanted forms of energy is wasted.

All energy transfers follow the law of conservation of energy:

Energy cannot be created or destroyed,just changed in form.

Law of conservation of energy

Efficiency

Efficiency is a measure of how good a device is at changing energy from one form to another.

All devices waste energy, so the efficiency of a device is never 100%.

energy efficiency = useful output energy

total input energy X 100

Calculating Efficiency:

0.45 or 45%

a) How much energy is wasted?

b) In what form is the energy wasted?

c) What is the efficiency of the bulb?

Efficiency = Useful Total

= 45 J100 J

= 0.45 or 45%

This filament bulb is supplied with 100 J of electrical energy, which it converts to 45 J of light energy.

55 J

heat

Wasted energy = Total – Useful = 100 J – 45 J

= 55 J

Calculating Efficiency

Energy Transfer diagramsConsider a light bulb. Let’s say that the bulb runs on 100 watts (100 joules per second) and transfers 20 joules per second into light and the rest into heat. We can show how energy is transferred into various forms using Sankey diagrams. Draw this as a diagram:

100 J/s electrical energy

“Input” energy “Output” energy

80 J/s heat energy (given to

the surroundings)

20 J/s light energy

Sankey diagrams

Consider a kettle:

1) Work out each energy value.

2) What is the kettle’s efficiency?

Sound energy

Wasted heat

Heat to water

2000 J/s electrical energy

Consider a computer:

150 J/s electrical energy

10 J/s wasted sound

20 J/s wasted heat

Useful light and sound

1) How much energy is converted into useful energy?

2) What is the computer’s efficiency?

Example questions

In which two forms is energy usually wasted?

A. Sound and light

B. Heat and sound

C. Heat and light

D. Light and chemical

What is the intended energy transfer for a Bunsen burner?

A. Sound light

B. Heat chemical

C. Chemical light

D. Chemical heat

What is the energy change for a falling rock?

A. Kinetic gravitational

B. Gravitational kinetic

C. Kinetic speed

D. Mass speed

In what form is energy wasted for all electrical devices?

A. Heat

B. Electrical

C. Sound

D. Light

The cost of electricityLearning objectives

• Understand that Power is calculated from IV and that energy consumed is calculated from:

Energy = Power x time.

• Know that mains electricity is charged for in units of kilowatthours.

• Use the equation to calculate the cost of electricity: cost = power x time x cost of 1kWh.

Buying electricity

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In your home will be an electricity meter.

It records how much electricity you use in units.

From the number of units you use your electricity bill is calculated.

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How many units have been used here? ______26

Demo to look at meter and different appliances

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

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

= 1 kilowatt hour (kWh)

Calculating the units of electricity

Cost of electricity

The cost of electricity is the number of units multiplied by the cost per unit:

Cost of electricity = Number of units x cost per unit

Example:

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

Cost = 10 x 9p

= 90 p

Cost of electricity = Power (kW) x time (h) x cost per kWh (p)

which is the same as……

Buying electricity – question 1:

A kettle uses 45.2 kWhr of energy.

If electricity costs 10p per unit, how much does it cost to use the kettle?

Number of units:

Number of units of electricity is 45.2 units

Cost of electricity:

Cost = 45.2 x 10 p

= 452p or £4.52

Buying electricity - question 2:

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 that it uses?

Number of units:

kWhr = 3kW x 4hr

= 12 kWhr

Number of units of electricity is 12 units

Cost of electricity:

Cost = 12 x 8 p

= 96p

Electricity is measured in units called “kilowatt hours” (kWh). For example…

A 3kW fire left on for 1 hour uses 3kWh of energy

A 1kW toaster left on for 2 hours uses 2kWh

A 0.5kW hoover left on for 4 hours uses __kWh

A 200W TV left on for 5 hours uses __kWh

A 2kW kettle left on for 15 minutes uses __kWh

Amount of electrical energy

To work out how much a device costs we do the following:

Cost of electricity = Power (kW) x time (h) x cost per kWh (p)

For example, if electricity costs 8p per unit calculate the cost of the following…

1) A 2kW fire left on for 3 hours

2) A 0.2kW TV left on for 5 hours

3) A 0.1kW light bulb left on for 10 hours

4) A 0.5kW hoover left on for 1 hour

48p

8p

8p

4p

The Cost of Electricity

The cost of electricityLearning outcomes

• Use the equation to calculate the cost of electricity:

cost = power × time × cost of 1kWh

where power is measured in kilowatts and time is measured in hours.