P1.2 energyefficiency

P1.2 Energy and efficiency

Appliances transfer energy but they rarely transfer all of the energy to the place we want. We need to

know the efficiency of appliances so that we can choose between them, including how cost effective

they are, and try to improve them.

✓ compare the efficiency and cost effectiveness of methods used to reduce ‘energy consumption’

✓ describe the energy transfers and the main energy wastages that occur with a range of appliances

✓ interpret and draw a Sankey diagram.

Key wordsApplianceEfficiencyCost effectivenessSankey diagram

What would life be like without electricity?

P1.2.1 Energy transfers and efficiencya) Energy can be transferred usefully, stored, or dissipated, but cannot be created or destroyed.

b) When energy is transferred, only part of it may be usefully transferred; the rest is ‘wasted’.

c) Wasted energy is eventually transferred to the surroundings, which become warmer. The wastedenergy becomes increasingly spread out and so becomes less useful.

d) To calculate the efficiency of a device using:

efficiency = useful energy out (x100%) total energy in

efficiency = useful power out (x100%) total power in

Useful energy = Light + Sound 119 + 1 = 120 Efficiency = (useful ÷ total) x 100%= (120 ÷ 200)x 100%= 60%

More efficient so less electricity needed Less fuel burned, so less CO2 produced

Changes in picture brightness and loudness of sound affect energy transfer

Sankey diagrams can be used to show efficiencies:

An efficient machine

An inefficient machine

Least energy wasted as heat – 45% efficient

28 J/s

16 J/s

6 J/s

16 + 6 = 22 J/s

Efficiency = (useful ÷ total) x 100%= (28 ÷ 50) x 100%= 56%

More efficient so less electricity needed Less fuel burned, so less CO2 produced

P1.3 The usefulness of electrical appliances

We often use electrical appliances because they transfer energy at the flick of a switch. We can

calculate how much energy is transferred by an appliance and how much the appliance costs to

run.

✓ compare the advantages and disadvantages of using different electrical appliances for a particular application

✓ consider the implications of instances when electricity is not available.

I can:

Key wordsApplianceTransducerDynamoPowerKilowatt-hours

Transferring electrical energya) Examples of energy transfers that everyday electrical appliances are designed to bring about.

Name one transducer (device) that changes:A. Sound energy to electrical energy

…………………………………………….

B. Chemical energy to electrical energy …………………………………………….

C. Electrical energy to light energy …………………………………………….

D. Potential energy to kinetic energy …………………………………………….

Draw a chain diagram to show the energy changes that occur when:

A. An electric drill is switched on

B. A Bunsen burner is ignited

C. A mass is lifted 1 metre

D. A bullet is fired from a rifle

E. A rubber ball is bounced on the floor.

b) The amount of energy an appliance transfers depends on how long the appliance is switched onand its power.

c) To calculate the amount of energy transferred from the mains using:

E = P x t

E is energy transferred in kilowatt-hours, kWhP is power in kilowatts, kWt is time in hours, h

ExampleHow much energy is transferred if a 1 kW fire is left on for 2 hours?

SolutionUsing E = P x tSubstituting in P = 1 kW and t = 2 h givesE = 1 x 2 = 2 kWh

d) To calculate the cost of mains electricity given the cost kilowatt-hour.

If 1 kWh cost 14p.How much does it cost to run a 1 KW fire for 2 hours?

We have already calculated the kWh aboveSo the cost will be 14 x 2 = 28p

energy = power × time

power = 850 W = 0.85 kW,time = 6 minutes = 0.1 h

Energy = 0.85 x 0.1

= 0.085 kWh

20 J 60 J

20 J

Efficiency = (useful ÷ total) x 100%= (20 ÷ 80)x 100%= 25%

Light and sound

Efficiency = (useful ÷ total) x 100%= (480 ÷ 800)x 100%= 60%

Transferred to the surroundings

Useful Power = 1200 x 0.8 = 960 WWasted energy = 1200 – 960 = 240 WEnergy = Power x Time = 0.24 kW x 0.5= 0.12 Cost = 0.12 x 15 = p

Transferred to the surroundings as heat and sound

Difference in kWh per year =350-225 = 125 kWh

Cost = 125 x 12 = 1500p = £15

Each year costs £15.So a reduction in (12-9) 3 years gives a saving of 3 x 15 = £45

YES

Less electricity used / energy neededLess fossil fuels burnedLess polluting gases emitted

NO

Old freezer must be disposed ofHazardous chemicals inside freezer eg CFCsLot of Energy used in producing new freezer

Iron

HairdryerKettle

sound