REVISION

• Y10 Revision of Energy and Electricity

infraredwaves

The Earth is warmed by heat energy from the Sun.

How does this heat energy travel from the Sun to the Earth?

There are no particles between the Sun and the Earth, so the heat cannot travel by conduction or by convection.

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The heat travels to Earth by infrared waves. These are similar to light waves and are able to travel through empty space.

How does heat travel through space?

Heat can move by travelling as infrared waves.

These are electromagnetic waves, like light waves, but with a longer wavelength.

This means that infrared waves act like light waves:

They can travel through a vacuum.

They travel at the same speed as light – 300,000,000 m/s.

They can be reflected and absorbed.

What are infrared waves?

Infrared waves heat objects that absorb them and are alsoknown as thermal radiation.

Investigating thermal absorption

Absorbing thermal radiation

Infrared waves heat objects that absorb (take in) them.

Certain surfaces are better at absorbing thermal radiation than others. Good emitters are also good absorbers.

best emitter worst emitter

best absorber worst absorber

white silvermattblack

Matt black surfaces are the best absorbers of radiation.

Shiny surfaces are the worst emitters because they reflect most of the radiation away.

Why are solar panels that are used for heating watercovered in a black outer layer?

Heat energy only flows when there is a temperature difference from a warmer area to a cooler area.

Why does heat transfer happen?

Heat is a type of energy called thermal energy.

Heat can be transferred (moved) by three main processes:

1. conduction

2. convection

3. radiation

During heat transfer, thermal energy always moves in the same direction:

HOT COLD

Why do objects get hotter or colder?

Temperature is a measure of how hot an object is.

Heat transfer only takes place when there is a temperature difference. The heat energy flows from a warmer area to a cooler area.

Why does an ice lolly melt on a warm tongue?

There is a temperature difference between the tongue and the lolly, so heat energy flows from the warm tongue into thecold ice lolly.

This heat transfer means that the ice lolly melts as it gets warmer, and the warm part of the tongue touching it gets cooler.

How might climate change cause the polar ice caps to melt?

How are the particles arranged in a solid, a liquid and a gas?

What is conduction?

solid liquid gasParticles that are very close together can transfer heat energy as they vibrate. This type of heat transfer is called conduction.

Conduction is the method of heat transfer in solids but not liquids and gases. Why?

What type of solids are the best conductors?

How do non-metals conduct heat?

Metals are good conductors of heat. The outer electrons of metal atoms are not attached to any particular atom. They are free to move between the atoms.

When a metal is heated, the free electrons gain kinetic energy.

This means that the free electrons move faster and transfer the energy through the metal.

This makes heat transfer in metals very efficient.

Insulators do not have free electrons and so they do not conduct heat as well as metals.

How do metals conduct heat?

heat

What is convection?

Warmer regions of a fluid are less dense than cooler regions of the same fluid.

The warmer regions will rise because they are less dense.

The cooler regions will sink as they are more dense.

This is how heat transfer takes place in fluids and is called convection.

The steady flow between the warm and cool sections of a fluid, such as air or water, iscalled a convection current.

Example: Adding silver reflectors behind radiators costs £25 and saves £50 per year.

Payback time is the time it takes for the cost of installing insulation to be equalled by the savings made from reduced energy costs.

= 0.5 years (6 months)

What is payback time?

payback time (in years) =cost of insulation

saving each year

payback time = 25 50

3 years

80 years

1 year

5 years

Why is double glazing popular if the payback time is so long?

What is the payback time for these types of insulation?

Calculating payback time

How heat escapes

Cost of heat escapingper year

Cost of insulation

Payback time

roof

windows

draughts

walls

£80

£40

£50

£100

£240

£3,200

£50

£500

electrical

light

sound

heat

An energy transfer diagram shows the input and output energies for a device. This includes all the useful and wasted forms of energy.

For example, in a television:

Energy transfer in a television

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

Remember that energy cannot be created or destroyed.

Wasted energy spreads out into the surroundings – this is called dissipated energy.

This dissipated energy is too spread out to do useful work and so cannot be reused.

What happens to the wasted energy produced when energy is changed from one form to another?

What happens to wasted energy?

For example, the heat and light energy produced by this light bulbare too spread out to be reused.

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?

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

Calculate the efficiency of a bulb

Non-renewable or renewable?

What happens in a coal/oil power station?

Carbon dioxide – This greenhouse gas is the main waste product of burning fossil fuels. Increased levels of carbon dioxide due to human activities are thought to be connected with global warming.

What waste do fossil fuels produce?

Burning fossil fuels creates waste products that can act as pollutants and have harmful environmental effects.

Sulfur dioxide and nitric oxides – These gaseous pollutants contribute to the formation of smog and acid rain.

Ash – This waste solid is disposed of in landfill sites.

What can be done to reduce the problems caused byburning fossil fuels?

What is the greenhouse effect?

What happens in nuclear power station?

How does a greenhouse capture solar energy?

Solar power stations use the Sun’s energy to heat water and make steam, which then drives a turbine to produce electricity.

Some solar power stations use a series of mirrors, called heliostats, to reflect light onto a boiler.

This solar power station in California consists of about 1800 heliostats, with an electrical output of 10 megawatts.© Sandia National Laboratory/NREL

What is a solar power station?

Solar cells (or photocells) turn light energy from the Sun directly into direct current electricity.

Manufacturing solar cells is very expensive and requires the use of highly toxic materials. However, once the solar cell is built it produces no pollution and requires little maintenance.

Solar cells are ideal for use in remote locations where maintenance is difficult and other sources of electricity would be expensive.

Satellites have been powered using solar cells since the 1950s.

Why use solar cells to power satellites?

Pros and cons of solar cells

Biomass, also known as biofuels or bioenergy, is material from living sources. The simplest biomass energy sources are plants, which can be burnt to produce steam to turn a turbine.

Traditionally, wood is burnt to give heat but trees grow slowly and require a lot of land. Other materials, such as waste from poultry farms, can also be burnt.

Biomass fuels are renewable as more plants can be grown, producing yet more biomass.

The carbon dioxide released during burning biomass is absorbed by the replacement plants. Biomass is said to be

‘carbon neutral’ and does not add to global warming.

What is biomass?

Using biofuels – true or false?

© David Parsons/NREL

A geothermal power plant can be built to utilize steam and hot water that rise naturally to the Earth’s surface.

How can hot rocks generate electricity?

The steam and hot water provide a way of turning a generator, which then produces electricity.

This is the largest producer of geothermal power in the world. It is in California and has an output of 750 MW of electricity.

Sometimes, the hot water is trapped in natural underground reservoirs and does not reach the Earth’s surface. This watercan be reached by drilling to depths of several kilometres.

Pros and cons of geothermal energy

Wind turbines have large blades that capture the kinetic energy of the wind.

This kinetic energy is used directly to turn the turbine and generate electricity.

How do wind turbines work?

Wind turbines are a source of 'clean energy’ as they do not produce any polluting waste.

However, some people consider this source of renewable energy to be noisy and an eyesore. There is also some concern that, if wind turbines are poorly located, they couldkill migrating birds.

Wind power – true or false?

How does a hydroelectric dam work?

Pros and cons of hydroelectric power

How does wave power work?

Wave power – true or false?