Physics Revisionhttp://www.bbc.co.uk/schools/gcsebitesize/science/aqa/

Which will melt the ice cube fastest? Why?

Metal block Wood block

Which will melt the ice cube fastest? Why?

Metal block Wood block

The Metal block! Why?

Energy transfers

• Conduction: heat energy transfer through solids e.g. metal rod(particles)

• Convection: heat energy transfer through fluids e.g. liquids and gases(particles)

• Radiation: heat energy transfer by waves through a vacuum and surface material

Kinetic theory• The kinetic particle theory explains the properties of the different

states of matter. The particles in solids, liquids and gases have different amounts of energy. They are also arranged differently and move in different ways.

More energy = more vibrations!

Which state of matter has the most energy?

Just for metals…

• The electrons in piece of metal can leave their atoms and move about in the metal as free electrons.

• The parts of the metal atoms left behind are now charged metal ions.• The ions are packed closely together and they vibrate continually. • The hotter the metal, the more kinetic energy these vibrations have.• This kinetic energy is transferred from hot parts of the metal to cooler

parts by the free electrons. • These move through the structure of the metal, colliding with ions as

they go.

Heat vs temperature

• Temperature and heat are not the same thing because:

• Temperature is a measure of how hot something is• Heat is a measure of the thermal energy contained in an object.• Temperature is measured in oC and heat is measured in J.

• Freezing point of water: 0oC

When materials change state (solid liquid) the temperature stays the same until all of the material has melted

Flow of energy is always hot to cold

How does a hot air balloon work? • A burner at the base of the balloon warms

the air inside giving the air molecules more kinetic energy.

• As the air warms up, it moves upwards as it becomes less dense, then cools once away from the heat source, becomes more dense and moves round in a circular pattern known as a convection current.

• When the balloon is full of hot air, it lifts off the ground because the hot air inside it is less dense, lighter and has more kinetic energy than the cold air outside it.

Infrared radiation• All objects emit (give out) and absorb (take

in) thermal radiation, which is also called infrared radiation. The hotter an object is, the more infrared radiation it emits.

• Infrared radiation is a type of electromagnetic radiation, which involves waves rather than particles.

• This means that, unlike conduction and convection, radiation can even pass through the vacuum of space. This is why we can still feel the heat of the Sun, although it is 150 million km away from the Earth.

Vacuum: Empty space which has NO particles!

Absorbing thermal radiation

best emitter worst emitter

best absorber worst absorber

white silvermattblack

• Matt black surfaces are the best absorbers and emitters of radiation.

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

• Shiny surfaces are the worst absorbers and emitters because they reflect most of the radiation.

U Values• U-values measure the effectiveness of a material as

an insulator in buildings.

• The lower the U-value is, the better the material is as a heat insulator.

• For example, here are some typical U-values for building materials:

• a cavity wall has a U-value of 1.6 W/m²• a solid brick wall has a U-value of 2.0 W/m²• a double glazed window has a U-value of 2.8

W/m².

Evaporation and condensation

Evaporation and condensation are changes of state:• Evaporation involves a liquid changing to a gas• Condensation involves a gas changing to a liquid

• Evaporation is the reason why damp clothes dry on a washing line.• Condensation is the reason why windows become foggy on a cold day.

If a …………….. is at the……………., and moving ………. enough, it may ……… the liquid. This is called …………..

Freedom!

Evaporation CondensationSurfaceSlowFastMolecule EscapeTrap

Evaporation

Evaporation

If a molecule is at the surface, and moving fast enough, it may escape the liquid. This is called evaporation.

Freedom!

4 factors that affect evaporation rate

1. Temperature2. Surface area3. Humidity 4. Air flow over the liquid surface

Which ones also affect condensation? 2, 4

Questions:1. Why are warm windy days the best days to hang out

your washing? Why should washing be stretched out on the washing line?

2. You have spilled some water on your kitchen floor and you do not have any mops, clothes or similar material to soak up the water. What can you do to increase the rate of evaporation?

3. In the tropical rainforest it is difficult to get washing to dry (even if it is not raining!). Explain why.

Keeping warm or cold• The bigger the difference in temperature between an object and its surroundings, the

greater the rate at which heat energy is transferred. • Other factors that affect the rate at which an object transfers energy by heating include:

• Surface area of object• Volume of the object• Material used to make the object• Nature of the surface that the object is touching

Animal Issues• Small animals like mice have a large surface area compared totheir volume. They lose heat to their surroundings very quickly and must eat a lot of food to replace the energy lost.

• Elephants have large ears with a large surface area compared to their volume. They lose heat to their surroundings more slowly and may even have difficulty avoiding overheating. Their ears allow heat to be transferred from the elephant to its surroundings, helping to keep the animal cool.

Human Issues

• Engineers design heat transfer devices so that they gain or lose heat energy efficiently. For example, car radiators are flat, with many small fins to provide a large surface area.

• Similarly, household radiators are thin and flat, and may have fins so that heat energy is transferred to the room quickly.

Types of energy and energy changes

• Energy cannot be created or destroyed, it can only be changed from 1 form to another.

• Nuclear• Chemical• Elastic • Gravitational • Kinetic • Thermal • Light• Sound• Electrical

Wasted energy: energy transformed that isn’t

requirede.g. light bulbsElectrical lightElectrical heat

Sankey diagrams

Efficiency Calculations Efficiency = Useful energy output x 100%

Total energy input

SPECIFIC HEAT CAPACITY

At the end of a sunny day at the beach, you often notice that while the sand has become quite hot, the water has stayed cool.

WATER SAND

SAME amount of HEAT ENERGY

Small TEMPERATURE

RISE

Large TEMPERATURE

RISE

Putting the SAME AMOUNT OF HEAT into some materials gives a BIGGER TEMPERATURE RISE than in other materials

Specific Heat Capacity

• The amount of energy needed to change the temperature of 1 kg of the substance by 1°C

• Water has a particularly high specific heat capacity. This makes water useful for storing heat energy, and for transporting it around the home using central heating pipes.

An equation…

Energy = mass x specific heat capacity x temperature change

E = mc∆Ө

Energy (E) JoulesMass (m) kilogramsTemperature change (∆Ө) oCSpecific heat capacity (c) J/kgoC

Q: Calculate the energy needed to heat 1.5kg water from 20oC to 60oC. The specific heat capacity of water is 4200J/kgoC.

Specific latent heat

• Fusion: The heat needed to change a mass of 1 kg the substance from a solid at its melting point into liquid at the same temperature.

• Vaporisation: The heat needed to change the substance from a liquid at its boiling point into vapour at the same temperature.

• An equation:

• Energy (J) = Mass (kg) × Specific latent heat (J/kg)

• E = ml

Energy calculations• The amount of electrical energy transferred to an appliance depends on its

power and the length of time it is switched on. The amount of mains electrical energy transferred is measured in kilowatt-hours, kWh. One unit is 1 kWh.

E = P × tE is the energy transferred in kilowatt-hours, kWhP is the power in kilowatts, kWT is the time in hours, h.

Different Units! Q: Change 2000W into kW and 7200s into hours…• On the last slide Power was measured in kilowatts instead of the more

usual watts. • To convert from W to kW you must divide by 1,000.e.g. 2,000 W = 2,000 ÷ 1,000 = 2 kW.

• Also time was measured in hours, instead of the more usual seconds.• To convert from seconds to hours you must divide by 3,600.e.g. 7,200 s = 7,200 ÷ 3,600 = 2 h.

Payback time = Initial cost Annual saving

We can calculate the amount of electrical energy transferred by an appliance and how much it costs to run. This is useful for comparing the advantages and disadvantages of using different electrical appliances for a particular purpose and to save money by comparing costs.

Q: Double-glazing might cost £2,500 and save £100 a year. What is the payback time?

Total cost = number of units × cost per unite.g. If 5 units of electricity are used at a cost of 8p per unit, the total cost will be 5 × 8 = 40p.

Energy Resources• Electricity is a very convenient form of energy that can be generated

using different energy resources. Some of these resources are renewable and some are non-renewable. Each resource has advantages and disadvantages.

The fossil fuels are coal, oil and natural gas. They were

formed from the remains of living organisms millions of

years ago and they release heat energy when they are

burned. They are non-renewable.

Generating electricityCarbon capture• Carbon capture and storage is a way to prevent carbon dioxide

building up in the atmosphere. It is a rapidly evolving technology that involves separating carbon dioxide from waste gases. The carbon dioxide is then stored underground, for example in old oil fields or gas fields such as those found under the

North Sea.

Nuclear fuels

• The main nuclear fuels are uranium and plutonium. These are radioactive metals. Nuclear fuels are not burnt to release energy. Instead, nuclear fission reactions (where the nuclei in atoms are split) in the fuels release heat energy.

• The rest of the process of generating electricity is then identical to the process using fossil fuels. The heat energy is used to boil water. The kinetic energy in the expanding steam spins turbines, which then drive generators to produce electricity.

Power Stations

• Power stations fuelled by fossil fuels or nuclear fuels are reliable sources of energy, meaning they can provide power whenever it is needed. However, their start-up times vary according to the type of fuel used.

• Nuclear power stations and coal-fired power stations usually provide 'base load' electricity - they are run all the time because they take the longest time to start up. Oil-fired and gas-fired power stations are often used to provide extra electricity at peak times, because they take the least time to start up.

• The fuel for nuclear power stations is relatively cheap, but the power stations themselves are expensive to build. It is also very expensive to dismantle old nuclear power stations and to store their radioactive waste, which is a dangerous health hazard.

National Grid and Transformers

• The National Grid• Electricity is distributed from power stations to consumers through the National Grid, which allows

distant power stations to be used. It also allows a mix of different energy resources to be used efficiently to supply the country’s electricity, whatever the local demand.

• Transformers• Electricity is transferred from power stations to consumers through the wires and cables of the

National Grid. When a current flows through a wire some energy is lost as heat. The higher the current, the more heat is lost. To reduce these losses, the National Grid transmits electricity at a low current. This needs a high voltage.

• Transformers are used in the National Grid. A transformer is an electrical device that changes the voltage of an alternating current (ac) supply, such as the mains electrical supply. A transformer that:

• increases the voltage is called a step-up transformer• decreases the voltage is called a step-down transformer.

Power stations produce electricity at 25,000 V. Step-up

transformers change the voltage to the very values needed to

transmit electricity through the National Grid power lines.

Electricity is sent through these at 400,000 V, 275,000 V or

132,000 V. This reduces energy losses during transmission but

the voltages would be dangerous in homes. Step-down

transformers are used locally to reduce the voltage to safe

levels. The voltage of household electricity is about 230 V.

Electricity from a power station goes to:

1. Step-up transformers

2. High voltage transmission

lines

3. Step-down transformers

4. Consumers, for example

homes, factories and shops.

What are waves? • Waves are vibrations that transfer energy from place to place without matter (solid, liquid or

gas) being transferred. Think of a Mexican wave in a football crowd: the wave moves around the stadium, while each spectator stays in their seat only moving up then down when it's their turn.

• Some waves must travel through a substance. The substance is known as the medium and it can be solid, liquid or gas. E.g. sound

• Other waves do not need to travel through a substance

They may be able to travel through a medium, but they

do not have to. Visible light, infrared rays, microwaves

and other types of electromagnetic radiation are like this.

They can travel through empty space. Electrical and

magnetic fields vibrate as the waves travel.

Transverse and longitudinal

• Light and other types of electromagnetic radiation are transverse waves. • In transverse waves the oscillations (vibrations) are at right angles to the

direction of travel and energy transfer.

• In longitudinal waves, the oscillations are along the same direction as the direction of travel and energy transfer.

• Amplitude: Maximum disturbance from its undisturbed position.

• Frequency: The number of waves produced by a source each second. It is also the number of waves that pass a certain point each second.

• Time period: Time taken for one wave to pass.

• Wavelength: Distance between a point on one wave and the same point on the next wave.

Sound WavesWhen an object or substance vibrates, it produces sound:• the greater the amplitude, the louder the sound• the greater the frequency, the higher the pitch.

Sounds 1 and 2:the sound waves have the same frequency, so the sounds have the same pitchsound 2 has a greater amplitude than sound 1, so sound 2 is louder.Sounds 2 and 3:the sound waves have the same amplitude, so the sounds have the same loudnesssound 3 has a greater frequency than sound 2, so sound 3 is higher pitched.

Hearing Range

Calculating wave speed

v = f × λ• v is the wave speed in metres per second, m/s• f is the frequency in hertz, Hz• λ (lambda) is the wavelength in metres, m.

Refraction • Sound waves and light waves change

speed when they pass across the boundary between two substances with different densities, such as air and glass. This causes them to change direction and this effect is called refraction.

• There is one special case you need to know. Refraction doesn't happen if the waves cross the boundary at an angle of 90° (called the normal) - in that case they carry straight on.

Refraction

Diffraction • When waves meet a gap in a barrier, they carry on

through the gap. However, the waves spread out to some extent into the area beyond the gap. This is called diffraction.

• The extent of the spreading depends on how the width of the gap compares to the wavelength of the waves. Significant diffraction only happens when the wavelength is of the same order of magnitude as the gap. For example:

• a gap similar to the wavelength causes a lot of spreading with no sharp shadow, eg sound through a doorway

• a gap much larger than the wavelength causes little spreading and a sharp shadow, eg light through a doorway.

Reflection • Sound waves and light waves reflect from surfaces. When waves

reflect, they obey the law of reflection:

the angle of incidence equals the angle of reflection

Ray diagram

• In a ray diagram, the mirror is drawn as a straight line with thick hatchings to show which side has the reflective coating. The light rays are drawn as solid straight lines, each with an arrowhead to show the direction of travel. Light rays that appear to come from behind the mirror are shown as dashed straight lines.

The image in a plane mirror is:• Virtual (it cannot be touched or projected onto a screen)• Upright (if you stand in front of a mirror, you look the right way up)• Laterally inverted (if you stand in front of a mirror, your left side

seems to be on the right in the reflection).

Electromagnetic Spectrum

• The electromagnetic spectrum is a continuous range of wavelengths.

• The types of radiation that occur in different parts of the spectrum have different uses and dangers, which depend on their wavelength and frequency.

Wavelengths range from 10-15m to 104m

Communicating with waves: radio waves and microwaves

Uses… Radio Waves

• Radio waves are used to transmit television and radio programmes. Television uses higher frequencies than radio.

• A radio programme receiver does not need to be directly in view of the transmitter to receive programme signals. Diffraction allows low-frequency radio waves to be received behind hills, although repeater stations are often used to improve the quality of the signals.

• The lowest frequency radio waves are also reflected from an electrically charged layer of the upper atmosphere, called the ionosphere. This means that they can still reach receivers that are not in the line of sight because of the curvature of the Earth's surface.

Uses… Microwaves

• Microwave radiation can also be used to transmit signals such as mobile phone calls.

• Microwave transmitters and receivers on buildings and masts communicate with the mobile telephones in their range. Some people think that mobile phones, which transmit and receive microwaves, may be a health risk.

• This is not accepted by everyone, as the intensity of the microwaves is too low to damage tissues by heating, and microwaves are not ionising.

• Certain microwave radiation wavelengths pass through the Earth's atmosphere and can be used to transmit information to and from satellites in orbit. Satellite television signals use microwaves.

Communicating with waves: Visible light• Visible light is the light we can see. It allows us to communicate with

one another through books, hand signals and video. • The use of visible light needs the transmitter and receiver to be in the

line of sight. But it is more secure against eavesdroppers than radio waves.

• Cameras let us record still pictures and movies, and photography is an important use of visible light. Very bright light can damage our eyes – you should never look directly into the Sun.

Communicating with waves: Infra Red• We cannot see infrared radiation, but we can feel it as heat energy. High

intensity infrared is used in heaters, toasters and grills, and it can cause burns. Infrared sensors can detect heat from the body. They are used in:

• security lights• burglar alarms.• Infrared radiation is also used to transmit information from place to place, including:• remote controls for television sets and DVD players• data links between computers.

Origins of the Universe- Big Bang Theory

• Current evidence suggests that the universe is expanding and that matter and space expanded violently and rapidly from a very small initial ‘point’. This has also led to the theory that the universe began with a ‘big bang’.

• Originally all the matter in the universe was concentrated into a single incredibly tiny point. This began to enlarge rapidly in a hot explosion (called the Big Bang), and it is still expanding today. The Big Bang happened about 13.7 billion years ago

Astronomers have even detected a cosmic

microwave background radiation, CMBR, that

is thought to be the heat left over from the

original explosion.

Doppler Effect• When an ambulance or police car goes past, its siren is high-pitched

as it comes towards you, then becomes low-pitched as it goes away.• This effect, where there is a change in frequency (and wavelength), is

called the Doppler effect.• When a source moves towards an observer, the observed wavelength

decreases and the frequency increases.• When a source moves away from an observer, the observed wavelength increases and the frequency decreases.

Red Shift- Why is it useful?

• Our Sun contains helium. We know this because there are black lines in the spectrum of the light from the Sun where helium has absorbed light. These lines form the absorption spectrum for helium.

• When we look at the spectrum of a distant star, we still see an absorption spectrum

• The positions of the lines have changed because of the Doppler effect. Their wavelengths have increased (and their frequencies have decreased).

• Astronomers have found that the further from us a star is, the more its light is red-shifted. This tells us that distant galaxies are moving away from us, and that the further away a galaxy is, the faster it's moving away.

• Since we cannot assume that we have a special place in the Universe, this is evidence for a generally expanding universe. It suggests that everything is moving away from everything else.