Gcse Core Physics Revision Guide

8/6/2019 Gcse Core Physics Revision Guide

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GCSE Science Revision Page 1 PHYSICS

P1a 1.1 Surfaces and Radiation

• Every object gives out (emits) thermal radiation

• Dark, matt surfaces emit more radiation thanlight, shiny surfaces

• Dark, matt surfaces are also better at absorbingradiation than light, shiny surfaces. This isbecause it has a lot of cavities, as the diagram to

the right shows – these cavities trap and absorbthe radiation

P1a 1.2 Conduction and Convection

• Metals conduct heat better than non-metals

• Metals contain a lot of free electrons, whichmove about at random inside the metal and holdthe positive ions together. They collide with eachother and with the positive ions (ions are chargedparticles)

• Think of a rod being heated. The free electrons at the hot end gain kinetic energyand move faster. These electrons then diffuse or ‘spread out’, and collide with other

free electrons and positive ions in the cooler end, and as a result they transfer kinetic energyto these electrons and ions, so energy has beentransferred from the hot end to the cooler end

• Convection takes place in fluids only (a fluid isa gas or liquid), and conduction only in solids

• The diagram to the left shows convection. Hotgases from the burning candle go straight up thechimney above the candle, and cold air is drawndown the other chimney to replace the cold air leaving the tank

P PP P1a 1 1a 1 1a 1 1a 1 Heat Transfer

1.11.11.11.1 Surfacesandradiation

1.21.21.21.2 Conductionandconvection

1.31.31.31.3 Artificiallymodifyingheattransferathome




• A good example of everyday convection is in the hot water tank you would find at home. Hot water from the boiler risesand flows into the tank where it rises to the top. The diagram

to the right shows the system. When you use a hot water tapat home, you draw off hot water from the top of the tank

• Convection takes place due to circulation currents in thefluids

P1a 1.3 Artificially Modifying Heat Transfer at Home

• The below diagram shows how we can cut back on our heating at home and save money at the same time

• Sometimes we need tocontrol heat transfer, for example, a car engine whichoverheats could go up inflames

P1a 2.1 Different Forms of Energy

•

These are the main forms of energy:- Chemical energy is energy stored in fuel (including food), and is released whenchemical reactions take place

- Kinetic energy is the energy of a moving object- Gravitational potential energy (or GPE) is the energy of an object due to its

position- Electrical energy is energy transferred by an electric current- Thermal (heat) energy is the energy of an object due to its temperature, partly

because of the random kinetic energy of the particles of the object- Elastic (or strain) energy is the energy stored in a springy object when we squash

or stretch it- Sound energy is energy created from sounds

P PP P1a 1a 1a 1a 2222 using energy

2.12.12.12.1 differentformsofenergy

2.22.22.22.2 energyconservation

2.32.32.32.3 usefulenergy,wastedenergyandefficiency




• We say that energy is transformed when it changes from one form into another

• Energy cannot be lost, it just changes forms

• An example of energy transformation is using a torch:

chemical energy in the battery → electrical energy → light energy + thermal energy

P1a 2.2 Energy Conservation

• Scientists are forever experimenting to see if the total energy after a change is thesame as the total energy from before the change

• All experiments so far show that it is the same

• This is known as the conservation of energy, which tells us that energy cannot becreated or destroyed

P1a 2.3 Useful Energy, Wasted Energy and Efficiency

• Useful energy is energy transferred to where we want it to go, in the form we want itto be in

• Wasted energy is energy which is not usefully transferred

• Wasted energy spreads out to its surroundings. For example, the gears of a car gethot when the car is running, so thermal energy transfers from the gear box to thesurrounding air

• Useful energy eventually transfers to itssurroundings too. For example, the usefulenergy supplied to the road wheels of a car changes into thermal energy of the tyres, theroad and the surrounding air

• Energy becomes less useful the more it spreadsout

•

The Sankey diagram to the left shows how wecan represent useful and wasted energy outputsfrom a machine or appliance

• We can sum this up by using this general equation:energy supplied = useful energy delivered + energy wasted

• For any device that transfers energy, we can use this to work out the energyefficiency of it:

P1a 3.1 Electrical Devices and Energy

• When we use an electrical device, some energy transformed is useful, and the rest iswasted

• The table on the next page shows the useful energies and wasted energies of various electrical appliances

efficiency =useful energy transferred by device

total energy supplied to device

P PP P1a 1a 1a 1a 3 33 3 Electrical Energy

3.13.13.13.1 electricaldevicesandenergy3.23.23.23.2 power

3.33.33.33.3 thenationalgrid




P1a 3.2 Power

• The energy supplied toan electrical device per

second is the power supplied to it

• In general, we say thatthe more powerful adevice, the faster therate at which ittransforms energy

• We measure the power of a device in watts (W)or in kilowatts (kW)

• For any device:- the input energy is the

energy supplied to it- the output energy is

the useful energy per second transferred byit

• 1 watt is a rate of transfer of energy of 1

joule per second (J/s)

• 1 kilowatt 1000 watts

• Power (in watts, W) = rate of transfer of energy

• The energy supplied to a 1kW device in an hour is 1 kilowatt-hour (kW h)

• Energy transferred (kW h) = power of device (kW) x time in use (hours, h)

• For example, a 2kW heater switched on for 4 hours uses 8kW h of electrical energy

• When paying the electricity bill, we use this equation:total cost = number of kW h used x cost per kW h

P1a 3.3 The National Grid

• Our electricity reaches our homes through the National Grid

• To supply the electricity, we use step-up transformers in power stations and step-

down transformers in sub-stations near homes• The National Grid’s voltage is 132000 volts plus, and power stations only produce

electricity at a voltage of 25000 volts. This is when we use step-up transformers atpower stations to step this voltage up to the National Grid voltage, and then we usestep-down transformers at the sub-stations near our homes to step the voltage backdown to 230 volts for homes and offices

• Some people constantly debate over whether they should be built above or belowground. If they are built above ground they are visible and not aesthetically pleasing,but campaigners for underground mains could be argued with, because undergroundcables are far more expensive, much more difficult to repair and are extremelydifficult to bury where they cross canals, rivers and roads

=energy transferred (in joules, J)

time taken (in seconds, s)




P1a 4.1 Making Electricity

• Almost all electricity is generated in power stations

• In a coal or oil-fired power station, the burning fuel heats water in a boiler to producesteam. The steam drives a turbine that turns an electricity generator

• In a gas-fired power station, we burn natural gas directly in a gas turbine engine.This produces a powerful jet of hot gases and air that drives the turbine

• We use nuclear fission to obtain electricity from a nuclear power station

• Nuclear fission works by the following process:- The fuel in a nuclear power station is uranium

- The uranium fuel is contained in sealed cans in the core of the reactor - The nucleus of the uranium atom is unstable and can split in two- When this does happen, energy is released

• Once nuclear fission has taken place, the thermal energy of the core is taken awayby a fluid (called the ‘coolant’) that is pumped through the core. The coolant is veryhot when it leaves the core. It flows through a pipe to a heat exchanger, then back tothe reactor core. The thermal energy of the coolant is used to turn water into steamin the heat exchanger. The steam drives the turbines which generates electricity

P1a 4.2 Energy from Wind and Water Sources

• A wind turbine is an electricity generator at the top of a narrow tower. The force of the wind drives the turbine’s blades around, turning a generator. The power

generated increases as the wind speed increases• We can also use waves to generate electricity, as well as hydroelectric power and

tidal power

• With wave power, the waves cause a floating material attached to a generator tmove up and down, moving the turbine delivering power to the grid system

• Hydroelectricity is water kept in reservoirs which flows downhill turning turbines atthe foot of the hill, which generate electricity

• Tidal power involves barraging coastal water and releasing it through turbines

P1a 4.3 Power from the Sun and Inside the Earth

• Solar radiation transfers energy from the Sun, using solar cells, or by using theSun’s radiation to heat water directly using solar heating panels

• Solar cells only generate <10% of the energy they absorb into electricity, and areexpensive and require the right weather to use

• A solar heating panel heats water that flows through it

• Geothermal energy comes from energy released by radioactive substances deepwithin the Earth. The energy released by these radioactive substances heats thesurrounding rock, and as a result, heat is transferred towards the Earth’s surface

P PP P1a 1a 1a 1a 3 33 3 Electrical Energy

4.14.14.14.1 makingelectricity

4.24.24.24.2 energyfromwindandwatersources

4.34.34.34.3 powerfromthesunandinsidetheearth




P1a 5.1 The Electromagnetic Spectrum

• The electromagnetic spectrum is as follows:

• The wavelength of a wave is the distance from one wave peak to the next wavepeak along the waves

• The frequency is the number of complete waves passing a point per second

• We measure frequency in the unit of hertz (Hz) when:- 1 Hz = 1 complete wave per second- 1 kilohertz (kHz) = 1000 Hz- 1 megahertz (MHz) = 1,000,000 Hz

• The speed of a wave is worked out by using:wave speed (m/s) = frequency (Hz) x wavelength (m)

• Alternatively, the frequency can be worked out by using:

P1a 5.2 Gamma Rays and X-Rays

• X-rays pass through soft tissue but are absorbed by bone

• X-rays produce radiographs

• Gamma radiation is electromagnetic radiation from radioactive substances

• Gamma rays can be used to kill harmful bacteria in food, sterilise surgical equipmentand to kill cancer cells

P PP P1 11 1b bb b 5 55 5 Electromagnetic Waves

5.15.15.15.1 theelectromagneticspectrum

5.25.25.25.2 gammaraysandx-rays

5.35.35.35.3 visiblelightandultravioletradiation

5.45.45.45.4 infra-redwaves,microwavesandradiowaves

frequency =wave speed (in m/s)

wavelength (in metres)




P1a 5.3 Visible Light and Ultraviolet Radiation

• Light from the Sun and light bulbs is called white light, because it has all colours of the visible spectrum on it

• Ultraviolet (UV) radiation makes some chemicals emit light • UV radiation is harmful to the human eye and can cause blindness

• UV radiation comes from the Sun, and too much of it can cause skin cancer

P1a 5.4 Infra-Red Radiation, Microwaves and Radio Waves

• All objects emit infra-red radiation

• The hotter an object is the more radiation it emits

• Infra-red radiation is absorbed by the skin, which kills skin cells as they heat up

• Infra-red devices include heaters and remote controls

• Microwaves are shorter than radio waves in wavelength, even though they comebefore radio waves in the electromagnetic spectrum. This is why they are calledmicrowaves

• We use microwaves for communication and heating food in microwave ovens

• Radio waves are longer in wavelength than microwaves – but appear after microwaves in the electromagnetic spectrum because they have a lower frequency

• We use radio waves to carry radio, TV and mobile phone signals

• Radio waves are emitted from an aerial

• Analogue and digital signals: page 88-9

P1a 6.1 An Introduction to Radioactivity

• The term radioactivity was given to uranium by Marie Curie who said that it gaveout radiation all the time

• We can use a Geiger counter to detect radioactivity, where the counter will clickeach time a particle from a radioactive substance enters the Geiger tube

• Ernest Rutherford discovered two types of radiation:- alpha radiation who’s symbol is α

- beta radiation who’s symbol is β• Scientists later discovered a third type, gamma radiation, written γ

• The nuclei of atoms of radioactive elements are unstable

• An unstable nucleus becomes stable by emitting alpha, beta or gamma radiation –this is known also as decaying

P1a 6.2 Alpha, Beta and Gamma Radiation

• Alpha particles are helium nuclei, and cannot even penetrate a human hand or piece of paper

• Beta particles are negative electron which can penetrate a thin aluminium sheet

• Gamma rays are electromagnetic waves which can penetrate some concrete

P PP P1 11 1b bb b 6666 Radioactivity

6.16.16.16.1 anintroductiontoradioactivity

6.26.26.26.2

alpha,betaandgammaradiation

6.36.36.36.3 radioactivehalf-life

6.46.46.46.4 usefulradiation




• Alpha radiation’s range is about 10cm in air

• Beta’s range is about 1m in air

• Gamma radiation’s range is approximately 10,000km – but we often say it is

unlimited• Radiation from a radioactive substance can knock electrons out of atoms. These

atoms become charged because they have lost electrons. This process is calledionisation

P1a 6.3 Radioactive Half-Life

• Atoms of elements can contain identical amounts of protons but different amounts of electrons – these are called isotopes

• The activity of a radioactive substance is the number of atoms that decay per second

• The number of clicks per minute that a Geiger tube detects from a radioactivesubstance is the count rate

• The time taken for the count rate of a radioactive substance to half is its half-life

• The half-life of a radioactive isotope is the time it takes:- for the number of nuclei of the isotope in a sample to half - for the count rare of the isotope to fall to half its initial value

P1a 6.4 Useful Radiation

• Radioactivity can be harmful to us and our skin, but itserves some uses

• Automatic thickness monitoring – looking at thediagram:- if the thickness of the foil increases too much, thedetector reading drops- the detector sends a signal to the rollers to increasepressure on the metal sheet

Alpha radiation

Beta radiation

Gamma radiation

Paper Aluminium Lead Concrete

OO

+

+

-

Alpha radiation

Beta radiation

Gamma radiation

Paper Aluminium Lead Concrete

OO

+

+

-




• Radioactive tracers can be used for medical purposes. Patients can drink a glass of water containing a small amount of a radioactive substance, if they want to test their kidneys. The substance would simply flow in and out of healthy kidney, but a

blocked kidney would not allow the substance to exit for a lot longer - This means that for a normal kidney the radioactive reading goes up then down- …and for a blocked kidney, the reading goes up and stays up

• Radioactive dating is a process where we can find out how old things are by usingradioactivity. There are two types…

• Carbon dating is used to find the age of ancient wood and trees (page 101)

• Uranium dating is used to find the age of igneous rocks (page 101)

P1a 7.1 The Ever-Expanding Universe

• We can find out lots of things about stars and galaxies by studying the light fromthem. We can use a prism to split the light into a spectrum. The wavelength of lightincreases across the spectrum from blue to red. We can tell from the spectrum if astar or galaxy is moving towards or away from us. This is because:- the light waves are stretched out if the star or galaxy is moving away from us. We

call this red shift because the spectrum of light is shifted towards the red part of the spectrum

- the light waves are squashed together if the star or galaxy is moving towards us.We call this blue shift because the spectrum of light is shifted towards the blue part

of the spectrum • The faster a star or galaxy is moving towards or away from us, the bigger the shift is

• Red shifts and blue shifts follow the Doppler effect (page 106-7)

• Red shift and blue shift show that all galaxies are moving away from each other,proving that the whole universe is expanding

P1a 7.2 The Big Band Theory

• The Big Bang Theory tries to explain the expanding Universe. It states:- the Universe is expanding after exploding suddenly in a Big Bang from a very

small initial point- space, time and matter were created in the Big Bang

• In 1965, it was discovered that microwaves were coming from every direction inspace. The existence of this background microwave radiation can only beexplained by the Big Bang Theory

P PP P1 11 1b bb b 7777 The Origins of the universe

7.17.17.17.1 theever-expandinguniverse

7.27.27.27.2 thebigbandtheory

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Gcse Core Physics Revision Guide