YEAR 9 Physics iGCSE: Revision Notes for Year 9  

CONTENTS (Formulae you will need to know are in bold type) :


 1.  How is heat (thermal energy) transferred and what affects the rate of transfer?

• Thermal (infra red) radiation is the transfer of energy by electromagnetic waves.Infrared image of a cat.

• All bodies emit and absorb thermal radiation.

• The hotter a body is the more energy it radiates.

• Dark, matt surfaces are good absorbers and good emitters of radiation.

• Light, shiny surfaces are poor absorbers and poor emitters of radiation.

• The transfer of heat energy by conduction involves vibrating particles: the vibrations are

 stronger at the hot end, strongly vibrating particles set the their neighbours vibrating so that

 energy moves along the material, see diagram below:

strong vibs.                                       weak vibs.

• The dimensions of a body affect the rate at which it conducts heat: heat moves faster along thicker wires.

• The bigger the temperature difference between a body and its surroundings, the faster the

   rate at which heat is transferred.

• The transfer of heat energy by convection happens when a hot fluid expands, gets less dense,

   particles get further apart and are pushed upwards.

•  Conduction happens in solids, liquids, and gases. Convection ONLY in liquids and gases.


2. Evaporation and Condensation

• evaporation is the escape of the most energetic particles from the surface of the liquid into the vapour state,

    so the total energy of the liquid falls, so the temperature falls.

• condensation is when very energetic vapour particles suddenly lose energy causing a drop down into

   the lower energy liquid state – liquid droplets appear:


3. What is meant by the efficient use of energy? You should be able to:

• describe energy transfers/transformations and the energy wastages that occur in a range of devices

• calculate the efficiency of a device: efficiency = useful energy transferred / total energy transferred

• Energy cannot be created or destroyed. It can only be transformed from one form to another form.

• When energy is transferred and/or transformed only part of it may be usefully transferred/transformed.

• Energy which is not transferred/transformed in a useful way is ‘wasted.’

• Both wasted energy and the energy which is usefully transferred/transformed are eventually

  absorbed by the surroundings which will become warmer.

• Energy becomes increasingly spread out and becomes increasingly more difficult to use for further

   energy transformations.


4. How should we generate the electricity we need?

• You need to be able to compare advantages and disadvantages of using different

  energy sources / fuels to generate electricity.

• In most power stations an energy source is used to heat water. The steam produced drives

  a turbine which is coupled to an electrical generator.

• Common energy sources include coal, oil and gas, which are burned to produce heat,

  and uranium / plutonium, in which nuclear fission produces heat.

(a) fossil fuel burning power station






(b) a nuclear (fission) power station





(c) renewable energy power station                                                 





• Energy from renewable energy sources can be used to drive turbines directly.

• Renewable energy sources used in this way include wind, the rise and fall of water

   due to waves and tides, and the falling of water in hydroelectric schemes.

• Electricity can be produced directly from the Sun’s radiation using solar cells.

• In some volcanic areas hot water and steam rise to the surface. The steam can

   be tapped and used to drive turbines. This is known as geothermal energy.

• Power stations can adversely affect the environment: eg. the release substances

   into the atmosphere, noise and visual pollution, and the destruction of wildlife habitats.

• The advantages and disadvantages of using fossil fuels, nuclear fuels and renewable energy.

   These include the cost of building power stations, the start-up time of power stations,

   the reliability of the energy source, the relative cost of energy generated and the location

   in which the energy is needed.


5. Waves

  a wave transfers energy away from a vibrating source

a) frequency f  is the number of waves passing a point per second, measured in Hertz.

Eg. If 10 waves go past in 5 secs, the frequency is 2 Hz.

b) Time Period T = the time taken for the source to produce 1 wave is called the period,

     T measured in seconds.

       frequency  f  =  1 / T  , units Hertz (Hz)

c) Wavelength λ = the distance between a particular point on a wave and the same point

     on the next wave (eg from crest to crest or from trough to trough)

d)  Amplitude A = the maximum displacement of a particle from its rest position (more amplitude means more wave energy)

e) Longitudinal wave = when particles vibrate parallel to the wave direction eg. sound waves

 f)  Transverse wave = when particles vibrate perpendicular to the wave direction eg. water waves

 g) Wave Equation:

wave speed = frequency Χ wavelength,    v  =  f λ

 (metre/second, m/s) (hertz, Hz) (metre, m)




6. Electromagnetic spectrum


• The electromagnetic spectrum is continuous but the wavelengths

within it can be grouped according to wavelength or frequency:


                      Increasing Frequency                  Increasing Wavelength

• EM radiation travels as waves and moves energy from one place to another.


• All types of EM waves travel at the same speed in a vacuum (space).


• EM radiation can be reflected, absorbed or transmitted


• When radiation is absorbed the energy it carries makes the

   substance which absorbs it hotter .


• Different wavelengths of EM radiation have different

   effects on living cells. Some radiations mostly pass through soft

   tissue without being absorbed, some produce heat, some may

   cause cancer, some may kill cells.


• Uses and hazards of each type of radiation - see table below:









few 100 m

Broadcasting radio, TV

No hazard



few cm

Mobile phones


Heating up food

Heating water in the body

Metal grid

Infra red

      few ΅m

Communication in optical fibres

Remote Controllers


Heating effect

Reflective surface


400-700 nm



No hazard


Ultra violet

few 100 nm


Sun tanning

Can cause cancer

or blindness

Sun cream


1 nm

Shadow pictures of bones or teeth

Causes cell damage

Lead screens

Gamma rays

< 1/100 nm

Killing bacteria, medical tracer, thickness monitoring

Causes cell damage

Lead screens / concrete


• Microwaves can pass through the Earth’s atmosphere and are used

  to send information to and from satellites for mobile phone networks.


• Infra red and visible light can be used to send signals along optical

  fibres and so travel in curved paths.


• Electromagnetic waves obey the wave formula:  wave speed = frequency Χ wavelength,    v  =  f λ




7. Atomic Structure and Radioactivity


• The basic structure of an atom is a small central nucleus

  composed of protons and neutrons surrounded by electrons.

•  In a neutral atom, number of electrons = number of protons


•  Atoms may gain or lose electrons to form ions.


•  The number of protons is called the atomic number.


•  The number of neutrons + the number of protons is called the mass number.


• The atoms of an element always have the same number of

  protons, but have a different number of neutrons for each isotope.


• Some substances give out radiation from the nuclei of their atoms

  all the time, whatever is done to them. These substances are said

  to be radioactive.


• Ionising radiations may be detected using a Geiger-Muller (GM) counter or photographic film.


• the activity of a radioactive material is measured in Becquerels (Bq), where

  1Bq  =  1 count/sec or 1 decay/sec.


•  Background (natural) sources include: radon in the air, cosmic rays, radioactive material in the ground,

   man-made sources.





Ionising Power

Effect of Electric or Magnetic field


Alpha (a)

He nucleus

2p + 2n

charge + 2 

Few cm air

Thin paper

very ionising

Same as a positive charge

Smoke detector

Beta (b)


charge = -1

Few mm of aluminium

Less than alpha

Same as a negative charge

Thickness monitor in paper mill - see below

Gamma (g)

short wavelength em radiation

Several cm lead, couple of m of concrete

A lot less than alpha

No effect.

Medical tracer, leaky pipe tracer, Killing bacteria


•  After alpha decay the mass number A of the nucleus will have gone down by 4, and the

    atomic number Z will have gone down by 2. see examples:




•  After β decay the mass number A of the nucleus will be unchanged, and the atomic number Z

    will have gone up by 1. see example (note that the β particle is the electron):


A paper thickness control system using a β source:

• Hazards of nuclear radiation: all 3 cause ionisation which can damage, mutation, or kill healthy cells


• You need to know how to reduce your exposure to nuclear radiations


• Decay is a random process as each unstable nucleus decays independently of its neighbours.


• The half-life is defined as the time taken for the number of active nuclei to halve or the

  time taken for the count rate to halve (no. of Becquerels to halve) eg.12s for barium-143:

•  half life of 14C can be used to date archaeological specimens, and a similar technique

   can be used in geology to date rocks.


8 Stretching Materials

•  describe an experiment to investigate how extension varies with the applied force


•  where the Extension against Force graph is a straight line through the origin, we say

    Force (F) is proportional to Extension (e), which is following Hooke's Law:








•  in the graph below, beyond the elastic limit the material does not obey Hooke's Law

   nor will it return to its original length when the applied force is removed.

•  elastic behaviour is when a material returns to its original length after the applied force is removed.


•  inelastic behaviour is when a material does not return to its original length after the applied force is removed.


• The type of energy stored in stretched materials is called elastic potential energy EPE