Edexcel IGCSE Physics

Gravity

• Gravity is the force of attraction between all masses.
• On the surface of the a planet, it makes all things accelerate towards the ground, with the same acceleration speed.
• It gives everything weight.
• It keeps planets, moons and satellites in orbit.

Weight

• Weight is caused by the pull of gravity.
• The weight of an object is just the force of gravity pulling it towards the centre of the Earth.
• Different on other planets - the force of gravity pulling on it is different.

Mass

• Mass is just the amoung of 'stuff' in an object .
• For any given object this will have the same value anywhere in the universe.

Equation

Weight = mass x gravitational field strength

• Weight (w) - N, Newtons
• Mass (m) - Kg, kilograms
• GFS (g) - N/kg, Newtons per kilogram

Moments

• A moment is the turning point of a force.
• The force of an object causes a turning effect or moment on the pivot. A larger force would mean a larger moment.
• Using a longer object, the same force can exert a larger moment because the distance from the pivot is greater.
• To get the maximum moment (or turning effect) you need to push at right angles (perpendicular) to the object.
• Pushing at any other angle means a smaller moment because the perpendicular distance between the line of action and the pivot is smaller.

Equation

Moment = force x perpendicular distance from pivot

• Moment - Nm, Newton metres
• Force (F) - N, Newtons
• Distance (d) - m, metres

Centre of Gravity

• The centre of gravity hangs directly below the point of suspension. 
• The centre of gravity of an object is the point through which weight of a body acts.
• A freely suspended object will swing until its centre of gravity is vertically below the point of suspension.

Finding Centre of Gravity of a Flat Shape

1) Suspend shape and a plumb line from the same point, and wait until they stop moving.

2) Draw a line along the plumb line.

3) Do the same thing again, but suspend the shape from a different pivot point.

4) The centre of gravity is where your two lines cross.

The Principle of Moments

• If an object is balanced then: Total Anticlockwise Moments = Total Clockwise Moments
• If Total Anticlockwise Moments does not equal the Total Clockwise Moments, there will be a Resultant Moment

Insulation

• If the appliance has a plastic casing, and no metal parts showing then it's said to be double insulated.
• The plastic is an insulator, so it stops a current flowing - which means you can't get a shock. Anything with double insulation doesn't need an earth wire - just a live and neutral.

Earthing and Fuses

• All appliances with metal cases must be "earthed" to reduce the danger of electric shock. "Earthing" just means the case must be attached to an earth wire. An earthed conductor can never become live.
• If a fault develops in which the live somehow touches the metal case, then because the case is earthed, a big current flows through the live wire, the case and the earth wire.
• This surge in current 'blows' (melts) the fuse (or trips the cicuit breaker) which cuts off the live supply.
• This isolates the whole appliance, making it impossible to get an electric shock from the case. It also prevents the risk of fire caused by heating effect of a large current.

Circuit Breaker

• Like fuses, they protect the circuit from damage if too much current flows.
• When circuit breakers detect a surge in current in a circuit, they break the circuit by opening a switch. 
• A circuit breaker (and the circuit they're in) can easily be reset by flicking a switch on the device. 
• This makes them more convenient than fuses - which have to be replaced once they've melted. 

Why a current resistor results in the electrical transfer of energy and an increase in temperature

• When there is an electric current in a resistor there is an energy transfer which heats the resistor.
• This happens because the electrons collide with the ions in the lattice that make up the resistor as they flow through it. This gives the ions more energy, which causes them to vibrate and heat up.
• This heating effect increases the resistor's resistance - so less current will flow, or a greater voltage will be needed to produce the same current. 
• The heating effect of an electric current can have other advantages. For example, if you want to heat something. 
• Toasters contain a coil of wire with a really high resistance. When a current passes throught the coil, its temperature increases so much that it glows and gives off infrared (heat) radiation which cooks the bread.

Power

• Electrical power is the rate at which an appliance transfers energy

Current

• Current is the flow of charge round the circuit.

Voltage

• Voltage is what drives the current round the cicruit. Kind of like "electrical pressure". You may also see it called potential difference.
• Voltage is the pressure from an electrical circuit's power source that pushes charged electrons (current) through a circuit.

Equation

Electrical power = current x voltage

• Electrical power (P) - W, watts
• Current (I) - A, amps
• Voltage (V) - V, volts

Wire

The current through a wire (at a constant temperature) is proportional to voltage.

Different resistors

The current through a resistor (at a constant temperature) is proportional to voltage. Different resistors have different resistances and different slopes on the graph.

Metal filament lamp

As the temperature of the metal filament increases, the resistance increases, and has a curve on the graph.

Diode

Current will only flow through a diode in one direction.

Resistance

Resistance is anything in the circuit which slows the flow down.

If you increase resistance, the less current will flow and voltage is increased because more will be needed to keep the same current flowing.

Light-dependent resistors (LDRs)

• LDRs are a special type of resistor that changes its resistance depending on how much light falls on it.
• In bright light, the resistance falls and in darkness, the resistance is highest.
• Useful for burglar detectors.

Thermistors

• A thermistor is a temperature-dependent resistor.
• In hot conditions, the resistance drops and in cool conditions, the resistance goes up.
• Thermistors make useful temperature detetctors e.g. fire alarms.

Equation

Voltage = current x resistance

• Voltage (V) - V, volts
• Current (I) - A, Amps
• Resistance (R) - Ohms

Graphs

• The steeper the I-V graph, the lower the resistance.
• If the graph curves, it means the resistance is changing.

Waves transfer energy and information without transferring matter.

Transverse

• In transverse waves, the vibrations are at 90 degrees to the direction energy is transferred by the wave.
• Examples: Light, water waves, slinky spring wiggled up and down

Longitudinal

• In longitudinal waves, the vibrations are along the same direction as the wave transfers energy.
• Examples: Sound, ultrasound, shock waves

1. Wavelength (upside down y) - the distance from one peak to the next.

2. Frequency (f) - how many complete waves there are per second (passing a certain point).

3. Amplitude - the height of the wave (from rest to crest)

4. Wave speed (v, for velocity) - how fast the wave goes

5. Period (T) - the time it takes (in s) for one complete wave to pass a point.

Equation

Wave speed = frequency x wavelength

• Speed (v) - m/s, metres per second
• Frequency (f) - Hz, hertz
• Wavelength (y) - m, metres

Equation

Period = 1/frequency

• Period (T) - s, seconds
• 1
• Frequency (f) - Hz, hertz

• All waves can be reflected
• Reflection of visible light is what allows us to see most objects. Light bounces off them into our eyes.

Law of relflection

• Angle of incidence = angle of reflection

Diagrams

• The normal is an imaginary line that's perpendicular to the surface at the point of incidence --> dotted line
• The angle of incidence is the angle between the incoming wave and the normal.
• The angle of reflection is the angle between the reflected wave and the normal.

Virtual images

• Virtual images are formed when light rays bouncing off an object onto a mirror are diverging, so the light from the object appears to be coming from a completely different place. 
• They cannot be projected onto screen
• Always upright

• All waves can be refracted
• Waves travel at different speeds in substances with diffrent densities. 
• Waves travel slower in denser media apart from sound which travels faster.
• So when a wave crosses a boundary between two substances, it changes speed.

Direction of wave travel

• If the wave hits the boundary 'face on', it slows down but carries on in the same direction.
• If a wave meets a different medium at an angle, this part of the wave hits the denser layer first and slows down while the outside bit carries on at the first, faster speed.
• So when the wave changes direction - it's been refracted.

• The angle between the ray and the normal is the angle of incidence.
• If the second material is denser than the first, the refracted ray bends towards the normal. 
• The angle between the refracted ray and the normal (the angle of refraction) is smaller than the angle of incidence.
• If the second is less dense, the angle of refaraction is larger than the angle of incidence (bends away from the normal).

• The refractive index of a transparent material tells you how fast light travels in that material.

Equation

Refractive index = sin i/ sin r

• Refractive index (n) 
• Angle of incidence (i)
• Angle of refraction (r)

Practical

1. Draw around a rectangular glass block on a piece of paper and direct a ray of light through it at an angle. 

2. Trace the incident and emergent rays, remove the block and draw in the frefracted ray between them.

3. Draw in the normal line at 90 degrees to the edge of the block, at the point where the ray enters the block.

4. Use a protractor to measure the i and r. These angles are made with the normal.

5. Calculate the refractive index using Snell's law.

• Total internal reflection (TIR) occurs when: The angle of incidence is greater than the critical angle and the incident material is denser than the second material

Explained

• Light going from a higher refractive index to a material with a lower refractive index speeds up and so bends away from the normal - from glass to air.
• If you keep increasing the angle of incidence (i), the angle of refraction (r) gets closer and closer to 90 degrees. 
• Eventually i reaches a critical angle C for which r = 90 degrees. The light is refracted right along the boundary.
• Above this critical angle, you get total internal reflection - no light leaves the medium

Equation

sin C = 1/n

• Critical angle (c)
• 1
• Refractive Index (n)

The higher the refractive index, the lower the critical angle.

For water-air boundary, c is 49 degrees.

Optical Fibres

• Made from plastic and glass
• Central core surrounded by cladding with a lower refractive index
• The core of the fibre is so narrow that light signals passing through it always hit the core-cladding boundary at angles higher than C - so the light is always totally internally reflected.
• It only stops working if the fibre is bent too sharply.

Periscopes

• The ray of light travels into one prism where it is totally internally reflected by 90 degrees.
• It then travels to another prism lower down and is totally internally reflected by another 90 degrees.
• The ray is now travelling parallel to its initial path but at a different height.

1. Connect two microphones to an oscilloscope

2. Place them about 2 m apart using a tape measure to measure the distance between them

3. Set up the oscilloscope so that it triggers when the first microphone detects a sound, and adjust the time base so that the sound arriving at both microphones can be seen on the screen

4. Make a large clap using the two wooden blocks next to the first microphone Use the oscilloscope to determine the time at which the clap reaches each microphone and the time difference between them

5. Repeat this experiment for several distances, e.g. 2 m, 2.5 m, 3 m, 3.5 m

• Sound waves will be reflected by hard flat surfaces. Things like carpets and curtains act as absorbing surfaces, which will absorb sounds rather than reflect them.
• Sound waves will also refract (change direction) as they enter different media. As they enter denser material, they speed up.

• A sound wave receiver, such as a microphone, can pick up sound waves travelling through the air. 
• To display these sound waves, and measure their properties, you can plug the microphone into an oscilloscope. The microphone converts the sound waves to electrical signals.
• An oscilloscope is a device which can display the microphone signal as a trace on a screen.
• The appearance of the wave on the screen tells you whether the sound is loud or quiet and high or low pithced. 
• Loudness increases with amplitude - bigger so carries more energy.
• The higher the frequency, the higher the pitch.

• Work done = energy transferred
• When a force moves an object through a distance, work is done on the object and energy is transferred

Equation

Work done = force x distance moved

• Work done (W) - J, Joules
• Force (F) - N, Newtons
• Distance (d) - m, metres

• Raised objects store energy in gravitational potential energy stores
• Lifting an object in a gravitational field requires work. This is causes a transfer of energy to gravitational potential energy store of the raised object.

Equation

Gravitational potential energy store = mass x gravitational field strength x height

• GPE - J, Joules
• Mass (m) - Kg, kilograms
• g - N/kg, Newtons per kilogram
• Height (h) - m, metres

• Movement means energy in an object's kinetic energy store.
• Anything that is moving has energy in its kinetic energy store. Energy is transferred to this store when an object speeds up and is transferred away from this store when an object slows down.

Equation

Kinetic energy store = 0.5 x mass x speed² 

• Kinetic energy (KE) - J, Joules
• Mass (m) - kg, kilograms
• Speed (v) - m/s, metres per second

The principle of conservation of energy

• Energy can be stored, transferred between stores, and dissipated - but it can never be created or destroyed. The total energy of a closed system has no net change.
• Energy is only useful when it is transferred from one store to a useful store.

Link Between KE and GPE

• Transfer of energy (usually mechanically) between GPE store and KE store.
• g x h = 0.5 x v²

• Power is the rate of energy transfer or rate of doing work.
• Power is the rate at which energy is transferred.

Equation

Power = Work done / time taken

• Power (P) - W, Watts
• Work done (W) - J, Joules
• Time taken (t) - s, seconds

• Pressure is a measure of the force being applied to the surface of something.
• The same force applied over a larger area creates a lower pressure.
• In gases and liquid, the pressure at any point acts equally in all directions.
• In gases and liquids, the pressure increases with depth. The pressure is higher at the bottom of the sea than at the surface, and it is lower high up in the atmosphere than close to the Earth.

Equation

Pressure = force / area

• Pressure (p) - Pa, pascals
• Force (F) - N, Newtons
• Area (A) - m², metres squared

• Pressure difference in liquids and gases depends on density.
• Pressure difference is the difference in pressure between two points in a liquid or gas.

Equation

Pressure difference = height x density x gravitational field strength

• Pressure difference (p) - Pa, pascals
• Density (curly P) - Kg/m³, kilograms per metres cubed
• Height (h) - m, metres
• g - N/kg, newtons per kilogram

Solids

• Strong forces of attraction hold the particles close together in a fixed, regular arrangement. The particles don't have much energy so they can only vibrate around their fixed positions.

Liquids

• There are weaker forces of attraction between the particles. The particles are close together, but can move past each other, and form irregular arrangements. They have more energy than the particles in a solid - they move in random directions at low speeds.

Gases

• There are almost no forces of attraction between the particles. The particles have more energy than those in liquids and solids - they are free to move, and travel in random directions and at high speeds. 

• The energy is a substance's thermal energy store is held be its particles in their kinetic energy stores - this is what the thermal energy store actaually is. 
• When you heat a liquid, the extra energy is transferred into the particles' kinetic energy stores, making them move faster. Eventually, when enough of the particles have enough energy to overcome their attraction to each other, big bubbles of gas form in the liquid - this is boiling.
• It's similar when you heat a solid. The extra energy makes the particles vibrate faster until eventually the forces between them are partly overcome and the particles start to move around - this is melting.
• When a substance is melting and boiling, you're still putting in energy, but the energy's used for breaking bonds between particles rather than raising the temperature. So the substance stays at a constant temperature.
• When a substance is condensing or freezing, bonds are forming between particles, which releases energy. This means the temperature doesn't go down until all the substance has changed state.