Edexcel Igcse Physics

Forces and Motion

Kilogram (kg): the metric way of measuring weight -

Metre (m): the metric measure for distance -

Metre/second (m/s): this is a measure of speed; it is how many meters you go in a second -

Metre/second² (m/s²): this is a measure of acceleration -

Newton (N): a newton is a measure of force -

Second (s): unit of time -

Newton per kilogram (N/kg): how much force for every kilogram of weight

A distance-time graph is a graph showing the relationship between distance travelled and time taken.

Interpreting

A horizontal line is a stationary object. A line upwards is an object moving away from the start; a downward line is an object moving towards the start. The steeper the line the faster the object. To get a speed see how much up it goes for across. Rise over Run.

You could plot the time it takes for a toy car to travel a distance and then plot a distance time graph. Then repeat at different speeds and compare the different graphs.

Alternatively, you could use a ticker tape attached to the back of the car. Distance/Dots = speed. For example, if you has 50 dots on a meter tape then it traveled at an average speed of (1/50) 0.02 meters per second.

A velocity-time graph is a graph showing the relationship between velocity and time taken.

Interpreting

A line going diagonally upwards shows an acceleration, if it is straight it is constant. A line going diagonally downwards shows a deceleration. If it is straight, it is constant. The steeper the line the more rapid the acceleration. A straight flat line is a constant velocity: you are travelling at one speed in one direction.

Acceleration is measured in meters per second²: m/s². Change in velocity / time. The time period in the graph is then divided by the change in velocity. Rise over Run.

Distance can be calculated by finding the area under the velocity time graph (line).

Changes in shape

Affects momentum. Force = change in momentum/ time taken. An example of this is crumple zones in car decrease the force on the passengers.

Changes in direction

Whichever direction the force is greatest in will be the direction the object travels in.

When an object is going at a constant speed it has forward and backward forces. The forward and backward forces are equal, so the speed is constant.

When an object is decelerating it has forward and backward forces, but the backward force is larger than the forward one, slowing the object down.

Gravity: Gravity gives everything a weight. It makes everything accelerate towards the ground. The weight of an object corresponds to the pull of its gravity. - Electrostatic force: force between two charged objects, similar charges repel, opposite charges attract. Thrust: or push or pull, occurs when a system expels mass in one direction and this mass will cause a force of equal magnitude but opposite direction on the system. - Friction/Drag: the force that resists movement between two surfaces that are in contact. - Tension - Tension passes through strings, cables, ropes or wires when they are being pulled in opposite directions. The tension force is directed along the length of the wire and pulls equally on the object at the opposite ends of it.- Lift: opposes the weight. Reaction to the surface, not allowing object to sink. - Reaction force (Contact force): when two objects are pushed together, come into 'contact,' they exert equal and opposite forces on each other. For example, the contact force from the ground pushes up when you stand and your weight pushes down.

Vector

Has magnitude and a direction. For example velocity is a speed in a given direction.

Scalar

Has a magnitude. For example speed.

Force has magnitude, it is measured in newtons but it acts in a direction.

Friction is a force that acts in the opposite direction to motion.

When first an object is falling it is accelerating- the force acting downward (gravity) is larger than the force acting upwards (air resistance). But when air resistance and gravity become equal the object will have reached its maximum speed; its terminal velocity.

Dropping parachutes from a given height; this shows us that gravity is acting on them. By increasing the size of the parachute and recording the results we can see that air resistance also has an effect on falling objects; plotting a graph should reveal that bigger surface area takes more time, from which we can infer that air resistance acts on the falling objects.

Stopping distance is thinking distance and braking distance added together, things that effect this are:

- The condition of the driver; drugs/ tiredness

- How worn the brakes/ tyres are

- If the weather conditions are poor

- How heavy the car is

- The speed the car is travelling at

If the time taken for momentum to change is increased, the overall force felt is decreased.

Crumple zones in cars increase the time it takes for the car's momentum to reach zero, meaning passengers feel less of the force. Air bags do the same thing; increasing the time till momentum of a body reaches zero reduces the force felt.

(Equation in 1.23)

“Each action has an equal and opposite reaction” the principle of this law is that two bodies interacting are both exerting a force on each other.

An objects centre of gravity is where all of its weight acts through.

This means you need to understand that if you have say a plank of wood being held in balance by springs pushing up at the ends and you put a weight on the beam, the springs would have to exert more force as they need to equal the downward force.

1. Attach a spring to a newton meter and measure its length

2. Add a 50g weight and measure again

3. Continue to add another weight and take another measurement

4. Do this up to 400g

5. By plotting a graph from the results from this you can see the extension increases with force; as each time a weight is added the spring gets longer.

A force extension graph shows how much a material stretches for the force applied. The initial linear region is the straight diagonal line showing a linear correlation between force and extension meaning that they increase at the same rate. This is Hooke's law. But at some point the graph will begin to curve, this is when an object reaches its elastic potential.

Elastic behaviour is the way that when you stretch an object with this behaviour it will return to its original shape after the forces stretching it stop stretching it. Eg when you stretch an elastic band and then let go it pings back to regains its original shape and size. However if you stretch it too far, it won't go back, this is its elastic limit.

Gravitational field strength is how strongly something pulls an object towards it.Earth has a higher gravitational field strength than the moon: on earth we are pulled down so much that we can jump only for a few seconds; on the moon the time you can jump for is longer as it is pulling you back in with a weaker gravitational field. Bigger planets than earth will have a higher gravitational field strength.

If an object is within the field of another objects gravitational force then it will travel around it in a path known as an orbit. In this way:

- Moons to orbit planets

- The planets to orbit the sun

- Artificial satellites to orbit the Earth

- Comets to orbit the sun

Planets orbit in circles, where as comets orbit in ellipses (ovals) as does the moon.

The first half of the equation works out the circumference of the circle, this is the distance, which is then divided by the time.

Orbital speed = 2×π ×orbital diameter/ time period

v = πd/ T

Electricity

Ampere (A): current

Coulomb (C): charge

Joule (J): energy

Ohm (Ω): resistance

Second (s): time

Volt (V): potential/energy

Watt (W): power

In frayed cabling the insulation has worn down exposing live wires, electricity can be conducted from these. Longer cables are at a higher risk of being damaged and there is more resistance with longer wires making them more at risk of overheating. Damaged plugs create a risk that some of the safety features may be broken. Water conducts electricity and can cause energy from the circuit to flow trough it creating a fire and electrocution risk. Metal objects in sockets have the same dangers.

Insulation

Insulation is when a non-conductor of electricity such as plastic or rubber is used as the casing for an appliance rather than a conductor. A non-conductor is called an insulator.

Double Insulation

Double insulation is when none of the electric parts of an appliance can be touched by the user; as well as the wiring being insulated, the outer casing of the appliance is also made of an insulating material such as plastic.

Fuses

A fuse is a safety device with a thin wire inside it. The wire has a low melting point. The fuse will blow if the wire gets too hot. This will shut down the circuit

Circuit Breakers

If the circuit overheats, the circuit breaker opens, breaking the circuit.

As a resistor slows down the movement of electrons, the kinetic energy that was moving them is converted into heat energy. This can be used, for example, in hair dryers or heaters.

energy transferred = current × voltage × time

E = I × V × t

Direct current flows in one direction only. It is supplied by cells and batteries. It comes out as a straight line on an oscilloscope.

Alternating current changes from one direction to another rapidly. Mains electricity is alternating.

In a series circuit everything is connected on one line. This means that the voltage is shared out between every component: this makes it useful for supplying low power things like fairy lights.

In a parallel circuit different components are connected separately to the supply. This means that of one component breaks the others can continue being powered as the whole circuit is still functioning, this makes it practical to use. It is also good for charging higher power things as the potential difference is equal all over a parallel circuit so each component receives the full voltage.

The current in a series circuit is the same through out all parts of the circuit. It is worked out using the equation I= V/R. So its the total of the voltages received by the components divided by the total of all the components resistances.

If you increase the resistance the current will decrease. Resistors, metal filament lamps and diodes all create resistance in a circuit and so will decrease the current.This can be investigated using an ammeter and measuring the current with and without these components, or with different voltage levels (measured by voltmeter.)

Increasing the resistance will decrease the current. This can be achieved by adding more components or ones with higher resistance.

Decreasing the resistance will increase the current. This can happen if components are removed or replaced by those with lower resistance.

An LDR is a light dependent resistor. Its resistance changes with the intensity of light: the brighter it is the less resistance; the less light the more resistance.

Thermistors are temperature dependent resistors. In hot conditions there will be less resistance where as in the cold the resistance is high.

For an LED to light up there must be a current in a circuit. If a LED is in a circuit but not emitting light then there must be no current. If an LED is illuminated then it will have a current flowing through it. By this we know that if the LED in our circuit is shining then there is a current, if it isn't then we don't.

Current is the rate at which charge is flowing through a circuit.'

It is like the flow of water through a set of pipes'

Electric current is a flow of electrons, so when there is an electric current in a metal, the electrons in the metal are flowing.

Often people think of voltage as if it were something pushing current through a circuit, which is helpful, but more accurately its the energy transferred per unit of charge passed.

The unit volt is a joule per coulomb. These are things that simply need to be learnt.

Electrical conductors are materials that allow a current to pass through them. To do this they need to have 'free' electrons, because current is a flow of electrons. Metals have free electrons because of the way they are bonded (atoms and electrons within a lattice) this means they are good electrical conductors.

Plastics are polymers which are bonded in a way that means electrons aren't free and so can't move. No flow of electrons means no electric current so they are insulators.

Get a polyethene rod and rip up some small pieces of paper; the rod will have no effect on the paper.

Rub the polyethene rod with a cloth, now the rod will attract the pieces of paper, this is because it now has a charge they are attracted to.

If two materials are rubbed along each other one will gain electrons from the other.The one that has gained electrons has a negative charge. The one that has lost electrons will have a positive charge. The charges are electrostatic because they are not flowing.

Opposite forces attract.

Similar forces repel.

Electrostatic phenomena is an event where static electricity has a specific effect: for example a static shock. Electrons move from one material to another, the material with a negative charge will then look for some way to earth its charge: like clouds through lightening or a car through your hand and body.

When a large electrostatic charge builds up it can create a spark. When refuelling vehicles the fuel rubbing along the pipe can cause an electrostatic charge, if this sparks if could ignite the fuel causing a fire or explosion. (This can be avoided if the charge is brought to earth by a wire attached to the plain or tanker)

There are some events in which having two objects of opposite charge is very useful. An example of this is in photocopiers and inkjet printers where the ink is given a charge, and the parts of the paper where it's wanted is given the opposite charge, so that the ink is automatically attracted to the right parts of the paper.

Waves

Degrees (°): a measurement of an angle or temperature (in this context angle)

Hertz (Hz): Cycles per second

Metre (m): the metric measure for distance

Metre/Second (m/s): measure of speed, how many metres you go in a second

Second (s): unit of time

Transverse

- Vibrations (oscillations) go up and down along the line of travel

- Light and electromagnetic waves travel in this way

- If you drop something in water the waves move up and down as they travel outwards

- If you lie a piece of string on a table and move one end up and down, the movement will pass through the object to the other end.

Frequency

The number of waves per second, it is measured in Hertz (Hz). You can think of it as how quickly the waves are travelling.

Period of a wave

Time taken for the source to produce one complete wave.

Waves are able to transfer energy and information without transferring matter. For example, if you are pushing a swing, you are able to transfer energy to it without expelling any of your body matter.

Manipulate these equations to answer questions that may ask in a different order; use the triangle method.

The electromagnetic spectrum is a range of different frequency waves, one section of the spectrum is visible light (light we can see.) All of the waves in the electromagnetic spectrum travel at the same speed when they are in a vacuum.

E = mc²

Microwaves

- internal heating of body tissue

- this can damage cells if they overheat

Infrared

- skin cells are damaged by overexposure, burns

Ultraviolet

- damage to surface cells and blindness

- can damage receptor cells in the retina

Gamma rays

- can cause cells to change their arrangement or mutate causing cancer

Reflection

Light hitting a reflective surface will 'bounce' back from at the same angle they hit the surface

Refraction

Light waves change speed when they pass through objects of different densities, (i.e. air and glass) this causes them to change direction.

Diffraction

When light meets a barrier, it will carry on through the gap and spread out in the area beyond.

The angle of incidence is the angle that light hits a mirror; it is taken between 90 degrees from the mirror and the incidence wave (the wave that hits the mirror.) The angle of reflection is the angle that light leaves the mirror; it is taken between 90 degrees from the mirror and the angle of reflection. The angle of incidence is always the same as the angle of reflection.

- Place a block of glass on a piece of paper, drawing an outline.

- At one point, draw the normal line.

- Draw a line at 30 degrees to the normal line, shine a ray of light down this line.

- Draw a line where the light comes out the other side.

- Connect the two lines, this is the refracted ray

- Measure the angle of the emergent ray.

- Repeat for different shaped glass.

Refractive index= sin (angle of incidence)/ sin (angle of reflection)

n= sin(i) / sin(r)

Shine a ray of light through a glass block, measure the angle of incidence and the angle of refraction.

Do sin(i) divided by sin(r) and you will have the refractive index of glass.

When light travels from one medium to another it is refracted; it changes angle due to change in density.

Past a certain angle the light will simply be refracted back into the medium it is in, this angle is the critical angle.

sin(critical angle) = 1/refractive index

sin(c)= 1/n

Radios may crackle or internet may looses connection. This effects analogue signals badly as each time it is amplified the noise also gets amplified, this alters the signal making it hard or impossible to identify as the original signal.In digital signals any noise picked up is likely to be of a smaller amplitude than that if the on state, this means something receiving it will ignore the noise as it is neither on nor off, this makes them less likely to be distorted.

Multiplexing

Many digital signals can be sent down 1 wire with little interference. On the other hand analogue signals receive large amounts of interference so cannot be sent down the same wire.

Energy Resources & Energy Transfer

Kilogram (kg): the metric way of measuring weight

Joule (J): energy

Metre (m): the metric measure for distance

Metre/Second (m/s): this is a measure of speed; it is how many meters you go in a second

Metre/second² (m/s²): this is a measure of acceleration

Newton (N): a newton is a measure of force

Second (s): unit of time

Watt (W): power

Energy can change from one form to another, and frequently does. Some examples include: Chemical energy in food turns into kinetic energy for movement; Electrical energy in a circuit turns into heat energy in a resistor; Kinetic energy in your muscles turns into sound energy from you voice. Elastic potential energy in a taut rubber band turns into kinetic energy when it sails through the air.

Energy can never be lost, only transferred. Energy will always carry on, just in a different form.

For example, when you switch on a light, you are not loosing energy from a battery (chemical), you are just converting it to light energy.

Efficiency is how efficient a thing is, it is a decimal.

Conduction is when energy is passed from one particle to another via contact.

Convection is when particles with energy rise, the space they leave is filled by other particles. If the source of energy continues these new particles will also gain energy, they will then rise and the process will be repeated.

Radiation is when heat is transferred as infrared waves. These waves can travel through space and be conducted or reflected.

These energy transfers are all for heat energy.

Convection is helpful as it distributes heat energy. This is useful in many situations, for example, a radiator in one place will be able to heat a whole room, as hot air will rise away from it creating a current of cool air to be heated.

An insulator is something that is bad at conducting. If something with heat energy is surrounded by an insulator, it won't lose heat by conduction. This is true in buildings where insulating materials are put in walls and on floors to stop heat being lost from inside. This is the same in humans where we wear clothes to stop heat being lost from conduction. Air is a poor conductor, so materials with many air gaps in are also poor conductors; air trapped between double glazing prevents heat loss through windows.

Work done and energy transferred are always the same.

gravitational potential energy = mass × g × height

GPE = m × g × h

Useful to think that mass, gravity and height are all things that increase GPE.