Physics Edexcel P2a Revision Cards

In bright light, the resistance falls

In darkness, resistance in highest

When in hot conditions, resistance drops.

When in cool conditions, resistance rises.

Resistors get hot when an electrical current passes through them.

• It is the energy transfer that heats the resistor.
• This energy transfer is due to electrons colliding the resistor, which is made up of ions in a lattice structure.
• This gives the ions extra energy, which is released as heat.
• The heating increses the circuits resistance, in turn reducing the current.
• This makes the circuit less efficient, as energy is being lost as heat.
• It could also cause components to melt, which will break the circuit.
• Fuses are designed to melt and break, to stop the circuit it the current gets too high

This effect can have it's advantages. For example, toasters contains a coil of wire with really high resistance, which heats up and cooks bread. 

Power, P, Watts = Potential Difference, V, Volts x Current, I, Amps

Energy Transferred, E, Joules = Power, P, Watts x Time, T, Seconds

Acceleration is measured in m/s^2 or ms^-2

Weight in Newtons = Mass in Kilograms x Gravitational Field Strength in N/Kg

Falling objects in a vacuum accelerate at the same rate, because there's no air resistance.

Terminal Velocity is when the force pulling you down (weight) matches the force of the air resistance, and you no longer accelerate.

If object A exerts a force on object B then object B exerts the exact opposite force on object A.

Object A's force is called the ACTION FORCE
Object B's force is called the REACTION FORCE 

Force in Newtons = Mass in Kg x Acceleration in m/s^2

Many factors affect stopping distance in a car.

• Thinking distance
  • Your reaction time
  • How fast you're going
• Braking distance
  • How fast you're going
  • The mass of your vehicle
  • How good your brakes are
  • How good the grip is
    • Road surface
    • Weather conditions
    • Tyres

Having at 1.6mm tread depth stops aquaplaning

Momentum (kg m/s) = Mass (kg) x Velocity (m/s)

In a collision, momentum is conserved. Momentum before = Momentum after.

A big change in momentum in a short period of time will create a large force. This happens in car crashes, making injury very likely. Car are equipped with safety features (crumple zones, seat belts and air bags) that increases the time for the driver to stop, subsequently reducing the forces acting on said person.

When a force moves and object, energy is transferred and work is done.
Work Done (Joules) = Force (Newtons) x Distance Moved in Force's Direction (Metres)

Power is just the rate of doing work. It's measured in Watts or J/s, they are the same thing.

Kinetic Energy = 1/2 x Mass x Velocity²

Stopping DIstances increase alarmingly with extra speed

1/2 x m x v² = f x d f = braking force d = braking distance

To stop a car, the kinetic energy must be converted to heat energy at the brakes and tyres.
If you double to speed, you double the value of V, but because this gets squared, the K.E. is increased by a factor of four. The max braking force cannot increase, so the braking distance is increased by facotrs of four.

Gravitational Potential Energy = Mass x G x Height

Energy can never be created nor destroyed - only transferred from one form to another.

Energy is only useful when it's transferred from on form to another

An isotope is an atom with the same number of protons, but a different number of neutrons.

Most istopes tend to be radioactive, which means they decay into other elements and give out radiation.

You cannot tell when the isotope is going to decay and give out ionising radiation, neither can you make decay happen.

Atoms gain or lose electrons, turning them into either a positvely or negatively charged ion. This is called ionisation. Three types of radiation cause ionisation:

• Alpha. Helium nuclei. Strongly positively charged, strong ionisation. Pulls electons out of orbit. Slow and heavy. Stopped by paper.
• Beta. Electrons. Negatively charged, moderate ionisation. Pushes electrons out of orbit. Quite fast and small. Stopped by thin aluminium. For every beta particle emitted, a neutron turns into a proton in the nucleus.
• Gamma. Electromagnetic wave. Weakly ionising. Transfers energy into the electron, until the electron have enough energy and breaks free from the atom. Stopped by thick lead.

This is the joining of small atomic nuclei.

For example, hydorgen nuclei can fuse to become helium nuclei.
This produces lots of energy, more than fission for a given mass, so people are trying to develop fusion reactors.
The problem is, it only happens at 10 million degrees C, otherwise fusion is prevented by the electrostatic repulsion of the protons. The hot hydrogen would also have to be in a magnetic field, not a physical container. Getting a reactor to this temperature would take more energy than it fusion released, so people are looking to cold fusion.

In 1989, STanley Pons and Martin Fleischmann said they'd conducted. The experiment has since been discredited by MIT. The work just hadn't been validated before the theory went public.

• Fire Alarms - A weak source of Aapha is inside the smoke detector, close to two electrodes. The source causes ionisations, and a current flows. The smoke from a fire will absorb the radiation, cutting off the current and sounding the alarm.
• Sterilisation of food and equipment. Food can be irradiated with gamma radiation which kills all the microbes, meaning the food won't go bad as quickly. Medical equipment can also be sterilised this way. The alternative would be to boil them, which can cause damage to plastic instuments. The source of gamma would need to be very strong and have a long half life.
• Medical Tracers. A radioactive isotope will be injected into the body, it MUST be beta or gamma so that the radiation passes completely through the body, also it must have a short half life.. The tracers can be tracked around the body and used to detect and diagnose medical conditions such as cancer. Leaks in underground pipes are detected similarly.
• Thickness gauges. The thickness of paper can be controlled using a negative feedback system involved a beta source. A detector senses how much beta is passing through the paper and a hydraulic control moves to change paper thickness accordingly.
• Treating cancer. High doses of gamma kills all living cells so radiotherapists can direct this at cancerous cells and kill it.

When Marie Curie discovered the radioactive properties of radium in 1898, nobody knew of it's dangers. It was used in medicines and paint, but when people exposed to this excessively began to get cancer, some doubts arose.

Ionising Radiation causes tissue damage and cell mutation.
Low doses mutate cells, causing them to divide uncontrollably. This is cancer.
High doses kill cell completely, which causes radiation sickness if many die at once.

You should protect yourself in a lab by following these steps:

• Never allow skin contact with the source. Handle with tongs.
• Keep source at an arms length away.
• Keep source pointing away from the body.
• Put the source back into a labelled lead box as soon as the experiment is over.

• Full protective suits to prevent particles being inhaled or lodged under fingernails.
• Lead lined suits, barriers or screen.
• Robot arms to carry out tasks in highly radioactive areas.

Nuclear Waste is a big problem
At the moment, it is dealt with by vitricfication, melting the waste with other materials to form a type of liquid glass, which is then buried deep underground.
Another solution is the put it in a thick metal container and bury it in a deep hole and fill it up with concrete.
It is important to ensure there are plenty of materials around it to absorb the radiation so it doesn't get tot the Earth's surface.

Nuclear Power has it's pros and cons

• It is a reliable source of energy and also very clean. The fuel, uranium, is cheap and readily available. Huge amounts of energy can be obtained from a small mass of fuel.
• However, disasters such as chernobyl can occur, and there's a risk of of radioactive waste leaking out. The cost of a nuclear power plant is high, and decommissioning it safely takes decades. It could also be used a smokescreen for developing nuclear weapons.