P4

We call the process of getting electricity from nuclear reactions nuclear fission. This involves a nuclear reaction, which produces heat. This heat is used to heat water into steam, which is then used to turn turbines turning a generator.

Uranium releases energy when the nucleas is hit by a neutron. This collision causes the nucleas to split so energy is released.

A chain reaction can occur because when a nucleas splits more neutrons are given out. These can cause even more nuclei to split. To stop this from going out of control we place rods in the reactor. These will absorb some neutrons but leave enough to enable the reaction to keep going.

This can be described in terms of the movement of electrons; if an atom gains electrons it will have a negative static charge, and vice versa. 

Static can be dangerous when using flammable materials or when large quantities can pass through the body. It can be a nuisance when causing clothes to cling or when dust/dirt gets attracted to insulators. But the chance of recieving a static shock can be reduced by earthing, insulating matts or shoes with insulating soles.

If enough charge builds up on an insulator, the charge can leap the gap, causing a spark. This can be prevented by discharging the object, gradually. This is called earthing. The Earth can soak up any excess charge. This is very important when unloading flammable materials from lorries.

Defibrillators use static. Two paddles with insulated handles are charged from a high voltage supply and are put in good electrical contact with the patient’s chest. It's important that no one else gets a shock, which is why the paddles have insulating handles. Charge is then passed through the patient to make the heart contract.

Paint Spraying involves a charged gun. This ionises the paint particles which then get attracted to the object which will have an opposite charge. This provides an even cover with little waste.

An Electostatic Precipitator uses a charged grid to ionise (charge) dust particles when smoke leave the chimney. These particles then get attracted to oppositely charged metal plates, which means there is much less polution.

Background radiation comes from unstable isotopes (in food, air etc), cosmic rays and human activity.

Radiation can be used as tracers. They can show leaks/blockages, track the dispersal of waste and find the route of underground pipes. To do this gamma (most penetrating, can reach the surface) is put into a pipe. This is then tracked by a detector above ground and reductions in the gamma can provide the location of blockages.

Smoke Detectors use a weak source of alpha to ionise the air between two electrodes. This makes charged particles which carry a current but smoke will absorb the radiation so the current stops.

Radioactive carbon can be used to date old materials.

This is because the amount of carbon 14 in the air hasn't changed for thousands of years and when an object dies gaseous exchange with the air stops. As the C14 in the wood decays the activity of the sample decreases, leading to a reasonably accurate date.

Half life is the time taken for the count rate to halve.

Decaying radioactive materials give off nuclear radiation in the form of alpha, beta and gamma.

Radioactivity comes from the nucleus of an unstable atom. An alpha particle is a helium nucleus. A beta particle is a fast moving electron. 

When an alpha particle is emitted the mass number decreases by 4, the nucleus has 2 less neutrons and 2 less protons, so the atomic number decreases by 2. This means that a new element is formed.

When a beta particle is emitted the mass number is unchanged, but the nucleus has 1 less neutron and 1 more proton. The atomic number decreases by 1.

Resistance = voltage/current

A variable resistor is one where you can alter the resistance.

If you have a given resistor, the current will increase as p.d. (voltage) increases. But if you have a fixed p.d. the current will decrease as resistance increases.

The live wire carries the high voltage. The neutral wire is the second wire to complete the circuit. The earth wire stops the appliance becoming live in case of a fault.

If a fault allows the live wire to touch the metal case a huge current will flow through the earth wire to the ground. This blows the fuse, cutting off the supply to the live wire and reducing the chance of electrocution.

A wire fuse melts if the current becomes too large, breaking the circuit. This is important because if the appliance develops a fault, the prevention of the flow of current prevents overheating and a fire, stopping further damage to the appliance.

Double insulated appliances don't need earthing because the case of the appliance is a non-conductor and can't become live.

Humans can't hear ultrasound because it is higher than the threshold of human hearing. It can be used for body scanning because when it hits boundaries between different media some of the wave is reflected back. These waves are then processed by a computer. Ultrasound is used instead of X-rays because it can produce images of soft tissue and doesn't damage living cells.

The wave length of a longitudinal wave is the distance between the centre of two compressions. A compression is the name given to the region where the particles are close together. Rarefaction is the name given to the region where the particles are far apart.

In longitudinal waves the particles oscillate back and forth, but in transverse waves they oscillate up and down.

Gamma rays come from the nucleus of certain radioactive materials. Gamma, beta and alpha can be used as tracers in the body. 

X-rays are made by firing high speed electrons at a metal target. They are easier to control than gamma, but they have similar wavelengths.