# edexcel physics P3

These cards are all you need for revising Edexcel extension science. They contain information on all of topic 5.

- Created by: Alex Judge
- Created on: 26-05-11 11:02

## Kinetic Theory

This is when gases consist of very small particles. E.G. oxygen consists of oxygen molecules.

The particles are constantly moving in random directions. They collide with each other and the walls of the container, bouncing off each other.

However, they hardly take up any space. Most of the gas is empty space.

Kinetic energy is propotional to tempertaure

If you increase the temperature of gas, you give the particles more energy.

So, if you double the temperature, you double the average kinetic energy.

The temperature of a gas (in Kelvin) is proportional to the average kinetic energy of it's paritcles.

As you heat up a gas, the average speed of particles increases. Anything that's moving has kinetic energy. Kinetic energy is 1/2mv squared.

## Kelvin and Absolute Zero

If you increase the temperature of something, it's particles get more energy, usually meaning they will move about more. Moreover, if you decrease the energy of something, you are reducing the kinetic energy of the particles.

The coldest anything can get is -273 degrees. This is also 0 degrees in Kelvin. It is known as absolute zero where the atoms have as little kinetic energy as possible.

To convert from degrees Celsius to Kelvin, just add 273. Similarly, to get from Kelvin to Celsius then subtract 273.

A change of one degree is also a change of one Kelvin. For example, if you went from 22 degrees to 23 degrees, then Kelvin would simply go from -251 to -250. This shows that the scales are very similar.

## Pressure in Gases

## Constant Volume Equation

In a sealed container, pressure divided by temperature = constant.

Alternatively, to work it out you can use P1 and T1 as your starting conditions and P2 and T2 as your final conditions.

Example: A container has a volume of 30 litres. It is filled with gas at a presure of 1 atm and a temperature 290 K. Find the new pressure if the temperature is increased to 315 K.

So P1 = 1, T1 = 290, so 1/290 (which still equals 290). Also, T2 = 315. To work it out, we now rearrange the equation is: P2 = T2/The answer to P1/T1 - P2 = 315/290, which equals 1.09 atm - P2.

Temperature for this equation must always be in kelvin, so if they give you it in Celsius, just convert it to Kelvin.

## 'General' Equation

If the pressure isn't constant, then this equation applies instead:

This is when Pressure (P1) X Volume (V1) Divided by Temperature (T1) equals constant. You can use the same hypothesis as before: the left hand side are your starting conditions, and the right your final conditions - P2 X V2 / T2.

Example: A gas at a pressure of 2.5 atmospheres is compressed from a volume of 300 cm^3 down to a volume of 175cm^3. During the compression, the temp increases from 230K to 280K. Find the new pressure in atmospheres.

Answer: P1V1/T1 = (2.5 X 300) / 230 = (P2 X 175) / 280. Then you rearrange the equation to look like this: P2 X 175 = [(2.5 X 300) / 230] X 280 = 913.04. Then, we rearrange it again: P2 = 913.04 / 175 = 5.22 atm

## Particles in Atoms

Alpha: Helium nucleus, slow + heavy, strongly ionising, stopped by paper etc Beta: An electron, light + fast, moderately ionising, stopped by thin metal. Gamma: Electromagnetic radiation, no mass, very fast, weakly ionising, stopped by thick lead or very thick concrete.

Positron Radiation's positively charged beta radiation. Positrons are like electrons, they've got the exact same charge, but they're positively charged. They're obliterated as soon as they meet an electron. This is called annihilation.

Neutron radiation is neutrons. Neutrons are more penetratating than alpha beta or gamma, but they aren't directly ionising. They can be absorbed by the nuclei of atoms they pass through. This can make a nucleus radioactive. They then emit ionising radiation, making neutrons 'indirectly ionising'.

Neutrons are best absorbed best by light nuclei - like hydrogen, so sheilding is made from hydrogen rich materials. Neutron absorbtion makes nuclei emit gamma radiation, so thick lead may be added to radiation sheilding.

## Particles in Atoms Continued

Neutrons are difficult to detect because because they're neutral. This makes a difference because charged particles knock electrons off atoms they bang into, making them ionising.

Charged particles are often detected by the trail of ions left behind. This doesn't happen with neutral particles.

Charged particles also move towards charged objects. You can use an electrical field to deflect a beam of alpha particles (or anything else that is charged). This doesn't work with neutral particles either.

Instead, neutrons are dected by looking for nuclear decays from the nuclei they make radioactive.

## Particles in Atoms 2

Some nuclei are more stable than others. A nucleus will be unstable if it has: too many neutrons, too few neutrons, too many protons and neutrons - it becomes too heavy or too much energy.

The curve of stability

If any isotope that doesn't lie on the curve is unstable/radioactive. Particles or radition are emitted. Any isotope that lies above the curve has too many neutrons to be stable. Any isotope that lies below the curve has too few neutrons to be stable

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