# Matter: Very Simple

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• Created by: helena
• Created on: 27-05-13 20:08

## Ideal Gases

An Ideal Gas is one that obeys the Ideal Gas Law: pV = NkT

The ratio pV/nT  is nearly the same for all gases at sufficiently low pressure and high temperature

As p decreases and T increases real gases get more ideal

At high pressures gases don't behave like Ideal gases due to the reduced volume available for the molecules to move in

At low temperatures gases don't behave ideally because they have little energy, meaning they move slowly enough for them to be affected by attractions between themselves, reducing the pressure

Boyles Law: pV is constant for constant n and T

Charles' Law: V/T is constant for constant p and T

The Pressure Law: p/T is constant for constant n and V

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## Kinetic Theory of Gases

The Kinetic Theory of Gases explains the behaviour of a gas as due to the motion of its molecules

The theory is based on the following assumptions

• A gas consists of molecules of negligible size
• The molecules conllide elastically with each other and the container, on average gaining or losing no  energy
• The molecules are in continual random motion
• There are negligible forces of attraction between the molecules
• The duration of an impact is much less than the time between impacts

It can explain the ideal gas relationship, pV = NkT

pV= 1/3 Nm<C2>

where <C2> is the mean of the square of the molecular speeds

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## Conservation of Momentum

Imagine a closed boundary drawn around any set of objects, large or small. In a given time, energy may enter across the boundary and energy may leave across the boundary

The principle of conservation of Energy states that the change in energy within the boundary is always exactly equal to the difference between that entering and that leaving. Energy is never manufactured or destroyed

The standard unit of energy is the Joule

The baic means of measuring the amount of energy passed from one part of a system to anotehr is the wrk done, equal to the force acting multiplied by the component in the direction of the force. For this reason energy is often said to be 'the capacity to do work'

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## Random Walk

There is no way to record the random motion of all the partilces in a gas. Instead you can use a model

A random walk assumes that each particle starts in one place, moves N steps in a random direction, and ends up somewhere else

The distance the particle moves before a collision is normally 10-7m. So it is no wonder diffusion is such a slow process, even if the particles are travelling at high speeds

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## Specific Thermal Capacity

Adding thermal energy to an object raises its temperature, as long as it doesn't change state

All molecules in a sample move randomly, with a variety of speeds

The temperature of a sample is a measure of the average kinetic energy of its molecules

If you took all of the kinetic energy out of a sample, its temperature couldn't fall any further. This is absolute zero

The specific heat capacity, c, of a substance is the amount of energy needed to raise the temperature of 1kg of the substance by 1K

E=mcΔT

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