Thermal physics

Define internal energy

The sum of the randomly distributed kinetic energy and potential energies of the particles in a body

How is the internal energy of a system increased

Transfer of thermal energy (system is heated)

Work is done on the system by an external force

How is internal energy of a system decreased

The system is cooled

System itself does work against an external force

Define specific heat capacity and unit

Amount of energy needed to heat 1kg of substance by 1K without any change of state

Jkg^-1°C^-1 (or Kelvin)

Determination of specific heat capacity by an electrical method

c = Q/mdeltatheta

So c = VIdeltat/mdeltatheta

Determination of specific heat capacity by the method of mixtures

Internal energy lost by substance = energy gained by the water

All energy gained by water is used to raise its temperature

Define continuous flow method

Where a heater is used to heat up flowing water

Energy in continuous flow method

Energy from heater = thermal energy given to water + energy lost to surroundings

Rates in cases of fluid flow

Rate of energy transfer = rate of mass flow X cdeltatheta

Define specific latent heat of vaporisation

The energy required to change the state of 1kg of liquid to gas without a change in temperature

Define specific latent heat of fusion

The energy required to change the state of 1kg of solid to liquid without a change in temperature

Describe process of substance changing state

Changing of state requires energy to break down bonds

Potential energy component of the substance’s internal energy changes but the average kinetic energy of the particles doesn't

Why is ice better at cooling drinks compared to cold water

Drink cola down because it loses thermal energy when it's in contact with something cooler

Ice has very high latent heat of fusion so it takes a lot of energy to melt

When ice melts it remains at 0°C so It stays colder for longer

Boyles law

The volume of a fixed mass of gas at a constant temp is inversely proportional to it's pressure

Charles law

The volume of a fixed mass of gas at a constant pressure is directly proportional to it's Kelvin temperature

Pressure law

The pressure of a fixed mass of gas at constant volume is directly proportional to the Kelvin temperature

Units for pressure

Pa or Nm^-2

Ideal gas equations n

pV=nRT n=number of moles

pV=NkT N is number of molecules

Work done by a gas when it expands at constant pressure

W = pdeltaV

Where p is pressure and deltaV is change in volume

Assumptions of an ideal gas

All molecules identical, moving in random motion

All collisions elastic

Volume of molecules negligible compared to volume of container

Time spent in collisions negligible compared to time between collisions

No forces between molecules except during collisions

Random motion of molecules

Molecules have a range of speeds

No preferred direction of movement

Gases at the same temperature

Have the same average kinetic energy per particle

What does Brownian motion prove

Evidence of the random motion of gas molecules and direct evidence for the existence of atoms

Energy in ideal gas

Internal energy is kinetic energy of the atoms

Define molecular kinetic theory model

Explains the behaviour of gas by considering the movement of gas particles in a box

Describe the pressure law

Average KE increases with temperature

Faster molecule exerts a bigger force on container wall because it experiences a larger momentum change on hitting and bouncing off the container walls

More collisions per second due to molecules moving faster

Increased force on walls and increased frequency of collisions causes gas pressure to increase

Describe Boyle's law

Average KE is unchanged because temp is constant

More collisions of molecules with container walls per second because molecules are contained in smaller space and travel smaller distances between collisions

Pressure increases because the force on walls has increased due to the rise in collisions per second

Describe Charles law

Average KE increase with temp

Average force on container walls increases as faster molecules experience a greater change of momentum on hitting and bouncing off walls

If the force exerted on walls by molecules increase but the pressure is unchanged, the molecules must hit the walls less frequently

Achieved by an increased in volume: molecules have to travel further distances between collisions

Less collisions per second means for the same change in momentum to occur, time is greater

Thermal equilibrium

No net flow of thermal energy

Bodies at same temperature

How is thermal equilibrium reached?

Energy exchanged in collisions until average KE of all molecules is the same

Existence of pressure

Molecules have momentum at wall

Leads to force: wall exert force on molecule

Molecules exerts equal and opposite force on wall

Pressure = force/area, pressure exists in the container

Amount of gas increases

Space between molecules and walls decreased

More collisions between molecules and walls per second

So F increases and pressure increases as p=F/A

Potential energy in ideal gas

None