3.4 - Thermal Physics

Internal energy of a system

The sum of kinetic and potential energies of the particles/molecules of a system

Symbol used to denote internal energy of a system

U

Absolute zero

Temperature of the system when it has minimum internal energy

Internal energy of an ideal, monatomic gas

Wholly kinetic so U=3/2 RT

Equation for U of an ideal monatomic gas

U=3/2RT

Explain why PE of an ideal gas is negligible

The attractive forces between the particles are negligible

Heat

Energy flow from a region at higher temperature to a region at lower temperature, due to the temperature difference

Heat energy

Process of energy entering or leaving a system

If no heat flows between systems in contact, they are said to be in

Thermal equilibrium and are at the same temperature

When two systems are in thermal equilibrium

No heat flow between systems in contact and they are the same temperature

Work

If the system is a gas, in a cylinder fitted with a piston, the gas does work of amount pΔV when it exerts a pressure, p, and pushes the piston out a small way, so the gas volume increases by ΔV

Equation for work done

W=pΔV

Work from a p-V graph

Even if p changes, W = area under the p-V graph

First law of thermodynamics

The increase, ΔU, in internal energy of a system is ΔU=Q-W in which Q is the heat entering the system and W is the work done by the system

ΔU is a positive value

Increase in internal energy of the gas

ΔU is a negative value

Decrease in internal energy of the gas

Q is a positive value

Heat enters

Q is a negative value

Heat leaves

W is a positive value

Work done by the gas

W is a negative value

Work is done on the gas

Equation for Q of a solid or liquid

W is negligible so Q=ΔU

Specific heat capacity

The heart required, per kilogram, per degree Celsius or kelvin, to raise the temperature of a substance

Q=mcΔT

Symbol used to denote specific heat capacity

c

Unit for specific heat capacity

Jkg^-1K^-1 or Jkg^-1•C^-1