Thermal energy transfer

What is the internal energy of a substance?

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

What is the symbol and unit for internal energy?

U and Joules.

What is the internal energy of a system determined by?

• Temperature

• The random motion of molecules

• The phase of matter

• Intermolecular interactions

How can the internal energy of a system increase?

• Doing work on it

• Adding thermal energy to it

How can the internal energy of a system decrease?

• Losing thermal energy to its surroundings

• The system doing work on its surroundings.

What is the first law of thermodynamics?

The internal energy of a system is increased when energy is transferred to it by heating or when work is done on it.

What happens to internal energy when a gas expands or compresses?

When a gas expands, work is done by the gas on the surroundings so internal energy decreases.

When a gas is compressed, work is done on the gas by the surroundings so internal energy increases.

What is specific heat capacity?

The amount of thermal energy required to raise the temperature of 1kg of a substance by 1 degree without a change in state.

What is the symbol and unit for specific heat capacity?

c and joules per kilogram per kelvin

What is the equation for specific heat capacity?

∆Q=mc∆θ

Q= change in thermal energy

c= specific heat capacity

∆θ= change in temperature

How can the specific heat capacity of a fluid be found?

By using a continous-flow calorimeter. A fluid flows through an electrical heating wire, change in temperature measure using θ= T₂-T₁. To find mass of fluid the flow rate is recorded and multiplied by the time taken to give the mass of the fluid that flows in as m₁. current and potential difference are also recorded. flow rate is then altered to give m₂ and the potential difference is changed so that temperature difference stays the same. c= Q₂-Q₁/ (m₂-m₁)∆θ

What happens when a substance changes state?

there is no temperature change. The potential energies of molecules change, but not their kinetic energies.

What is latent heat?

The thermal energy required to change the state of 1kg of mass of a substance without any change in temperature.

What are the two types of latent heat?

specific latent heat of fusion (melting and freezing)

specific latent heat of vaporisation (boiling and condensing)

What is the equation for latent heat?

Q= mL

Q= thermal energy

m= mass

L= latent heat (j kg-¹)

What requires more energy, evaporating 1kg of water or melting 1kg of ice?

Evaporating 1kg of water requires roughly seven times more energy than melting the same amount of ice. When ice melts energy is required to just increase the molecule separation until they can flow freely over each other, when water boils energy is required to completely seperate the molecules until there are no longer forces of attraction between the molecules, hence requires much more energy. Vaporisation is also doing work against atmospheric pressure. (more energy has to be supplied to separate molecules than break a solid bond)

What is thermal equilibrium?

When two or more substances are mixed and both have the same final temperature.

What is the equation for thermal equilibrium?

∆Q (colder)= -∆Q (warmer)

if one of the materials changes phase: ∆Q (colder solid) + ∆Q (melting) + ∆Q (colder liquid) = -∆Q (warmer)