Thermal energy transfer

Question 1

What is internal energy?

Answer 1

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

Question 2

What is the symbol for internal energy and what is it measured in?

Answer 2

• Symbol is U
• Measured in Joules

Question 3

What is the internal energy of a system determined by?

Answer 3

• Temperature
• Random motion of molecules
• Phase of matter (solid, liquid, gas)
• Intermollecular interactions between the particles

Question 4

What is the first law of thermodynamics?

Answer 4

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

Question 5

When is mechanical work done on an object?

Answer 5

• When distance is covered against a force

Question 6

What happens when a gas expands and contracts (work done)?

Answer 6

• When a gas expands (volume increases), work is done BY the gas ON the surroundings. This decreases the internal energy of the gas
• When a gas contracts (volume decreases), work is done ON the gas BY the surroundings. This incereases the internal energy of the gas

Question 7

What is thermal equilibrium?

Answer 7

• When there is no longer an exchange of heat energy and both region reach an equal temperature

Question 8

What is the specific heat capacity of a material?

Answer 8

• The amount of thermal energy required to raise the temperature of 1kg of a substance by 1 C without a change of state

Question 9

Why do good electric conductors have low speciic heat capacities?

Answer 9

• The lower the specific heat capacity of a material, the faster the material will heat up or cool down
• Good electric conductors are great conductors of heat and hence have a low specific heat capacity

Question 10

Why is there no change in temperature when there is a change in state?

Answer 10

• All of the thermal energy is being used to break the bonds in the substance
• The potential energies change but not the kinetic energy

Question 11

What is specific latent heat?

Answer 11

• The amount of thermal energy required to change the state of 1kg of mass of a substance without a change of temperature

Question 12

Why is the SLH of vaporisation higher than of fusion?

Answer 12

• The molecules must be completely seperated with no more forces of attraction between them