Thermal Physics

Question 1

Give the 2 conditions required for the transfer of energy between 2 objects

Answer 1

• One object exerts a force on the other one and males it move (work is done)
• One object is hotter than the other so heat transfer takes place by means of conduction, convection or radiation

Question 2

Define internal energy

Answer 2

The internal energy of an object is the energy of its molecules due to their initial movement and positions

“The sum of the random distribution of the kinetic and potential energies of its molecules”

Question 3

Define temperature

Answer 3

A measure of the degree of hotness of an object

Question 4

Define thermal equilibrium

Answer 4

The condition under which two substances in physical contact with each other exchange no heat energy

Question 5

Give the fixed points for the Celsius scale

Answer 5

1) Ice point (0°C) which is the temperature of pure melting ice
2) Steam point (100°C) which is the temperature of steam at standard atmospheric pressure

Question 6

Give the fixed points for the absolute scale

Answer 6

1) Absolute zero (0K) which is the lowest possible temperature
2) The triple point of water (273.16K) which is the temperature at which ice, water and water vapour are in thermal equilibrium

Question 7

Give the equation to covert °C to K

Answer 7

Temp. in °C = absolute temp. in kelvin - 273.15

Question 8

State and describe absolute zero

Answer 8

No object can have a temperature below absolute zero

An object at absolute zero has minimum internal energy

Question 9

State 3 factors that effect the change in temperature of an object

Answer 9

• The mass of the object
• The amount of energy supplied to it
• The substance or substances from which the object is made

Question 10

Define specific heat capacity

Answer 10

The specific heat capacity of a substance is the energy needed to raise the temperature of unit mass of the substance by 1K without change of state

Question 11

Give the symbol and units for specific heat capacity

Answer 11

Symbol: c
Unit: Jkg⁻¹K⁻¹

Question 12

Give the equation for specific heat capacity

Answer 12

Energy needed ΔQ = mc(T₁-T₂)

Question 13

Describe the inversion tube experiment and give the equation to calculate SHC

Answer 13

Lead shots in a known length L of cardboard tube with a thermometer at one end.
The temperature of the lead shots is measured before and after n number of inversions
Assuming all the gravitational potential energy lost is transferred to internal energy of the lead shot:
c = gLn / ΔT

Question 14

Describe the electrical method to finding the SHC of a metal and give the equation for SHC

Answer 14

A block of metal of known mass m is in an insulated container and heated by a 12V heater inserted into a holed drilled in the metal and used to heat the metal. The change in temperature is measured by a thermometer in a second hole with a small amount of water or oil to improve the thermal contact between the thermometer and the metal
Since electrical energy = IVt, assuming no heat is lost to the surroundings:
c = IVT / mΔT

Question 15

Describe the electrical method to finding the SHC of a liquid and give the equation for SHC

Answer 15

A known mass of liquid is used in an insulated calorimeter of known mass and known SHC. A 12V heater is placed in the liquid to heat it directly, and a thermometer is inserted into the liquid to measure the temperature rise ΔT
Since the energy needed to heat the calorimeter = mass of calorimeter x SHC of cal. x temp. rise (ΔT), assuming no heat is lost to the surroundings
IVt = m₁c₁ΔT + mᶜᵃᶥcᶜᵃᶥΔT

Question 16

Define specific latent heat of fusion

Answer 16

The energy needed to change the state of unit mass of the substance from solid to liquid without change of temperature

Question 17

Define specific latent heat of vaporisation

Answer 17

The energy needed to change the state of unit mass of the substance from liquid to vapour without change of temperature

Question 18

Define sublimation

Answer 18

Where some solids vaporise directly when heated

Question 19

Give the equation for specific latent heat and its units

Answer 19

Q = ml
l = Q/m
units: Jkg⁻¹

Question 20

Give the equation for the energy transferred to the substance at constant rate P:

i) before the solid melts
ii) after the solid melts

Answer 20

i) P = mc[solid] x (ΔT / Δt)[solid]

ii) P = mc[liquid] x (ΔT / Δt)[liquid]

Question 21

For a pure substance with constant energy supplied (P), describe the effect on the rate of change of temperature for a solid and a liquid if the solid has a greater SHC

Answer 21

Since the solid has a greater SHC, the rate of temperature rise of the solid is less than that of the liquid, therefore the liquid heats up faster than the solid