THERMODYNAMICS

Question 1

What is meant by the temperature of a body

Answer 1

A measure of the mean, random KE of its vibrating atoms.

Question 2

What is meant by heat

Answer 2

The flow of energy from a region of higher temperature to a region of lower temperature.

Question 3

Specific heat capacity

Answer 3

The amount of energy needed to raise the temperature of 1 Kilogram of substance by 1 Kelvin.

Question 4

What are the assumptions we make in the kinetic theory

Answer 4

1. A gas consists of a very large number of molecules and are in continuous rapid motion - Brownian Motion.
2. Collisions between molecules and between molecules and walls are perfectly elastic meaning KE in conserved. This must be true as Brownian motion observed continuous motion if KE was not conserved than they would gradually slow down
3. The volume occupied by the gas molecules themselves compared to the container is negligible- as compressing a gas by a large amount is easy
4. Intermolecular forces are negligible except during a collision as they are very far part and very small
5. The duration of the collisions is negligible compared to the time spent between collisions as the molecules are far apart form one another

Question 5

What is meant by absolute zero

Answer 5

The temperature at which the pressure and volume of an ideal gas becomes 0 as it has no Kinetic energy

Question 6

What is the absolute temperature scale

Answer 6

The Kelvin or thermodynamic temperature scale with zero at 273 degrees.

Question 7

Define fluid

Answer 7

A medium that can flow e.g. gases and liquids.

Question 8

What is an Ideal gas

Answer 8

Gas that obeys the gas laws

Question 9

Define Internal energy

Answer 9

The total kinetic and potential energy of a molecule.

Question 10

Define Potential energy

Answer 10

Energy stored in the stretched or compressed bonds between molecules of a material

Question 11

Define kinetic energy

Answer 11

Energy due to movement of molecules

Question 12

What is mean square speed

Answer 12

The average of the squares of the speeds of the particles in a gas.

Question 13

Define pressure

Answer 13

Force per unit area (Pa or Nm^-2).

Question 14

What is the pressure law

Answer 14

For a constant mass of gas at constant volume, the pressure exerted by the gas is directly proportional to its absolute temperature

Question 15

What is boyle’s law

Answer 15

For a constant mass of gas at constant temperature, the pressure exerted by the gas is inversely proportional to the volume it occupies

Question 16

What is Charles law

Answer 16

For a constant mass of gas at constant pressure, the volume occupied by the gas is directly proportional to its absolute temperature

Question 17

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Answer 17

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Question 18

What is the ideal gas law

Answer 18

Pv= NKT

Question 19

Explain why the pressure of a gas increases when the volume decreases

Answer 19

1. When the volume decreases, the frequency of collisions between the molecules of the gas and the container walls increases
2. This causes a greater rate of change of momentum (because more collisions are happening per second)
3. Force = rate of change of momentum

4· So the greater rate of change of momentum causes a greater force on the walls of the container

5· Pressure = force /area so the greater force causes a greater pressure on the walls of the container for the same area

Question 20

Describe the energy transfers of a melting ice cube

Answer 20

1. Energy from the surroundings is transferred to the ice cube
2. The ice molecules gain potential and kinetic energy
3. The gain in kinetic energy increases the temperature of the ice
4. This continues until the ice reaches 0C
5. The ice gains energy E=ml as it changes state to water that is zero degrees ,
6. Kinetic energy is constant whereas potential energy increases
7. The water then gains energy E=mcdelta T until thermal equilibrium is reached

Question 21

Measuring the specific heat capacity of a liquid/ solid method:

Answer 21

1. Measure the mass the block of aluminium/ mass of water
2. (ensure there is appropriate insulation/lagging to prevent energy transfer from the material to the surroundings)
3. Record the initial temperature
4. Switch power supply on and start the stopwatch
5. Record the current and potential difference
6. After a fixed time (e.g. 4 minutes), switch power off and record highest temperature reached, use this to calculate the change in temperature Δθ
7. Calculate the energy transferred E= IVt
8. Calculate c = E/mΔθ

Question 22

Sources of error in measuring the specific heat capacity of a liquid/ solid method

Answer 22

1. Energy is absorbed by heater, larger c value.
2. Energy lost to surrounding despite lagging, larger c value.
3. Little energy lost to lagging/ expanded polystyrene cup, larger c value.
4. Thermometer may have low resolution.
5. Finding the SHC of a liquid means we have to find the energy taken by the container, however an expanded polystyrene cup is light and a good insulator.

Question 23

Improving accuracy in measuring the specific heat capacity of a liquid/ solid method

Answer 23

1. Use a data logger and temperature sensor

2. Repeat and take average

Question 24

Specific Latent heat

Answer 24

The energy required to change 1 Kg of a substance from one state to another without change in temperature ( Q=ml)

where:
E= The heat energy(Joules)
m= Mass of substance (Kg)
l= Specific latent heat ( J kg-1 )

Question 25

Specific Latent heat of Fusion

Answer 25

Energy required to change 1 kg of Solid to liquid.

Question 26

Specific Latent heat of Vaporization

Answer 26

Energy required to change 1 kg of liquid into gas.