Thermal Physics

Question 1

Behavior of molecules during increase in K.E and P.E

Answer 1

If molecules move around more freely and faster, their kinetic energy has increased. If they break free of their neighbours and become more disordered, their electrical potential energy has increased. (Work must be done; energy must be put in to separate neighbouring atoms)

Question 2

When water is heated, each change of state (melting, boiling) involves:

Answer 2

• there must be an input of energy
• the temperature does not change
• the molecules are breaking free of one another
• their potential energy is increasing
• their kinetic energy remains the same

Question 3

When water is heated, every stage between changes of state (slopes in the graph) involve:

Answer 3

• the input of energy raises the temperature of the substance
• the molecules move faster
• their kinetic energy is increasing.
• potential energy remains the same

Question 4

Where is the energy utilised if temperature does not change during state change

Answer 4

During any change of state; the energy goes to breaking the bonds between neighbouring molecules. At a change of state, there is no change in kinetic energy, so there is no change in temperature. The energy supplied to cause a change of state is called the ‘latent heat’.

Question 5

Temperature

Answer 5

Temperature is a measure of the average kinetic energy of the molecules. When a thermometer is put in some water to measure its temperature, the water molecules collide with the thermometer and share their kinetic energy with it. At a change of state, there is no change in kinetic energy, so there is no change in temperature.

Question 6

Why does melting ice take less energy than boiling water of same amount?

Answer 6

When a solid melts, the molecules are still bonded to most of their immediate neighbours. When a liquid boils, each molecule breaks free of all of its neighbours. Melting may involve the breaking of one or two bonds per molecule, whereas boiling involves breaking eight or nine.

Question 7

Define Evaporation

Answer 7

A process by which a liquid becomes a gas at a temperature below its boiling point.

Question 8

Mechanism of evaporation

Answer 8

When a liquid evaporates, it is the most energetic molecules that are most likely to escape. This leaves molecules with a below-average kinetic energy. Since temperature is a measure of the average kinetic energy of the molecules, it follows that the temperature of the evaporating liquid must fall. There is a net outflow of energetic molecules from the liquid, and eventually it will evaporate away completely.

Question 9

Define Internal energy

Answer 9

The internal energy of a system (such as the heated stone) is defined as the sum of the random distribution of kinetic and potential energies of its atoms or molecules.

Question 10

Heating a gas (contribution to internal energy rise)

Answer 10

The walls of the container become hot and so its molecules vibrate more vigorously. The molecules of the cool gas strike the walls and bounce off faster. They have gained kinetic energy, and we say the temperature has risen.

Question 11

Doing work on a gas (contribution to internal energy rise)

Answer 11

Wall of the container is being pushed inwards. The molecules of the cool gas strike a moving wall and bounce off faster. They have gained kinetic energy and again the temperature has risen.

Question 12

How to lower internal energy (briefly)

Answer 12

If it loses heat to its surroundings, or if it expands so that it does work on its surroundings.

Question 13

First law of Thermodynamics & Equation

Answer 13

The increase in internal energy of a body is equal to the thermal energy transferred to it by heating plus the mechanical work done on it.
ΔU = q + W

Question 14

What do positive and negative values of Internal energy, ΔU refer to?

Answer 14

A positive value of ΔU means that the internal energy increases, a positive value of q means that heat is added to the system, and a positive value of W means that work is done on the system. Negative values mean that internal energy decreases, heat is taken away from the system or work is done by the system.

Question 15

When does first law equation become ΔU = q

Answer 15

a gas heated from the outside in a sealed container of constant volume. In this case, no work is done on the gas as the heat is added, so W is 0 and the first law equation ΔU = q + W becomes ΔU = q.

Question 16

Example of a gas doing work

Answer 16

If a gas expands, the walls are pushed outwards – the gas has done work on its surroundings (W is negative, if the gas is the system). In a steam engine, expanding steam pushes a piston to turn the engine, and in a car engine, the exploding mixture of fuel and air does the same thing.

Question 17

Work done Equation (& condition)

Answer 17

W = pΔV
Work done when the volume of a gas changes at constant pressure

Question 18

Compressing a gas - Condition 1 : Not allowing heat to enter or leave the system

Answer 18

By pushing the piston into the syringe very fast or by insulating the syringe. In this case,
q is zero and ΔU = W. All the work done by pushing in the piston increases the internal energy of the molecules. In this case, the kinetic energy of the molecules increases and the temperature increases, unless there is a change of state.

Question 19

Compressing a gas - Condition 2 : At constant temperature

Answer 19

Pushing the piston very slowly into a syringe containing gas; so slowly that the temperature stays constant at room temperature. This change is known as an isothermal change. The kinetic energy of the molecules remains constant.
Explanation :
The molecules become slightly closer together and this may mean that their internal energy U becomes slightly less but the change is very small (unless the gas becomes a liquid). If U is constant, then ΔU is zero and 0 = q + W. This means that, if the piston is pushed in and does positive work W, then q is negative, and heat is lost from the syringe. It can be thought of as doing positive work on the system and, with no extra internal energy, the system must lose some heat to the surroundings, perhaps by conduction of heat through the walls of the syringe. Similarly, if the piston is pulled out very slowly, W is negative and q is positive and heat enters the system.

Question 20

Define Isothermal change

Answer 20

A change of a system in which the temperature remains constant

Question 21

Define Thermal energy

Answer 21

Energy flowing from a region of higher temperature to a region of lower temperature or Energy transferred from one object to another because of a temperature difference another term for heat energy.

Question 22

Define Thermal equilibrium

Answer 22

When two objects are at the same temperature they are in thermal equilibrium with each other there will be not net as transfer of thermal energy between them when they are in contact with each other.

Question 23

Absolute zero, 0K

Answer 23

For any matter at absolute zero, it is impossible to remove any more energy from it. Hence, absolute zero is the temperature at which all substances have the minimum internal energy. (The kinetic energy of the atoms or molecules is zero and their electrical potential energy is minimum.)

Question 24

Conversion between Celsius scale and Thermodynamic (Kelvin) scale

Answer 24

θ (°C) = T (K) − 273.15
T (K) = θ (°C) + 273.15
(A change in temperature of 1 K is thus equal to a change in temperature of 1 °C)

Question 25

Thermodynamic scale

Answer 25

The thermodynamic scale is said to be an absolute scale as it is not defined in terms of a property of any particular substance. It is based on the idea that the average kinetic energy of the particles of a substance increases with temperature. The average kinetic energy is the same for all substances at a particular thermodynamic temperature; it does not depend on the material itself.

Question 26

Two fixed points on the thermodynamic scale

Answer 26

• absolute zero, which is defined as 0 K
• the triple point of water; the temperature at which ice, water and water vapour can co-exist, which is defined as 273.16 K (equal to 0.01 °C).

Question 27

Physical properties that can be used as the basis of thermometers

Answer 27

• the resistance of an electrical resistor or thermistor
• the e.m.f. (voltage) produced by a thermocouple
• the colour of an electrically heated wire
• the volume of a fixed mass of gas at constant pressure

Question 28

Liquid in glass thermometer

Answer 28

The length of a column of liquid in a tube, which gets longer as the temperature increases because the liquid expands. It depends on a change in density of a liquid.

Question 29

Thermocouple thermometer

Answer 29

A device consisting of wires of two different metals across which an e.m.f. is produced when the two junctions of the wires are at different temperatures.

Question 30

Thermistor thermometer

Answer 30

For a thermistor, the resistance changes rapidly over a relatively narrow range of temperatures. A small change in temperature results in a large change in resistance, so a thermometer based on a thermistor will be sensitive over that range of temperatures.
(p.s. Compared to that resistance for metals increases with temperature at a fairly steady rate)

Question 31

Define Linearity, Sensitivity, Calibration & Range

Answer 31

Linearity - the extent to which equal rises in temperature give equal changes in the thermometer’s output.
Sensitivity - how big a change in output is produced by a given change in temperature.
Calibration - adding a scale to a thermometer.
Range - all the temperatures, from lowest to highest, which a thermometer can measure.

Question 32

Define Specific heat capacity & Equation

Answer 32

E = mcΔθ — UNIT : J kg−1 K−1 or J kg−1 °C−1
The energy required per unit mass of the substance to raise the temperature by 1 K (or 1 °C).

Question 33

How is specific heat capacity is related to the gradient of the sloping sections of the graph (state change graph from melting to boiling) ?

Answer 33

The steeper the gradient, the faster the substance heats up and hence the lower its specific heat capacity must be.

Question 34

Define Specific latent heat & Equation

Answer 34

E = mL (Same for fusion & vaporisation)
Specific latent heat of a substance is the energy required per kilogram of the substance to change its state without any change in temperature.
When a substance melts, this quantity is called the specific latent heat of fusion; for boiling, it is the specific latent heat of vaporisation.

Question 35

Define Specific heat of fusion

Answer 35

The energy required per unit mass of a substance to change it from solid to liquid without a change in temperature. Unit: J kg−1

Question 36

Define Specific heat of vaporisation

Answer 36

The energy required per unit mass of a substance to change it from liquid to gas without a change in temperature. Unit: J kg−1.

Question 37

Comparing Resistance (thermistors & conducting wires) and Thermocouple thermometers

Answer 37

RESISTANCE ////////////// THERMOCOUPLE
1. robustness : very robust /// robust
2. range : thermistor - narrow range, resistance wire - wide /// can be very wide
3. size : larger than thermocouple; has greater thermal capacity therefore slower acting /// smaller than resistance thermometers; has smaller thermal capacity so quicker acting and can measure temperature at a point.
4. sensitivity: thermistor - high sensitivity over narrow range, resistance wire - less sensitive /// can be sensitive if appropriate metals chosen
5. linearity : thermistor - fairly linear over narrow range, resistance wire - good linearity /// non-linear so requires calibration
6. remote operation : long conducting wires allow the operator to be at a distance from the thermometer /// long conducting wires allow the operator to be at a distance from the thermometer