3.6.2 Thermal Physics

Question 1

What is internal energy?

Answer 1

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

Question 2

What affects the internal energy of a system?

Answer 2

Temperature - higher temp, higher kinetic energy.
Random motion of the particles.
Phase of matter - gasses have highest internal energy.
Intermolecular forces between particles - greater intermolecular forces, higher potential energy. Also linked to state of matter.

Question 3

How can internal energy be increased?

Answer 3

Doing work
Adding heat

Question 4

How can internal energy be decreased?

Answer 4

Losing heat to its surroundings
Changing state (solid to liquid or liquid to gas)

Question 5

What is the first law of thermodynamics?

Answer 5

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

Question 6

Explain the first law of thermodynamics in terms of gasses

Answer 6

When a gas expands, work is done by the gas on the surroundings - this decreases the internal energy of the gas. e.g. moving a piston - molecules are further apart, so have lower kinetic energy as they move slower, so lower internal energy.
When a gas is compressed, work is done on the gas by the surroundings. This increases internal energy

Question 7

What is meant by specific heat capacity?

Answer 7

The amount of thermal energy required to raise the temperature of 1kg substance by 1degree without changing its state.

Question 8

What is the equation for specific heat capacity?

Answer 8

Q=mc∆θ

• the heavier the material, the more thermal energy required to raise its temperature
• the larger the change in temperature, the higher the thermal energy required to achieve this change.

Question 9

Describe the continuous flow method

Answer 9

A fluid flows continuously over a heating element where energy is transferred to the fluid. Flow rate and potential difference is changed, keeping the change in temperature of fluid constant. As it flows through an electrical heating wire, the rise in temperature is measured using the electric thermometers. Flow rate is altered to give mass m_2 and potential difference is changed so temp difference stays the same. c is found by assuming heat loss to surroundings is same for both flow rates.

Question 10

How is flow rate calculated?

Answer 10

First flow rate is calculated by
I_1V_1t_1 = m_1c∆θ + E_lost
Second flow rate is
I_2V_2t_2 = m_2c∆θ + E_lost

E_lost is assumed to be the same, hence, equating the equations, to obtain
c= (Q_2 - Q_1)/(m_2-m_1)∆θ

Question 11

What is meant by specific latent heat?

Answer 11

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

Question 12

What is the formula for specific latent heat?

Answer 12

Q=mL

Question 13

What is absolute zero?

Answer 13

The temperature at which molecules in a substance have zero kinetic energy ( -273 C)

Question 14

What is Boyle’s law?

Answer 14

Pressure is inversely proportional to the volume of a gas

Question 15

What is Charles’ law?

Answer 15

Volume of a gas is proportional to the temperature of a gas
V1/T1 = V2/T2

Question 16

What is the pressure law?

Answer 16

Pressure of a gas is proportional to the temperature of a gas
P1/T1=P2/T2

Question 17

What is an ideal gas?

Answer 17

One which obeys Boyle’s law

Question 18

What is meant by the pressure of an ideal gas?

Answer 18

The frequency of collisions of the gas molecules per unit area of a container

Question 19

What are the idea gas equations?

Answer 19

pV = nRT
pV=NkT

k = Boltzmann constant (1.38*10^-23)

Question 20

What is the derivation of the formula for work done by a gas?

Answer 20

F=pA
F=W/s

pA = W/s
W = psA
W=pV

Question 21

How is the Boltzmann constant defined?

Answer 21

k =R/N_A

Question 22

What is the difference between gas laws and kinetic theory?

Answer 22

Gas laws are empirical in nature (they are based on observation and evidence). All based on how gas responds to changes in environment.
Kinetic theory is based on assumptions and derivations from existing theories (used to explain why gas laws behave the way they do)

Question 23

What type of energy do ideal gasses have?

Answer 23

Assumed to have no intermolecular forces, meaning they have no potential energy, only kinetic energy

Question 24

What is the formula for change in internal energy for ideal gas?

Answer 24

∆U = 3/2k∆T

Question 25

What are the assumptions made by the kinetic theory?

Answer 25

Molecules of a gas do not interact with eachother Molecules of a gas are shard, perfectly elastic spheres The time of a collision is negligible compared to the time between collisions Molecules move in continuous random motion The volume of the molecules is negligible compared to the volume of the container There are a very large number of molecules

Question 26

What is the formula linking pressure and mean square speed of molecules?

Answer 26

pV=1/3Nm(c_rms)^2

Question 27

What is the formula for average molecular kinetic energy?

Answer 27

pV = NkT pV = 1/3Nm(c_rms)^2 equate E_k = 3/2kT can replace Boltzmann constant E_k = 3RT/2N_A

Question 28

What is meant by Brownian motion?

Answer 28

Small particles suspended in a liquid or gas are observed to move around in random, erratic motion

Question 29

Describe Brownian motion

Answer 29

Can be observed under a microscope and provides evidence for existence of molecules in gas or liquids. Particles in random motion have a range of speeds and no preferred direction of movement. Larger particles change their speed and directions randomly, provides evidence concerning the behaviour of molecules in a gas. Small molecules are able to affect the larger particles because they are travelling at a higher speed than them and have a lot of momentum, which they transfer to the large particles.

Question 30

Evolving gas models

Answer 30

Democritus; concept of the atom Boyle; Discovered the relationship between pressure and volume. Amontons; relationship between temperature and pressure of gas at constant volume. Charles; relationship between volume of a gas and its temperature at constant pressure. Bernoulli; assumed gasses were made up of tiny particles, sparked the beginning of kinetic theory and Bernoulli's principle. Brown; discovered Brownian motion, helped support kinetic theory. Einstein; produced a paper on how kinetic theory was used to make predictions for Brownian motion.