3.6.2 - Thermal Physics

Question 1

What are the two ways internal energy of a system can change

Answer 1

By giving the gas heat energy, U will increase (or the gas losing heat to its surroundings)
If the gas does work W, U will increase

Question 2

What is the total energy of a gas also known as?

Answer 2

the internal energy U

Question 3

What law do we get from the principle of conservation of energy?

Answer 3

the first law of thermodynamics

Question 4

What happens when a substance is heated (without changing state)?

Answer 4

the KE of particles increases, and the internal energy increases also

Question 5

What sort of questions will the first law of thermodynamics (conservation of energy) be like?

Answer 5

Qualitative (no numbers involved)

Question 6

What is the shape of a temp-time graph when an object is changing state? Why?

Answer 6

it stays flat - the temperature does not change since the heat lost to the environment is creating/ breaking bonds.

Question 7

What is the number of moles equal to?

Answer 7

mass/ relative atomic (or molecular) mass

Question 8

what is 1 atmosphere in pascals approx. ?

Answer 8

1000 kPa

Question 9

what is 1m^3 equal to?

Answer 9

1000 litres
1,000,000 cm^3

Question 10

What is 0K in celcius?

Answer 10

-273.15 degrees

Question 11

How do you demonstrate the pressure law?

Answer 11

have a fixed vol. of gas in a (jolly) bulb and heat it up in a water bath.
measure the temperature and the pressure, plot a graph of pressure against temperature

Question 12

What will the pressure-temperature graph look like?

Answer 12

a straight line and extrapolates to get 0K of -273.15 degrees

Question 13

How do we demonstrate Charles’ Law?

Answer 13

gas in a syringe in a water bath, heat it up, gas expands so volume gets bigger pressure stays same.
plot graph of volume against temp.

Question 14

What does the volume-temperature graph look like?

Answer 14

straight line which an extrapolation goes to -273 degrees

Question 15

How do we demonstrate Boyles’ Law?

Answer 15

fixed mass of gas, compress it with a pump and measure pressure and volume. Leave time between readings to allow temp to stabilise

Question 16

What does the graph of pressure against volume look like?

Answer 16

curve suggesting inverse relationship - can do pressure against 1/volume to get a straight line

Question 17

What do we call a gas which obeys Boyle’s law at all times? How does this compare to real gases?

Answer 17

an ideal gas
they obey it generally quite well under most conditions

Question 18

What do we assume in the ideal gas/ Kelvin scale?

Answer 18

that pV = constant x T

Question 19

What does the k in pV=NkT stand for?

Answer 19

Boltzmann’s constant

Question 20

What is work done by an expanding gas equal to?

Answer 20

W = p x change in V (p = constant pressure)

Question 21

What is kinetic theory?

Answer 21

a mathematical model representing an ideal gas

Question 22

What is Brownian motion?

Answer 22

Robert Brown 1827 observed pollen grains in water and the displayed random motion; evidence that air is made up of a large number of tiny particles moving randomly and very quickly

Question 23

What causes gas pressure?

Answer 23

when the particles collide with the walls of the container, there is a change in momentum.
from Newton’s second law, a force is exerted on the wall.
the average force divided by the surface area=pressure

Question 24

Explain in terms of the motion of the particles in a car tyre will increase when it is warmer?

Answer 24

a higher temperature means that the particles have more KE and so are moving faster. the change in momentum per collision will be greater and there will be more collisions per second. the total change in momentum per second will be greater and so the pressure will increase.

Question 25

how do you find the root mean square of the particles’ velocities?

Answer 25

square all the velocities
take the average of these squares
square root this

Question 26

What is the average kinetic energy of particles?

Answer 26

1/2 x m x (Crms)^2