Thermal Physics (unit 3)

Question 1

What is internal energy?

Answer 1

the sum of the potential and kinetic energies of the particles in that system.
U = KE + PE

Question 2

What is the internal energy of an ideal gas?

Answer 2

total TRANSITIONAL kinetic energy of the gas particles.
3/2 nRT or 3/2 NKt

Question 3

Why for an ideal gas its internal energy is just it’s kinetic energy?

Answer 3

the attractive forces between the particles are negligible. This means that the amount of potential energy that they have is negligible.

Question 4

When is the internal energy of a system at it’s minimum?

Answer 4

absolute zero

Question 5

How does heat enter or leave a system?

Answer 5

heat enters or leaves a system through its boundary or container
wall

Question 6

How does heat react to in the context of it’s temperature and it’s surroundings?

Answer 6

Heat enters a system if the temperature of the system is lower than the temperature of its surroundings.

Heat leaves a system if the temperature of the system is higher than the temperature of its surroundings.

Question 7

What do we conclude as heat enters and leaves a system?

Answer 7

heat is energy in transit and not contained within the system

Question 8

What happens if temp is the same at both sides of the BOUNDARY?

Answer 8

said to be in thermal equilibrium and there is no heat flow between them

Question 9

What happens when to systems are placed into contact?

Answer 9

heat flows from one to the other until thermal equilibrium is reached and both systems are at the same temperature.

Question 10

What is energy also?

Answer 10

-Energy can also enter or leave a system by means of work.
-work is also energy in transit.

Question 11

How to calculate work done by a gas or on a gas?

Answer 11

W= p∆V (constant pressure)

Question 12

How can you also work out work done?

Answer 12

Area under P V graph

Question 13

Q context in Q=∆U+W ?

Answer 13

Q is positive (+ve) when heat enters a gas and negative (-ve) when heat leaves a gas.

Question 14

W context in Q=∆U+W ?

Answer 14

W is positive (+ve) when work is done by the gas (i.e. it expands) and negative (-ve) when work is done on the gas (i.e. it is compressed).

Question 15

ΔU Context in Q=∆U+W ?

Answer 15

ΔU is positive (+ve) when the internal energy of the gas increases and negative (-ve) when the internal energy of the gas decreases.

Question 16

If volume is constant in graph?

Answer 16

Work done is zero

Question 17

If temp is constant in graph?

Answer 17

ΔU =0

Question 18

How to work out NET WORK DONE?

Answer 18

-area inside loop

Question 19

If volume increases?

Answer 19

Work done by gas

Question 20

Volume decreases?

Answer 20

Work done on gas

Question 21

Amount of external work on a solid or liquid?

Answer 21

for a solid (or liquid), W is usually negligible, so Q = ΔU

Question 22

Equation for heat capacity?

Answer 22

Energy = mass x heat capacity x rise in temp

Question 23

How to use heat capacity?

Answer 23

If a substance has a specific heat capacity of 2000 J kg -1°C-1, then:

2000 J will raise the temperature of 1 kg by 1°C
2 x 2000 J will raise the temperature of 2 kg by 1°C
3 x 2 x 2000 J will raise the temperature of 2 kg by 3°C

Question 24

How to calculate heat flow?

Answer 24

Work done
-Negative when FLOW OUT OF GAS
-Positive when FLOWN INTO SYSTEM

Question 25

Estimation of absolute zero by gas laws? (Charles Law)

Answer 25

-Heat the water using a Bunsen burner and stir regularly. Measure the length of the trapped air every 10 oC up to 80 oC
-Plot a graph of the length of trapped air (y-axis) against temperature (x-axis). The temperature scale should cover the range -400 oC to 100 oC
-The length scale should start at zero
- Draw a line of best fit extended back until it cuts the x-axis, this is absolute zero.

Question 26

What does estimation of absolute zero using gas laws practical look like?

Answer 26

• sulfuric acid with length of trapped air with thermometer in a container of water

Question 27

Measurement of the specific heat capacity for a solid practical?

Answer 27

-Use a cylindrical block of the metal to be tested (such as copper or aluminium)
- The block should be well lagged using an insulator such as polystyrene and it needs two pre-drilled holes, one for a heater and one for a thermometer. -Measure the mass, m, of the block and
record its initial temperature, θ1.
-Switch the heater on and start the stopwatch. Record the voltmeter and ammeter readings. When the temperature has risen by 30 oC switch the heater
off and record the time taken, t.
-The formula can then be used to determine a value for c.

Question 28

What does measurement of specific heat capacity for a solid look like?

Answer 28

• circuit with power supply with ammeter in series to voltmeter and heater parallel to voltmeter
• heater is in a metal block with square insulation alongside a thermometer

Question 29

What equations for measurement of specific heat capacity?

Answer 29

-Assuming no energy losses:
-Electrical energy supplied by the heater = heat received by the block
-ItV = mc(θ2 – θ1)
- c = specific heat capacity

Question 30

Specific heat capacity units?

Answer 30

• J kg^-1 C^-1

Question 31

What is the energy at absolute zero?

Answer 31

• minimum internal energy