1.Thermal

Question 1

(EX)U13PH4 (f) The student repeats the experiment but uses a container that is not such a good insulator. Readings are obtained at the same time intervals as before. State what happens to the: (i) values of temperature; (ii) gradient of the graph; (iii) value obtained for the specific heat capacity

Answer 1

Temp↓ grad↓ c↑

Question 2

U12PH4 (i) Derive in clear steps a formula that shows how the internal energy of the ideal gas depends on the temperature of the gas. [4]

Answer 2

Internal energy of gas (U) = KE + PE and PE = 0 (for ideal gas) (1)

Question 3

U16PH4 (a) Explain what is meant by thermal equilibrium. [2]

Answer 3

If two (or more) systems are in thermal equilibrium then the systems are in thermal contact (1)

but there is no (net) heat flowing between them (1)

. Alternative No heat flow between the systems (1) because they are at the same temperature (1)

Question 4

U16PH4 Vegetables of mass 1.1kg and temperature 17°C are placed in the water, and the system is left to reach thermal equilibrium once again. Describe in terms of heat flows how thermal equilibrium is reached between the saucepan, water and vegetables. (Calculations are not required.) [3]

Answer 4

[Saucepan and water initially at higher temperature than vegetables], so net heat will flow from the saucepan to the water (1)

and from the water to the vegetables (1)

Accept heat flows from the saucepan to the vegetables (1) and from the water to the vegetables (1)

Eventually heat transfer will stop and all three (saucepan, water and vegetables) will be in thermal equilibrium / be at the same temperature (1).

Question 5

U16PH4 (d)saucepan-water-vegetable system Explain what will happen to the final temperature if the system is not completely isolated from the surroundings. [2]

Answer 5

Lower final temperature (or eventually temperature of surroundings) (1)

Heat will be lost to the surroundings from the system [as the temperature of the surroundings is lower] (or temperature gradient between saucepan-water-vegetable system and surroundings is driving transfer of heat) (1)

Question 6

12Explain why your answer to (d) is also the heat flowing into the gas during the cycle ABCA. [2]

Answer 6

ΔU = Q − W quoted or by clear implication or 1st law quoted (1); and ΔU = 0 (1)

Question 7

The first law of thermodynamics is given by: ∆U = Q – W State what is represented by:

Answer 7

∆𝑈 increase (or change) in internal energy of a system (1)

Q heat flowing into the system (1)

W work done by the system (1)

Question 8

(a) Heat is supplied to an ideal gas in a sealed container of fixed volume. Explain carefully the physical processes that cause an increase in the temperature and pressure of the gas. [6 QER]

Answer 8

Increase in temperature - kinetic theory effects Molecules move randomly

Collisions become more frequent [when heat supplied]

[No change in volume so heat flowing in] causes increase in U / kinetic energy No work done [as constant volume]

T increases with U as temperature proportional to U, or equivalent

Newton’s laws of motion

Momentum of molecules increase

Force on molecules = rate of change of momentum (during collision with wall)

Force on wall is equal and opposite to force on molecules

Greater forces during the collisions

Increase in pressure

Molecules collide with walls exerting force on walls and / or pressure

Pressure increases with temperature

Pressure = force on walls per unit area Mean pressure due to many collisions [and many molecules]