THERMODYNAMICS LONG ANSWER RECALL

Describe and explain the changes to the internal energy of ice when it is heated from -20⁰C to 120⁰C. (Internal energy and heating)

1. From -20 to 0: temp increases, avg KE of molecules increases

2. At 0: temp is constant during melting; energy supplied used to overcome intermolecular forces, PE increases

3. From 0 to 100: temp increases, avg KE of molecules increases

4. At 100: temp constant during boiling; energy supplied used to overcome remaining intermolecular forces, PE increases

5. From 100 to 200: temp increases, avg KE of molecules increases

6. Overall: both KE and PE contributions to internal energy increase but at different stages.

Explain, in terms of the kinetic theory, why the pressure of a gas increases when it is heated at constant volume. (Pressure & Kinetic Theory)

1. Heating increases the avg KE of molecules

2. Molecules have a higher avg speed

3. Molecules collide with the wall more frequently (2s=vt)

4. Each collision involves a greater change of momentum

5. Rate of change of momentum on the walls increases

6. Therefore, force on walls increases (F = m(v-u)/ t) and since area is fixed, pressure increases (p = F/A)

Describe an experiment to investigate the relationship between pressure and volume of a fixed mass of gas at constant temperature (Boyle’s Law Experiment CP14)

1. A fixed mass of air is trapped in a sealed syringe or pressure apparatus

2. Pressure is measured using a calibrated pressure sensor or gauge

3. Volume is measured from the scale on the apparatus used

4. The experiment is carried out slowly to allow temperatures to remain constant

5. Pressure is recorded for different volumes of gas

6. A graph of pressure against I/volume is plotted and the straight line confirms Boyle’s Law (but state the law!!)

1. Explain how Wien’s law and the Stefan Boltzmann law together account for the brightness and colour of stars of different temperatures (Black body radiation)

Wien’s Law: λ_maxT = constant. Define terms

Hotter stars → smaller λ_max-→ peak emission shifts to shorter wavelengths (blue)

Stefan Boltzmann law: L = σAT⁴ . Define terms

Hotter stars emit much greater power per unit area

Therefore hotter stars appear both brighter and bluer

Cooler stars appear dimmer and redder, with peak shifted to longer λ

Describe an experiment to determine the specific latent heat of vaporisation of water (Latent heat practical CP13)

1. Measure the mass of a beaker of water

2. Place an immersion heater in the water and boil for a known time, keeping the beaker lagged to reduce energy loss

3. Measure current and voltage across the heater; power = IV . Name equipment. How do you connect?

4. Energy supplied = ItV

5. After boiling, find the new mass of the beaker to find mass of water lost

6. Calculate L = E/m