Thermal physics Banker

1. (a) Which statement describes the particles in a liquid?A They are far apart and move freely at high speed.B They are close together and vibrate about fixed positions only.C They are close together and move around each other at random.D They are far apart and vibrate about fixed positions. [1]

A student in a school laboratory in Peshawar fills a small glass cell with smoke and lights it from the side. She looks at the cell through a microscope.

1. (a) Describe the motion of the bright specks of smoke seen through the microscope. [2]
2. (b) Explain what causes this motion. [3]
3. (c) State what this observation suggests about the speed of the air molecules compared with the speed of the smoke particles. [1]

A delivery rider in Lahore checks the air in his motorcycle tyre before a long ride.

1. (a) Explain, in terms of the air molecules, how the air inside the tyre exerts a pressure on the tyre wall. [3]
2. (b) The rider pumps more air into the tyre. Its volume and temperature do not change. Explain why the pressure increases. [2]

1. (a) State two differences between evaporation and boiling. [2]
2. (b) On a hot day a cricketer sweats. Explain, in terms of molecules, why the evaporation of sweat cools his skin. [3]

A sealed plastic syringe contains 60 cm³ of air at a pressure of 100 kPa. The plunger is pushed in slowly until the volume of the air is 24 cm³.

1. (a) Calculate the new pressure of the air. [3]
2. (b) State one assumption you made in (a). [1]
3. (c) Explain, in terms of molecules, why the pressure increases when the volume is reduced. [2]

A sealed metal flask of fixed volume contains air. The flask is placed in a water bath and heated slowly.

1. (a) Explain, in terms of the gas molecules, why the pressure in the flask increases as it is heated. [3]
2. (b) The room temperature is 20 °C. Convert this temperature to kelvin. [1]
3. (c) State what is meant by absolute zero and give its value in °C. [2]

1. (a) A fixed mass of gas has a volume of 3.0 m³ at a pressure of 120 kPa. The pressure is increased to 180 kPa at constant temperature. What is the new volume?A 1.5 m³B 2.0 m³C 4.5 m³D 7.2 m³ [1]

In many homes in Sindh, drinking water is stored in an unglazed clay pot called a matka. The clay has tiny pores, so the outside of the pot is always slightly damp.

1. (a) Explain why the water inside the matka becomes cooler than the surrounding air. [3]
2. (b) Suggest two ways the family could make the water cool faster. Give a reason for each. [4]
3. (c) The matka works less well during the humid monsoon season. Suggest why. [1]

A student investigates how the pressure p of a fixed mass of air depends on its volume V. The air is trapped in a sealed tube connected to a pressure gauge. She changes the volume, waits one minute, then reads the gauge. Results:V / cm³: 50.0, 40.0, 30.0, 25.0, 20.0p / kPa: 102, 128, 150, 204, 255

1. (a) Calculate p × V for each reading. [2]
2. (b) Identify the anomalous reading and suggest one cause. [2]
3. (c) State whether the results support the relationship p ∝ 1/V. Explain your answer. [2]
4. (d) Suggest why the student waited one minute after each change of volume before reading the pressure. [1]

A diver near Astola Island breathes out an air bubble at a depth where the total pressure is 250 kPa. The bubble has a volume of 1.2 cm³. At the surface the pressure is 100 kPa.

1. (a) Calculate the volume of the bubble at the surface. Assume its temperature does not change. [3]
2. (b) Explain, in terms of molecules, why the bubble expands as it rises. [3]
3. (c) Divers are trained never to hold their breath while rising quickly to the surface. Use your answer to (a) to suggest why. [1]

An aerosol can of air freshener carries the warning: 'Do not expose to temperatures above 50 °C.' On a summer afternoon in Jacobabad the inside of a parked car rises from 25 °C to 60 °C.

1. (a) Convert 25 °C and 60 °C to kelvin. [1]
2. (b) Explain, in terms of the molecules of gas inside the can, why the can could burst if it is left in a hot car. [3]
3. (c) A student says: 'The kelvin temperature has nearly doubled, so the pressure has nearly doubled.' Explain why the student is wrong. [2]
4. (d) Explain why there is a lowest possible temperature. [1]

1. (a) Water boils at 100 °C at normal atmospheric pressure. What is this temperature in kelvin?A −173 KB 100 KC 273 KD 373 K [1]

Overhead power cables between pylons near Multan are hung so that they sag slightly. In summer the afternoon temperature can reach 48 °C.

1. (a) Explain, in terms of particles, why the cables sag more on a hot afternoon. [2]
2. (b) Suggest why engineers do not hang the cables tight in winter. [1]
3. (c) Equal volumes of a solid, a liquid and a gas are heated through the same temperature rise. State which expands the most and which expands the least. [2]

1. (a) Define specific heat capacity. [2]
2. (b) Calculate the energy needed to raise the temperature of 2.0 kg of water by 35 °C. The specific heat capacity of water is 4200 J/(kg °C). [2]

A block of ice taken from a freezer at −18 °C is heated steadily until it has all melted and the water is at 30 °C.

1. (a) Describe what happens to the temperature of the substance while it is melting. [1]
2. (b) Explain, in terms of particles, why energy must still be supplied while the substance melts. [2]
3. (c) Define specific latent heat of fusion. [2]

An electric kettle has a power of 2200 W. It contains 1.5 kg of water at 25 °C. The specific heat capacity of water is 4200 J/(kg °C).

1. (a) Calculate the energy needed to heat the water to 100 °C. [2]
2. (b) Calculate the minimum time this takes. [2]
3. (c) The actual time is longer. Suggest two reasons. [2]

1. (a) Blocks of aluminium and copper have the same mass. Each receives the same quantity of thermal energy. The specific heat capacity of aluminium is 900 J/(kg °C) and that of copper is 390 J/(kg °C). Which statement is correct?A Both blocks have the same temperature rise.B The aluminium block has about 2.3 times the temperature rise of the copper block.C The copper block has about 2.3 times the temperature rise of the aluminium block.D The copper block has about 0.43 times the temperature rise of the aluminium block. [1]

Water in a kettle is already boiling. The kettle stands on a balance. The 2000 W heater is left on for 100 s and the reading on the balance falls by 0.090 kg.

1. (a) Calculate the energy supplied by the heater in 100 s. [1]
2. (b) Calculate a value for the specific latent heat of vaporisation of water. [2]
3. (c) Explain why the temperature of the water stays at 100 °C even though the heater is still switched on. [2]
4. (d) Suggest whether your answer to (b) is likely to be greater or smaller than the true value. Explain. [2]

A cook in a café in Islamabad mixes 0.3 kg of water at 80 °C with 0.2 kg of water at 20 °C in an insulated jug. She then adds 0.05 kg of ice at 0 °C.Specific heat capacity of water = 4200 J/(kg °C); specific latent heat of fusion of ice = 3.34 × 10⁵ J/kg.

1. (a) Show that the temperature of the mixture is 56 °C. Assume no energy is lost to the surroundings or the jug. [2]
2. (b) Calculate the final temperature after the ice has melted and the water has mixed. Give your answer to 3 significant figures. [4]
3. (c) Explain why a cube of ice at 0 °C cools a drink more than the same mass of water at 0 °C. [2]

A student finds the specific heat capacity of aluminium. A 1.00 kg aluminium block has two holes: one for a 12 V heater and one for a thermometer. The heater current is 4.0 A. The heater is switched on for 300 s.Temperature at start = 22.0 °C; highest temperature = 37.5 °C.

1. (a) Calculate the electrical energy supplied by the heater. [2]
2. (b) Calculate the specific heat capacity of aluminium given by these results. [2]
3. (c) The accepted value is 900 J/(kg °C). State whether the student's value agrees with it within 10 %. Show your working. [2]
4. (d) Suggest two improvements to the experiment, explaining how each improves the result. [2]

A boiling tube of a waxy substance X is melted in hot water, then removed and allowed to cool in air. A student records its temperature every minute:time / min: 0, 1, 2, 3, 4, 5, 6, 7, 8temperature / °C: 80.0, 74.0, 69.5, 69.0, 69.0, 69.0, 68.5, 62.0, 56.0

1. (a) State the freezing point of substance X shown by these results. [1]
2. (b) Describe how the results show that X is changing state between 3 min and 6 min. [1]
3. (c) Explain, in terms of particles, why the temperature stays constant while X freezes even though it is losing energy to the surroundings. [2]
4. (d) Calculate the average rate of cooling, in °C/min, of the liquid between 0 and 2 min and of the solid between 7 and 8 min. [2]
5. (e) Suggest why taking a reading every 30 s, instead of every minute, would give a more reliable freezing point. [1]

1. (a) Thermal energy reaches the Earth from the Sun through space. Which method of transfer is this?A conductionB convectionC evaporationD infrared radiation [1]

A karahi (cooking wok) is made of steel, but its two handles are made of wood.

1. (a) Explain why the metal body of the karahi is a good choice for cooking. [1]
2. (b) Explain, in terms of particles, how thermal energy is conducted through the metal. [3]
3. (c) Explain why the handles are made of wood. [1]

In a flat in Karachi, a split air conditioner is fixed high up on the wall near the ceiling.

1. (a) Explain why the air conditioner is mounted high on the wall. [3]
2. (b) In winter the family uses a gas heater. State where in the room the heater should be placed and explain why. [2]

1. (a) Four identical metal cans are filled with the same volume of water at 90 °C. Each can has a different outer surface. Which can cools fastest?A dull blackB shiny blackC dull whiteD shiny silver [1]

A student tests how the colour of a surface affects the rate of cooling. Two identical aluminium cans with lids are used: one painted dull black and one polished shiny silver. Each is filled with 250 cm³ of water at 80.0 °C and left on a wooden bench. After 15 minutes:dull black can: 61.5 °Cshiny silver can: 70.0 °C

1. (a) Calculate the temperature drop of each can. [1]
2. (b) State what the results show about the two surfaces. [1]
3. (c) State three variables that must be kept the same for this to be a fair test. [3]
4. (d) Suggest why both cans were fitted with lids made of the same material. [1]

A vacuum flask keeps tea hot during a family trip to Murree. It has a double-walled glass container with a vacuum between the walls. The glass surfaces facing the vacuum are silvered. The flask is closed with a hollow plastic stopper.

1. (a) Explain how the vacuum between the two glass walls reduces thermal energy transfer. [2]
2. (b) Explain how the silvered surfaces reduce thermal energy transfer. [2]
3. (c) Explain why the stopper is made of hollow plastic. [2]

1. (a) State the type of radiation that the Earth's surface mainly emits. [1]
2. (b) Explain why the average temperature of the Earth stays roughly constant over many years. [2]
3. (c) Greenhouse gases such as carbon dioxide absorb some of the infrared radiation emitted by the Earth. Explain why an increase in these gases causes the Earth's average temperature to rise. [3]

A solar water heater on a roof in Quetta has a dull black absorbing panel of area 2.0 m², with copper pipes fixed to its front and a layer of foam behind it. On a sunny day the Sun delivers 600 W to each square metre of panel. Only 40 % of this power reaches the 80 kg of water in the tank. Specific heat capacity of water = 4200 J/(kg °C).

1. (a) Explain why the absorbing panel is painted dull black. [1]
2. (b) Explain why the copper pipes are fixed to the panel and why the panel is backed with foam. [2]
3. (c) Calculate the time needed to heat the water by 20 °C. Give your answer in hours to 2 significant figures. [4]
4. (d) The tank is placed above the panel. Explain how the water circulates without a pump. [2]

On summer days at Clifton Beach, Karachi, a steady breeze blows in from the Arabian Sea during the afternoon.

1. (a) The specific heat capacity of sand is much lower than that of water. Explain why, on a sunny morning, the land becomes warmer than the sea. [2]
2. (b) Explain why a breeze blows from the sea towards the land during the afternoon. [3]
3. (c) Suggest and explain the direction of the breeze late at night. [2]
4. (d) Car radiators have many thin metal fins. Explain how the fins increase the rate of energy loss by radiation. [1]