2. Thermal physics
A section of Physics, 5054
Listing 10 of 446 questions
10 The average input power to a freezer is 80 W. The cost of 1 kW h is 25 cents. Explain what is meant by the kilowatt-hour (kW h). Calculate the cost of running the freezer for one week. cost = A large jug containing 1.5 kg of water is placed in the freezer. The water cools from 25 °C to 0 °C in a time of 60 minutes. The specific heat capacity of water is 4.2 J / (g °C). Calculate the thermal energy removed from the water as it cools from 25 °C to 0 °C. energy removed = After the water has reached 0 °C, thermal energy is removed from the water at the same rate as in . The specific latent heat of fusion of water is 3.3 × 105 J / kg. Calculate the mass of water at 0 °C that becomes ice in 60 minutes. mass = Describe the arrangement and the movement of the molecules 1. in liquid water, 2. in ice. Ice at 0 °C becomes water at 0 °C. State what, if anything, happens to the kinetic energy and the potential energy of the molecules as this happens. kinetic energy: potential energy:
5054_s13_qp_21
THEORY
2013
Paper 2, Variant 1
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A student makes a diagram of the molecules in a liquid, as shown in . motion molecule Suggest two ways in which the student’s diagram does not show the positions and motion of the molecules accurately. 1. 2. Some of the molecules of a liquid evaporate. Describe what happens during evaporation. Explain why evaporation has a cooling effect on the liquid. The beaker shown in contains pure water at a temperature of 80 °C. A thermometer, initially at room temperature, is placed in the air just above the water. The reading on the thermometer starts to increase slowly. water thermometer Explain how convection in the air causes the reading on the thermometer to increase. A heater is placed underneath the beaker and the water boils vigorously to produce steam. The thermometer reading increases quickly. Explain how the water in the gas state gives thermal energy to the thermometer. The heater placed underneath the beaker has a power of 200 W. The mass of water in the beaker is initially 100 g. The water in the beaker is heated for a time of 120 s. During this time, the temperature of the water in the beaker rises from 80 °C to 100 °C and then the water boils. At the end of this time, the mass of water in the beaker is 95 g. The specific heat capacity of water is 4.2 J / (g °C). The specific latent heat of vaporisation of water is 2250 J/ g. Calculate the thermal energy 1. produced by the heater in 120 s, thermal energy = 2. used to warm the water from 80°C to 100°C, thermal energy = 3. used to reduce the mass of water in the beaker, thermal energy = 4. produced by the heater that is not transferred to the water. thermal energy =
5054_s15_qp_21
THEORY
2015
Paper 2, Variant 1
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State what is meant by melting point. The temperature of a liquid is between its melting point and its boiling point. Using ideas about molecules, describe the changes that occur when the liquid evaporates. The boiling point of nitrogen is –196 °C. Liquid nitrogen, below its boiling point, is stored in a vacuum flask, as shown in . glass walls vacuum liquid nitrogen The flask has two glass walls with a vacuum between them. Suggest where, in the apparatus shown in , evaporation occurs. State and explain the advantage of having a vacuum between the two glass walls. The liquid nitrogen reaches –196 °C, its boiling point. A small piece of metal at 20 °C is lowered slowly into the liquid nitrogen. Bubbles form within the liquid nitrogen as it boils. State what is inside the bubbles. The small piece of metal has a mass of 50 g. When it is lowered into the liquid nitrogen, the metal cools to –196 °C. The specific heat capacity of the metal is 0.39 J / (g °C). The specific latent heat of vaporisation of nitrogen is 200 J / g. Calculate 1. the thermal energy lost from the metal as it cools, thermal energy = 2. the mass of nitrogen that boils away. mass =
5054_s16_qp_21
THEORY
2016
Paper 2, Variant 1
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is a diagram of a soldering iron. Solder is a mixture of metals used to make a permanent contact between electrical wires. heating element metal tip The heating element raises the temperature of the metal tip. When solder is placed against the tip, the solder melts over the wires to be joined. When the solder cools, it solidifies and the permanent connection is made. The working temperature of the metal tip is 380 °C. The boxes in show two materials and some different melting points. melting point 1000 °C solder metal of the metal tip 380 °C 200 °C 20 °C 0 °C material On , draw a line from the metal of the metal tip and a line from the solder to a suitable melting point for each. The heating element is rated at 24 V, 3.3 A. The heating element is switched on. The temperature of the metal tip rises from 20 °C to 320 °C in the first 10 s. Calculate the electrical energy supplied to the heating element in the first 10 s. energy = The metal tip is made of copper and has a mass of 2.3 g. The specific heat capacity of copper is 0.39 J / (g °C). Calculate the thermal energy gained by the metal tip in the first 10 s. thermal energy = Describe, in terms of free electrons, the process by which heat transfers through the metal tip. Heat is lost from the metal tip by convection in the air. Explain how convection occurs in the air. The temperature of the metal tip is measured with a thermocouple. In the space below, draw a labelled diagram of a thermocouple. Mark with a letter H the part of the thermocouple that is placed on the metal tip. State two reasons why a thermocouple thermometer is suitable for measuring how the temperature of the metal tip varies during the first 10 s. 1. 2.
5054_s19_qp_22
THEORY
2019
Paper 2, Variant 2
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Questions Discovered
446