Thermal Physics Problems
*Equal masses of two different solid substances A and B are at the same temperature. The specific heat capacity of substance A is GREATER THAN the specific heat capacity of substance B. The two substances now have their temperatures raised by the same amount. Explain which substance will have the greater increase in internal energy assuming both remain in the solid phase.
(Internal energy of the substance is the sum of all potential and kinetic energy within the substance. The greater the specific heat capacity, the more energy is required for the unit of mass to raise the temperature of a substance by 1 degree.) More energy would be required in order to raise the temperature by the same amount for substance A than for substance B because it has a higher specific heat capacity.
A heater of constant power heats a liquid of mass m and specific heat capacity c. The graph below shows how the temperature of the liquid varies with time (NOT HEAT). The GRADIENT of the graph is k and no energy is lost to the surroundings. What is the power of the heater?
(P= energy/time = mcΔT/t, slope = k= ΔT/t, P=mck) kmc
Estimate a value for the average separation of the gas atoms.
(Since it is a cube, to get the distance of separation you must cube root the volume of 1 atom.) The cube root of 2.0 x 10^-28 is 5.8x10^-10m
A metal block of mass M is heated. The graph shows the variation with thermal energy H and supplied to the block of its temperature rise θ. The gradient of the straight line graph is n. The specific heat capacity of the metal is
(c -> Mc -> Mn =) 1/Mn
The specific heat capacity of a metal block of mass m is determined by placing a heating coil in its centre, as shown in the diagram above. The block is heated for time t and the maximum temperature change recorded is Δθ (=ΔT). The ammeter and voltmeter readings during the heating are I and V respectively. The SPECIFIC HEAT CAPACITY is best calculated using which one of the following expressions?
(current x voltage = electric power, P = IV, P= mcΔθ/t, IV =mcΔθ/t, tIV/mΔθ=mcΔθ/mΔθ, c = IVt/mΔθ) c=VIt/mΔθ
State what is meant by and ideal gas.
A gas with no forces between molecules that obeys the ideal gas equation PV=nRT for all pressures, volumes, and temperatures.
A substance changes from solid to liquid at its normal melting temperature. What change, if any, occurs in the average kinetic energy and the average potential energy of its molecules?
AKE: constant APE: increases (solid->liquid = molecules have to be separated)
*Explain, with reference to the energy of the molecules, the constant temperature region of the graph.
All input is used to separate the molecules so the potential energy is increasing and the kinetic energy remains constant. Since temperature is a measure of the average kinetic energy of the molecules, the temperature is also constant.
Outline what is meant by thermal equilibrium in this context.
At a constant temperature, for copper the amount of energy ABSORPTION is EQUAL to the amount of energy EMISSION
Two objects X and Y are made of the same material. Object X is more massive than object Y. Both objeccts are the same temperature. Which of the following correctly compares the average kinetic energy and also the total energy of the molecules in the objects?
Average KE of the molecules in X and Y: same Total energy of the molecules in X and Y: greater in X than in Y (X has more mass)
Which of the following is an assumption made in the kinetic model of ideal gases?
Collisions between molecules are elastic
The temperature rise calculated in (b) would be dangerous for the runner. Outline three mechanisms, other than evaporation, by which the container in the model would transfer energy to its surroundings.
Conduction- transfer of thermal energy via intermediate collisions (contact). Usually occurs with metals. Convection- transfer of thermal energy via bulk movement of fluid due to density(?). Usually occurs with gases and liquids. Radiation- transfer of thermal energy via electromagnetic waves. No medium is required.
Suggest one reason why your answer to (c)(i) (shower problem) is only an estimate.
Energy is lost to the environment OR flow rate is not a constant rate
State two reasons, other than evaporation, why the answer to (b)(ii) is likely to be different from the actual mass of condensed steam.
Energy lost to surroundings Energy required to heat the cup
Distinguish between evaporation and boiling
Evaporation occurs at any temperature and only on the surface. Boiling occurs at only a single temperature but throughout the liquid. Evaporation occurs when vapor pressure is less than atmospheric pressure while boiling occurs when it is equal.
*Explain whether the temperature of the flame is likely to be greater or less than your answer to (b)(i)
Greater- Energy lost to the environment during transfer from the flame into the water
State two factors that cause an INCREASE in the rate of evaporation of a liquid.
Increased draft larger surface area higher temperature lower vapour pressure humidity
*Distinguish between internal energy and thermal energy
Internal energy is the total kinetic and potential energy of the molecules in the substance. Thermal energy is the amount of energy transferred between two bodies as a result of a temperature difference.
Suggest why, in terms of the molecular model, the energy associated with melting is less than that associated with boiling.
Melting requires energy to break bonds, but not as much energy is needed compared to boiling because with boiling the bonds not only must be broken, but the molecules have to be separated further.
Describe, in terms of molecular behaviour, why evaporation causes cooling.
More energetic molecules leave the surface so there is more space to spread out and so less collisions occur between molecules making there be less KE since temperature is a measure of the AVERAGE kinetic energy of the molecules the temperature decreases as well.
Explain why, when a liquid evaporates, the liquid cools unless thermal energy is supplied to it.
More energetic molecules leave the surface so there is more space to spread out and so less collisions occur between molecules, making there be less kinetic energy. Since temperature is a measure of the average kinetic energy of the molecules, the temperature decreases as well.
State the relation between the pressure P and the volume V for a change at CONSTANT TEMPERATURE
Pressure is inversely proportional to volume.
The theory of the experiment would suggest that the graph line should pass through the origin. Explain briefly why the graph does not pass through the origin.
Systematic error due to energy lost to the environment.
Explain what thermal energy and temperature mean. Distinguish between the two concepts.
Temperature is a measure of the AVERAGE kinetic energy of the molecules within the substance and it determines the direction of flow of thermal energy. It is measured in Kelvin. Thermal energy is the kinetic energy of the particles of an object and is measured in joules.
A liquid is contained in a dish open to the atmosphere. Which one of the following contains three factors that affect rate of evaporation of the liquid?
Temperature of the liquid, surface area, specific latent heat of vaporization
The kelvin temperature of an object is a measure of
The AVERAGE kinetic energy of the molecules of the object
*Define Specific Heat Capacity
The amount of energy PER UNIT MASS required to CHANGE the temperature of a substance by 1 DEGREE
A sample of an ideal gas is contained in a cylinder. The volume of the gas is suddenly decreased. A student makes the following statements to explain the change in pressure of the gas.
The average kinetic energy of the gas atoms increases. The atoms of the gas hit the walls of the cylinder more frequently.
*Define thermal capacity
The energy required to CHANGE the temperature of a substance by 1 DEGREE
Describe, with reference to the energy of the molecules, the difference in internal energy of a piece of iron and and the internal energy of an ideal gas.
The internal energy of an ideal gas is the total kinetic energy of the molecules since an ideal gas has only kinetic energy whereas for the iron it is the total kinetic energy plus the potential energy of the molecules.
Which of the following is NOT an assumption on which the kinetic model of an ideal gas is based?
The mean-square speed of the molecules is proportional to the kelvin temperature.
The temperature of an ideal gas is REDUCED. Which one of the following statements is true?
The molecules collide with the walls of the container less frequently.
Which of the following is the internal energy of a system?
The sum of the potential and the kinetic energies of the particles of the system
Suggest how the students would know when the water was boiling steadily
The temperature is constant/bubbling constantly
By reference to your answer in (a), suggest whether hailstones are likely to melt on hitting the Earth's surface.
This is extremely fast and would not have a chance to melt. In fact, this is moving over 2x the speed of sound. I would die.
Explain why a reading of the mass lost in the first 200 seconds and then a reading of the mass lost in the next 200 second interval were taken, rather than one single reading of the mass lost in 400 seconds.
This shows that if the masses are equal that it is boiling at a constant rate.
State the relation between the volume V and kelvin temperature T for a change at a CONSTANT PRESSURE.
Volume is directly proportional to the temperature.
A fixed quantity of an ideal gas is compressed at constant temperature. The best explanation for the increase in pressure is that the molecules
are colliding more frequently with the container walls
State two factors that affect the rate of evaporation of sweat from the skin of the runner.
area of exposed skin- increased surface area gives more evaporation presence or absence of wind- surrounded causes more evaporation
The specific latent heat of vaporization of a substance is the quantity of energy required to
convert a unit mass of liquid to vapour at a temperature of 100°C and a pressure of one atmosphere
A gas is contained in a cylinder fitted with a piston and shown below. When the gas is compressed rapidly by the piston its temperature rises because the molecules of the gas
gain energy from the moving piston
The energy of the molecules of an ideal gas is
kinetic only
The mole is defined as
the amount of a substance that contains as many elementary entities as the number of atoms in 12g of the isotope carbon-12
Two different objects are in thermal contact with one another. The objects are at different temperatures. The temperatures of the two objects determine
the direction of transfer of thermal energy between the objects
