Thermal properties of materials — OCR A-Level Physics
Test yourself on Thermal properties of materials with OCR A-Level practice questions.
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Thermal properties of materials explained
Specific heat capacity c is the energy needed to raise the temperature of 1 kg of a substance by 1 K, measured in J kg⁻¹ K⁻¹.
Read the full explanation
The energy transferred for a temperature change is E = mcΔθ, where m is mass in kilograms and Δθ is the temperature change in kelvin or degrees Celsius. For example, heating 0.50 kg of aluminium (c = 900 J kg⁻¹ K⁻¹) by 20 K needs E = 0.50 × 900 × 20 = 9.0 × 10³ J. A question may ask for the definition, the unit, or a value of E, m, c or Δθ.
(b)
This row is a sub-heading, not an assessed statement. It introduces the practical work in section 5.1.3: an electrical experiment to determine the specific heat capacity of a metal or a liquid, and the techniques and procedures for the electrical method applied to a metal block and to a liquid. Read it as a signpost telling you that the following rows are practical skills you must be able to describe, justify and evaluate, not facts to memorise. As you read, note that the electrical method supplies energy electrically to the sample, so you should be able to link power, time, temperature change, mass and specific heat capacity, and to identify the main sources of error in each version.
(i) an electrical experiment to determine the specific heat capacity of a metal or a liquid
You must be able to describe an electrical experiment that determines the specific heat capacity of either a metal or a liquid. In the metal version, an insulated metal block contains a heater and a thermometer; you measure the mass m of the block, supply electrical energy using a heater of power P for time t, and record the temperature rise Δθ. The energy supplied is E = P t, and if all of it raises the block's temperature, then P t = m c Δθ, so c = P t / (m Δθ). For a liquid, the same electrical method is used but the liquid is held in an insulated container, so the container also gains energy; you either account for the container's heat capacity or minimise its effect. You should describe how each quantity is measured, how heat loss is reduced, and how the result is evaluated.
(ii) techniques and procedures used for an electrical method to determine the specific heat capacity of a metal block and a liquid
This row asks for the techniques and procedures of the electrical method for both a metal block and a liquid. For a metal block, drill holes for the heater and thermometer, measure the mass, insulate the block, record the initial temperature, switch on the heater and start timing, record the current and potential difference to find power, and record the maximum steady temperature. For a liquid, place a known mass of liquid in an insulated container, use a heater and thermometer or temperature sensor, stir to keep the temperature uniform, and account for the container's heat capacity. In both cases, calculate energy supplied as P t and use P t = m c Δθ. The techniques include reducing heat loss, avoiding parallax when reading thermometers, and repeating measurements to improve reliability.
(c) specific latent heat of fusion and specific latent heat of vaporisation; E = m l
Specific latent heat of fusion is the energy needed to change unit mass of a substance from solid to liquid at its melting point without a change in temperature. Specific latent heat of vaporisation is the energy needed to change unit mass from liquid to vapour at its boiling point without a change in temperature. The energy transferred is E = m l, where m is the mass and l is the specific latent heat. For example, melting 0.50 kg of ice with a specific latent heat of fusion of 3.34 × 10⁵ J kg⁻¹ requires E = 0.50 × 3.34 × 10⁵ = 1.67 × 10⁵ J. During a change of state the temperature stays constant, so the energy supplied changes the arrangement of particles rather than their kinetic energy.
(d)
This row is a specification sub-heading, not an assessed statement. It introduces the practical work in 5.1.3: electrical experiments to determine specific latent heat of fusion and of vaporisation, and the techniques and procedures for an electrical method applied to a solid and to a liquid. Read it as a signpost telling you that the following two statements are practical skills you must be able to describe, justify and evaluate. When reading them, note what is measured directly (mass, current, potential difference, time, temperature), what is calculated (energy supplied, specific latent heat), and what corrections are needed (thermal energy gained from or lost to the surroundings). Link each procedure to the equation E = mL and to the electrical energy relation E = VIt, and check that the state change matches fusion or vaporisation.
(i) an electrical experiment to determine the specific latent heat of fusion and vaporisation
An electrical experiment supplies energy to a substance changing state to find the specific latent heat L using E = mL. For fusion, ice is melted in a funnel by an embedded heater; for vaporisation, a liquid is boiled by an immersed heater. You measure the mass m that changes state, current I, potential difference V, and time t, calculating energy as E = VIt. The specific latent heat is L = VIt/m. Thermal exchange with the surroundings affects results. During fusion, ice gains heat from warmer surroundings, melting extra mass, making the calculated L too small. During vaporisation, heat is lost to the surroundings, so less mass vaporises, making the calculated L too large. A control experiment helps correct for these exchanges.
(ii) techniques and procedures used for an electrical method to determine the specific latent heat of a solid and a liquid.
The electrical method uses a heater to supply a known energy to a substance while it changes state, then relates that energy to the mass changed using E = mL. For a solid, a heater melts ice at 0 °C; the melted water is collected and weighed, and the energy supplied is VIt. For a liquid, a heater boils water in an insulated container; the mass boiled away is found from the decrease in mass, and again E = VIt. Key techniques are: measure I and V with ammeter and voltmeter, time the heating with a stopwatch, find the mass change with a balance, keep the substance at its melting or boiling point throughout, and insulate the apparatus to limit energy exchange with the surroundings. A control or correction run, or a graph of energy supplied against mass changed, can reduce the effect of losses.
Your focus
- Define specific heat capacity and state its unit.
- Apply E = mcΔθ to calculate energy, mass, specific heat capacity or temperature change.
- Convert mass to kilograms and use temperature differences correctly in kelvin or degrees Celsius.
Show all 24 objectives
- Identify that this row is a sub-heading introducing the electrical determination of specific heat capacity for a metal and a liquid.
- Describe the purpose of the practical work signposted by this sub-heading.
- Explain how the following rows build from a general experiment to specific techniques and procedures.
- Describe an electrical experiment to determine the specific heat capacity of a metal or a liquid.
- Apply the energy balance P t = m c Δθ to calculate specific heat capacity from measured quantities.
- Evaluate the experiment by identifying heat loss and, for a liquid, the effect of the container.
- Describe the techniques and procedures for the electrical method applied to a metal block and to a liquid.
- Explain how measurements of mass, temperature, current, potential difference and time are used to find specific heat capacity.
- Evaluate the procedures by identifying how insulation, stirring and repetition improve the result.
- Define specific latent heat of fusion and specific latent heat of vaporisation.
- Apply E = m l to calculate energy transferred during a change of state.
- Explain why temperature remains constant during melting and boiling.
- Identify that this sub-heading introduces two practical statements about electrical methods for specific latent heat.
- Describe the quantities measured in an electrical determination of specific latent heat.
- Explain why energy losses matter and how the method can be improved.
- Describe an electrical experiment that determines the specific latent heat of fusion and of vaporisation.
- Calculate specific latent heat from measured values of mass, current, potential difference and time.
- Evaluate the effect of thermal energy exchange with the surroundings on the calculated specific latent heat.
- Describe the techniques and procedures of an electrical method for determining specific latent heat of a solid and of a liquid.
- Explain how insulation and control of temperature improve the accuracy of the determination.
- Apply E = VIt and E = mL to calculate a specific latent heat from experimental data.
Thermal properties of materials exam tips
Marking Points
- Specific heat capacity is the energy needed to raise the temperature of 1 kg of a substance by 1 K.
- The unit of specific heat capacity is J kg⁻¹ K⁻¹.
- The equation E = mcΔθ relates energy transferred, mass, specific heat capacity and temperature change.
- Mass must be in kilograms and Δθ is the temperature change in kelvin or degrees Celsius.
- A temperature difference in kelvin equals the same difference in degrees Celsius.
- States that electrical energy supplied is calculated from power and time, E = P t, with P measured using a voltmeter and ammeter or a power meter.
- Describes measurement of the mass m of the metal block or of the liquid, for example using a balance, and the temperature change Δθ using a thermometer or temperature sensor.
- Applies the energy balance P t = m c Δθ to obtain c = P t / (m Δθ), stating the assumption that energy supplied is transferred to the sample.
- Explains how heat loss to the surroundings is reduced, for example by insulating the block or container and by starting the timing when the temperature is steady.
- For a liquid, explains that the container also absorbs energy, so either the container's heat capacity is included or its effect is minimised and acknowledged as a limitation.
- Describes a suitable procedure, including allowing the heater to warm the sample, stirring a liquid to keep the temperature uniform, and reading the maximum or steady temperature rise.
- Describes the metal-block procedure: holes for heater and thermometer, measurement of mass, insulation, initial temperature, heating for a measured time, and final steady temperature.
- Describes the liquid procedure: known mass of liquid in an insulated container, heater and temperature sensor, stirring, and measurement of temperature rise.
- Explains how power is determined, for example from potential difference and current, P = V I, or from a power meter, and how time is measured.
- Applies P t = m c Δθ to calculate c, and states the assumption that energy supplied is transferred to the sample.
- Explains techniques that improve accuracy, such as insulating the apparatus, stirring a liquid, reading the thermometer at eye level, and repeating the experiment.
- For a liquid, explains how the container's heat capacity is handled, either by including it in the energy balance or by minimising its effect.
- Defines specific latent heat of fusion as the energy per unit mass to change from solid to liquid at constant temperature.
- Defines specific latent heat of vaporisation as the energy per unit mass to change from liquid to vapour at constant temperature.
- States and applies E = m l, using the correct specific latent heat for the change of state.
- Recognises that temperature remains constant during a change of state, so the energy supplied does not raise the temperature.
- Uses correct units, J kg⁻¹ for specific latent heat and J for energy, and converts mass to kg where necessary.
- States that electrical energy supplied is calculated from E = VIt, with I, V and t measured during the state change.
- Measures the mass m of substance that changes state, for example ice melted or water boiled away.
- Uses L = E/m, equivalently L = VIt/m, to determine the specific latent heat.
- Distinguishes fusion (solid to liquid) from vaporisation (liquid to gas) and selects the matching latent heat.
- Explains that for fusion, heat gained from the surroundings melts extra ice, making the calculated L too small.
- Explains that for vaporisation, heat lost to the surroundings reduces the mass vaporised, making the calculated L too large.
- Connects the ammeter, voltmeter and heater so that the electrical energy supplied is VIt.
- Measures the mass of solid melted or liquid vaporised using a balance, taking care to record only the mass that changes state.
- Keeps the substance at its melting point or boiling point during the measurement so that all supplied energy goes into the state change.
- Insulates the apparatus, for example with a lagged container or a lid, to reduce thermal energy transfer to or from the surroundings.
- Uses E = mL, with E = VIt, to calculate the specific latent heat of the solid or the liquid.
- Describes a correction for energy losses, such as a control run or plotting energy supplied against mass changed.
Examiner Tips
- 💡Write down E = mcΔθ, rearrange for the unknown, then substitute values with units.
- 💡Convert mass to kilograms and check whether the question gives a temperature change or two temperatures.
- 💡Check the magnitude of your answer: a small temperature change of a small mass should give a modest energy value.
- 💡Turn the sub-heading into a checklist: apparatus, measurements, control of variables, energy transfer calculation, and sources of error for both a metal and a liquid.
- 💡For each version, write the energy supplied electrically and the energy gained by the sample, then state which quantity is assumed to be lost to the surroundings.
- 💡Practise describing the procedure in a logical order so that a reader could repeat it, including how mass and temperature change are measured.
- 💡Write the energy balance explicitly before substituting numbers, so the examiner can see the physics even if the arithmetic goes wrong.
- 💡State the assumption that all electrical energy raises the temperature of the sample, then identify at least one way the assumption fails.
- 💡When describing a liquid experiment, mention insulation and stirring, and explain how the container affects the result.
- 💡Structure your answer as apparatus, measurements, procedure, calculation and evaluation so that both the metal and liquid versions are covered.
- 💡Use the same energy equation for both versions, but state clearly what is included in the thermal store for each.
- 💡Mention at least two specific techniques, such as insulation and stirring, and explain what error each one reduces.
- 💡Check whether the question involves a temperature change or a change of state before choosing between E = m c Δθ and E = m l.
- 💡Write down the value and unit of the specific latent heat you are using, and make sure it matches fusion or vaporisation.
- 💡Convert mass to kilograms and keep powers of ten consistent when substituting.
- 💡Read the sub-heading together with the two statements beneath it so you see the practical work as one coherent topic.
- 💡For each method, write down the measured quantities and the equation linking them before you consider improvements.
- 💡Check whether the state change is fusion or vaporisation, because that decides which latent heat you calculate.
- 💡Write the energy equation and the latent heat equation before substituting values, so the examiner can follow your reasoning.
- 💡State each measured quantity with its unit and say how it is measured, for example mass by a balance and time by a stopwatch.
- 💡When asked to evaluate, specify the direction of heat exchange (gain for fusion, loss for vaporisation) and its exact effect on the calculated value of L.
- 💡Describe the procedure in a logical order: set up, measure I and V, start timing, collect the mass change, then calculate.
- 💡Name the measuring instrument for each quantity, for example ammeter for current and balance for mass.
- 💡Explain how each improvement reduces a named error, rather than listing improvements without reasons.
Common Mistakes
- Using mass in grams instead of kilograms: convert grams to kilograms by dividing by 1000 before substituting.
- Using the final temperature instead of the temperature change: Δθ is the difference between final and initial temperatures.
- Confusing specific heat capacity with specific latent heat: specific heat capacity applies to a temperature change, not a phase change.
- Writing the unit as J kg⁻¹ °C⁻¹ only: J kg⁻¹ K⁻¹ is the standard unit, and a temperature difference in kelvin equals one in degrees Celsius.
- Treating the sub-heading as a fact to be memorised rather than a signpost to the practical skills in the following rows; instead, use it to organise your revision around the electrical method for a metal and for a liquid.
- Assuming the metal-block and liquid versions use identical apparatus and procedure; instead, note that a liquid needs a container and insulation, which changes the measurements and the corrections required.
- Reading only the method and ignoring evaluation; instead, prepare to explain how each measurement is made and how each source of error is reduced or accounted for.
- Using the final temperature instead of the temperature change; the correction is to calculate Δθ as the final temperature minus the initial temperature.
- Forgetting to convert units, for example using grams in place of kilograms or minutes in place of seconds; the correction is to convert mass to kg and time to s before substituting.
- Ignoring heat loss to the surroundings and treating the calculated value as exact; the correction is to insulate the apparatus and to explain that the measured c is likely to be higher than the true value because some energy is lost.
- For a liquid, ignoring the energy absorbed by the container; the correction is to include the container's heat capacity or to state that this is a source of systematic error.
- Confusing the metal-block and liquid procedures and omitting the container from the liquid method; the correction is to state that the liquid is in a container that also absorbs energy.
- Recording the temperature as soon as the heater is switched on rather than allowing the sample to reach a steady state; the correction is to wait for a steady or maximum reading.
- Failing to stir a liquid, so the measured temperature is not representative; the correction is to stir gently and continuously during heating.
- Using the power rating printed on the heater instead of measuring the actual power; the correction is to measure current and potential difference or use a power meter.
- Using the specific heat capacity equation instead of the latent heat equation during a change of state; the correction is to use E = m l when the temperature is constant.
- Mixing up fusion and vaporisation; the correction is to remember that fusion is solid to liquid and vaporisation is liquid to gas.
- Forgetting to convert grams to kilograms; the correction is to divide the mass in grams by 1000 before substituting.
- Assuming the temperature rises while the substance is melting or boiling; the correction is to state that the temperature stays constant during the change of state.
- Treating the sub-heading as a fact to memorise rather than a signpost to the two practical statements that follow. Correction: use it to organise your reading of the fusion and vaporisation methods.
- Assuming the same apparatus and procedure suit both a melting solid and a boiling liquid. Correction: identify the shared electrical method and the different arrangements needed for each state change.
- Ignoring energy losses to the surroundings when reading the method. Correction: note where insulation, a lid or a correction for losses is needed and why.
- Using the total mass of substance present instead of the mass that actually changes state. Correction: measure only the mass melted or vaporised during the timed heating.
- Stating that heat is lost to the surroundings during the fusion of ice. Correction: ice is colder than room temperature, so it gains heat from the surroundings, causing extra melting.
- Assuming the measured specific latent heat is exact despite thermal exchanges. Correction: explain how heat gain makes the calculated L for fusion too small, and heat loss makes L for vaporisation too large.
- Letting the temperature of the substance rise above its melting or boiling point. Correction: keep the substance at the change-of-state temperature so the energy supplied is used only for the latent heat.
- Recording the mass of the container or the total mass rather than the mass that changed state. Correction: find the mass difference due to melting or boiling.
- Ignoring heat exchange with the surroundings. Correction: lag the apparatus, use a lid and, where possible, apply a correction run or graph method.