Temperature — OCR A-Level Physics
Test yourself on Temperature with OCR A-Level practice questions.
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Temperature explained
Thermal equilibrium is the state reached when two or more objects in thermal contact have no net transfer of thermal energy between them.
Read the full explanation
Their temperatures are equal, so there is no resultant heat flow. For example, a thermometer placed in a beaker of water reaches thermal equilibrium with the water when the thermometer reading becomes constant; at that point the thermometer and water are at the same temperature. Thermal equilibrium is a dynamic condition: energy may still be exchanged at a microscopic level, but the average rates of transfer in each direction are equal, so the net transfer is zero. This concept underpins the definition of temperature and the operation of thermometers. In an MCQ, you may be asked to identify when equilibrium is reached or to distinguish it from steady-state conditions where a temperature difference is maintained by a continuous energy input.
(b) absolute scale of temperature (i.e. the thermodynamic scale) that does not depend on property of any particular substance
The absolute scale of temperature, also called the thermodynamic scale, is defined independently of any particular substance. It is based on fundamental thermodynamic principles, such as the efficiency of a Carnot engine, rather than on the expansion of a specific material like mercury or alcohol. The kelvin (K) is the unit of this scale, with 0 K being absolute zero, the temperature at which particles have minimum possible kinetic energy. Because the scale does not rely on a material property, it provides a universal and consistent way to measure temperature. For example, the triple point of water is defined as 273.16 K, and other temperatures are realised using primary thermometers that operate on fundamental principles. In an MCQ, you may be asked why the thermodynamic scale is preferred or how it differs from empirical scales.
(c) temperature measurements both in degrees Celsius ( ° C) and in kelvin (K)
Temperature can be measured in degrees Celsius (°C) or in kelvin (K). The Celsius scale is defined by the ice point (0 °C) and steam point (100 °C) of water at standard atmospheric pressure. The kelvin is the SI unit of thermodynamic temperature, with 0 K defined as absolute zero. The two scales have the same size degree, so a temperature change of 1 °C equals a change of 1 K. To convert from Celsius to kelvin, add 273 (approximately). For example, 25 °C is approximately 298 K. In an MCQ, you may be asked to convert between the scales or to identify the correct unit for a given measurement. Always check whether the question requires an exact conversion or an approximate one, and ensure you use the correct symbol and unit.
(d) T(K) ≈ θ(°C) + 273
The relationship T(K) ≈ θ(°C) + 273 allows you to convert a temperature from degrees Celsius to kelvin. Here, T is the thermodynamic temperature in kelvin and θ is the Celsius temperature. The approximation is used because the exact offset is 273.15, but for many calculations 273 is sufficient. For example, 20 °C is approximately 293 K. To convert from kelvin to Celsius, rearrange the equation: θ(°C) ≈ T(K) − 273. This relationship is valid for temperature differences as well, since a change of 1 °C equals a change of 1 K. In an MCQ, you may be asked to perform a conversion or to identify the correct rearrangement. Always check the direction of conversion and whether the question expects the approximate or exact value.
Your focus
- Define thermal equilibrium in terms of equal temperatures and zero net thermal energy transfer.
- Explain why a thermometer reading becomes constant when it reaches thermal equilibrium with its surroundings.
- Distinguish thermal equilibrium from steady-state conditions where a temperature difference is maintained.
Show all 12 objectives
- Describe the absolute scale of temperature as independent of any particular substance.
- Explain why the thermodynamic scale is based on fundamental principles rather than material properties.
- State the unit of the absolute scale and the value of the triple point of water.
- State the defining points of the Celsius scale and the unit of the kelvin scale.
- Convert temperatures between degrees Celsius and kelvin using the approximate relationship.
- Recognise that a temperature change of 1 °C equals a change of 1 K.
- Use the equation T(K) ≈ θ(°C) + 273 to convert temperatures.
- Rearrange the equation to convert from kelvin to Celsius.
- Apply the relationship to temperature differences, recognising that 1 °C equals 1 K.
Temperature exam tips
Marking Points
- Thermal equilibrium occurs when two objects in thermal contact have equal temperatures and no net transfer of thermal energy.
- At equilibrium, the average rates of energy transfer in both directions are equal, so the net energy transfer is zero.
- A thermometer reaches thermal equilibrium with its surroundings when its reading becomes constant.
- Thermal equilibrium is distinct from steady-state conditions where a temperature difference is maintained by a continuous energy input.
- The absolute scale of temperature is independent of the properties of any particular substance.
- It is based on fundamental thermodynamic principles, such as the efficiency of a Carnot engine.
- The kelvin (K) is the unit of the absolute scale, with 0 K defined as absolute zero.
- The triple point of water is a fixed point on the scale, defined as 273.16 K.
- Temperature can be measured in degrees Celsius (°C) or in kelvin (K).
- The Celsius scale is defined by the ice point (0 °C) and steam point (100 °C) of water at standard atmospheric pressure.
- The kelvin is the SI unit of thermodynamic temperature, with 0 K defined as absolute zero.
- A temperature change of 1 °C is equal to a temperature change of 1 K.
- To convert from Celsius to kelvin, add 273 (approximately).
- The equation T(K) ≈ θ(°C) + 273 converts a Celsius temperature to kelvin.
- T represents the thermodynamic temperature in kelvin and θ represents the Celsius temperature.
- The approximation uses 273 instead of the exact 273.15.
- To convert from kelvin to Celsius, rearrange to θ(°C) ≈ T(K) − 273.
- A temperature change of 1 °C equals a temperature change of 1 K, so the equation also applies to temperature differences.
Examiner Tips
- 💡Read the question carefully to distinguish between 'no net transfer' and 'no transfer at all'.
- 💡Recall that a constant thermometer reading indicates thermal equilibrium with the object being measured.
- 💡Eliminate options that confuse thermal equilibrium with steady-state conditions or with equal internal energies.
- 💡Remember that 'absolute scale' and 'thermodynamic scale' are the same thing.
- 💡Focus on the key phrase 'does not depend on property of any particular substance' when evaluating options.
- 💡Recall that the kelvin is the SI unit of temperature and that 0 K is absolute zero.
- 💡Remember the approximate conversion: T(K) ≈ θ(°C) + 273.
- 💡Check that you are using the correct unit symbol: °C for degrees Celsius and K for kelvin (no degree symbol).
- 💡When converting a temperature difference, the numerical value is the same in °C and K.
- 💡Write down the equation and substitute values carefully, checking the direction of conversion.
- 💡Remember that the approximation is usually acceptable unless the question specifies otherwise.
- 💡For temperature differences, no conversion factor is needed; the numerical value is the same in °C and K.
Common Mistakes
- Misunderstanding: thermal equilibrium means no energy transfer at all. Correction: energy is still transferred microscopically, but the net transfer is zero because the rates in each direction are equal.
- Misunderstanding: thermal equilibrium requires the objects to have the same internal energy. Correction: equilibrium requires equal temperatures, not equal internal energies; internal energy also depends on mass and material.
- Misunderstanding: a steady-state system with a constant temperature difference is in thermal equilibrium. Correction: a steady state can have a constant temperature difference with a continuous net energy flow, so it is not in thermal equilibrium.
- Misunderstanding: the thermodynamic scale depends on the expansion of mercury. Correction: it is independent of any material property; empirical scales depend on such properties.
- Misunderstanding: absolute zero is the freezing point of water. Correction: absolute zero is 0 K, the temperature at which particles have minimum kinetic energy, not the freezing point of water.
- Misunderstanding: the thermodynamic scale is the same as the Celsius scale. Correction: the Celsius scale is defined using the ice and steam points of water, while the thermodynamic scale is defined independently of any substance.
- Misunderstanding: the Celsius and kelvin scales have different degree sizes. Correction: the size of one degree Celsius is the same as one kelvin, so temperature differences are numerically equal.
- Misunderstanding: 0 °C is the same as 0 K. Correction: 0 °C is approximately 273 K, while 0 K is absolute zero.
- Misunderstanding: to convert from kelvin to Celsius, add 273. Correction: to convert from kelvin to Celsius, subtract 273 (approximately).
- Misunderstanding: to convert from kelvin to Celsius, add 273. Correction: subtract 273, so θ(°C) ≈ T(K) − 273.
- Misunderstanding: the equation gives an exact conversion. Correction: it is an approximation because the exact offset is 273.15.
- Misunderstanding: the equation cannot be used for temperature differences. Correction: because 1 °C equals 1 K, the same numerical difference applies.