Lesson summary
14. Temperature — Cambridge AS & A Level Physics 9702 — CAMBRIDGE AS & A LEVEL PHYSICS 9702 — COMPLETE NOTES notes: Explain thermal equilibrium, how temperature is measured, the Kelvin scale, and calculate energy changes using specific heat capacity and specific latent heat.Therm…
- Thermal energy
- Conservation
- Final temperature
- Thermometer
- Calibration
- Celsius scale
Section 1 of 8Thermal energy transfer
- Thermal energy = the energy an object has because of its temperature.
- Direction
- thermal energy always transfers from a region of higher temperature to a region of lower temperature, so temperature shows which way energy flows.
- A thermometer in boiling water warms up because energy flows from the water to the thermometer.
- Ice in room-temperature water melts because the water gives energy to the ice; no "cold" travels the other way.
- Methods
- thermal energy moves by conduction, convection or radiation.
- Conservation
- energy is not created or destroyed, only transferred from one place or form to another.
Section 2 of 8Thermal equilibrium
- Thermal equilibrium = two objects in contact have no net transfer of thermal energy between them, which happens when they are at the same temperature.
- Approach
- the hotter region cools and the cooler region warms until both reach one temperature.
- Final temperature
- it lies between the two starting temperatures and depends on the temperature difference, masses and materials.
Section 3 of 8Measuring temperature
- Thermometer = any device that measures temperature using a physical property that changes with temperature.
- Calibration
- a thermometer is marked at known fixed points, usually 0 °C and 100 °C for the melting and boiling points of water,
- and the scale between is divided equally.
- a thermometer is marked at known fixed points, usually 0 °C and 100 °C for the melting and boiling points of water,
| Property used | Thermometer | Behaviour and notes |
|---|---|---|
| Volume of a liquid | Liquid-in-glass (often mercury) | The liquid expands up a thin capillary, and the reading follows its length; expansion is not always perfectly linear |
| Volume of a gas at constant pressure | Gas thermometer | V ∝ T for an ideal gas, so volume increases with temperature |
| Resistance of a metal | Resistance thermometer | Resistance rises steadily with temperature |
| Resistance of a thermistor | Thermistor thermometer | Resistance falls as temperature rises, changing rapidly over a narrow range; the curve needs calibration |
| E.m.f. of a thermocouple | Thermocouple | Two dissimilar wires joined at two junctions, with a voltmeter on the free ends; a temperature difference gives an e.m.f. that is non-linear and needs calibration |
- Link
- the reading is tied to the property, such as temperature increasing as the thermocouple e.m.f. increases.
Section 4 of 8Celsius and Kelvin scales
- Celsius scale
- defined using properties of water, 0 °C at freezing and 100 °C at boiling.
- Thermodynamic (Kelvin) scale
- an absolute scale, not based on any one substance, whose SI unit is the kelvin (K).
- Conversion
- θ / °C = T / K − 273.15 and T / K = θ / °C + 273.15.
- 0 °C = 273.15 K, so add 273.15 to go from °C to K.
- Equal divisions
- a change of 1 K equals a change of 1 °C, so temperature differences need no conversion.
Section 5 of 8Absolute zero
- Absolute zero = 0 K, the lowest possible temperature, so a temperature in kelvin is never negative.
- Molecular meaning
- at 0 K molecules have minimum kinetic energy and no more energy can be removed.
- Proportionality
- the absolute temperature of a body is directly proportional to the average kinetic energy of its molecules.
- Space
- even empty space is about 2.7 K, just above absolute zero.
Section 6 of 8Specific heat capacity
1Specific heat capacity c
= the thermal energy needed to raise the temperature of 1 kg of a substance by 1 K (or 1 °C), in J kg−1 K−1.
2Equation2 points
- Q = mcΔθ, where Q is the energy transferred, m the mass in kg and Δθ the temperature change.
- A larger mass or temperature change needs more energy.
3Meaning3 points
- a low c heats and cools quickly and a high c heats and cools slowly, which guides the choice of material for a task.
- Specific heat capacity is mainly used for liquids and solids.
- Good electrical conductors such as copper and lead have low c.
| Substance | c / J kg−1 K−1 |
|---|---|
| Aluminium | 910 |
| Copper | 390 |
| Lead | 126 |
| Glass | 500–680 |
| Water | 4200 |
| Mercury | 140 |
Section 7 of 8Changes of state and latent heat
- Changes of state
- melting, vaporisation (liquid to gas), sublimation (solid to gas), freezing (liquid to solid) and condensation (gas to liquid).
- (solid to liquid)
- melting, vaporisation (liquid to gas), sublimation (solid to gas), freezing (liquid to solid) and condensation (gas to liquid).
- No temperature change
- the energy supplied changes the state, not the temperature, so the temperature–energy graph is flat.
- Specific latent heat L = the energy needed to change the state of 1 kg of a substance without a change of temperature, in J kg−1.
- Fusion is solid to liquid (and back) and vaporisation is liquid to gas (and back).
- Equation
- Q = mL.What the symbols mean: E or Q = energy to change state without a change of temperature (J) · m = mass (kg) · L = specific latent heat (J kg−1)
- Q = mL.
Section 8 of 8Why vaporisation needs more energy
| Quantity for water | Value |
|---|---|
| Specific latent heat of fusion | 330 kJ kg−1 |
| Specific latent heat of vaporisation | 2.26 MJ kg−1 |
- Comparison
- boiling 1 kg of water needs roughly seven times the energy of melting 1 kg of ice.
- Melting
- energy only increases the separation of molecules enough for them to flow past each other.
- Boiling
- energy separates the molecules completely, overcoming almost all intermolecular attraction.
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