Lesson summary
8.1 Temperature — AS PHYSICS 9702 notes: Explain thermal equilibrium and the thermodynamic scale, and use specific heat capacity and specific latent heat.Direction of flow — thermal energy is transferred from a region of higher temperature to one of lower temperatureThermal equil…
- Final temperature
- Thermometric property
- Calibration
- Disagreement between scales
- Kelvin scale
- Conversion
Section 1 of 11Thermal energy transfer and equilibrium
1Direction of flow
- Direction of flow thermal energy is transferred from a region of higher temperature to one of lower temperature
2Thermal equilibrium
- Thermal equilibrium bodies at the same temperature have no net transfer of thermal energy between themEnergy is exchanged at equal rates in both directions
3Final temperature
- Final temperature lies between the initial temperatures, as when ice is placed in warm water
Section 2 of 11Measuring temperature
Measuring temperature
Thermometric property
- Thermometric property any physical property that varies with temperature can be usedThe density (volume) of a liquidThe resistance of a metal or thermistorThe e.m.f. of a thermocoupleThe pressure of a gas at constant volume
Calibration
- Calibration the thermometer is calibrated at known fixed points, such as 0 °C and 100 °C, and the scale is divided into equal steps
Disagreement between scales
- Disagreement between scales thermometers using different properties agree only at the fixed points
Section 3 of 11The thermodynamic scale
Kelvin scale
- Kelvin scale it does not depend on the property of any particular substance
Conversion
- T/K = θ/°C + 273.15
Size of the unit
- Size of the unit a change of 1 K equals a change of 1 °C
Absolute zero
- 0 K is the temperature at which a system has minimum internal energyThe temperature cannot be lower than 0 K
Section 4 of 11Specific heat capacity
Definition
- Definition the energy required per unit mass to raise the temperature of a substance by one kelvin
Equation
- E = mcΔθ, with c = E/(mΔθ) in J kg⁻¹ K⁻¹What the symbols mean: E, Q or q = energy transferred (J) · m = mass · c = specific heat capacity (J kg−1 °C−1; water 4.18 J g−1 °C−1) · ΔT or Δθ = temperature change (°C or K)
- (kg; g in chemistry calorimetry)
Meaning of c
- Meaning of c a low value heats up and cools down quickly and a high value heats up and cools down slowly
Section 5 of 11Specific latent heat
Definition
- Definition the energy required per unit mass to change the state of a substance at constant temperature
Equation
- E = mL, with L = E/m in J kg⁻¹What 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)
Fusion
- Fusion solid to liquid, with specific latent heat of fusion Lf
Vaporisation
- Vaporisation liquid to gas, with specific latent heat of vaporisation Lv
Values for water
- Lf = 330 kJ kg⁻¹ and Lv = 2.26 MJ kg⁻¹
Section 6 of 11Why vaporisation needs more energy
Melting
- Melting only some bonds are broken and the particle separation barely changes
Boiling
- Boiling the particles are separated completely, so the forces of attraction are overcome fully
Expansion
- Expansion work is also done pushing back the atmosphere as the volume increases greatly
| Feature | Temperature change | Change of state |
|---|---|---|
| Equation | E = mcΔθ | E = mL |
| Temperature | Changes | Constant |
| Molecular kinetic energy | Changes | Unchanged |
| Molecular potential energy | Little change | Changes |
Section 7 of 11Electrical method
Electrical method
Measure the mass of the substance
Supply energy with an electrical heater of power P = VI
- Supply energy with an electrical heater of power P = VI record V, I and the time t
Record the temperature rise, or the mass that changes state
Apply the energy equation
- VIt = mcΔθ for heat capacity, or VIt = mL for latent heat
Reduce energy losses
- Reduce energy losses insulate the apparatus, or repeat at a second power so the losses cancel
- Latent heat with two powers
- L = (P₁ − P₂)/
- (rate of mass change₁ − rate of mass change₂)
- L = (P₁ − P₂)/
Section 8 of 11More from the specification
Joule
- The standard SI unit of energy and work, equivalent to one newton-metre (1 J = 1 N·m = 1 kg·m²/s²).
Absolute zero
- The lowest theoretically possible temperature (0 K or -273.15 °C), at which particles have minimal internal energy and zero kinetic motion.
Thermal equilibrium
- The state where two bodies in thermal contact cease exchanging net heat because they share the identical thermodynamic temperature.
Triple point of water
- The precise thermodynamic temperature (273.16 K) and pressure at which pure liquid water, ice, and water vapour coexist in stable equilibrium.
Section 9 of 11More from the specification
More from the specification
Temperature vs Thermal Energy
- Temperature determines the direction of spontaneous net thermal energy transfer between bodies, measured in units of degrees Celsius or kelvin.
Specific Heat Capacity Formula and Units
- The amount of thermal energy needed to raise the temperature of one kilogram of a material by one degree Celsius is calculated via Delta E = m * c * Delta T, expressed in joules per kilogram degree Celsius (J / (kg * C)).
Domestic Heating in an Electric Kettle
- When an electric kettle heats water, the electrical energy delivered (E = P * t) is absorbed by the liquid, with water's high specific heat capacity requiring substantial energy to reach boiling.
Thermal Properties of Solids
- In crystalline solids, heating increases the vibrational kinetic energy of particles fixed in rigid lattices,
- causing thermal expansion before overcoming intermolecular forces at the melting point.
Specific heat capacity units
- Specific heat capacity units measured in joules per kilogram kelvin, defining thermal energy needed to raise unit mass temperature.
Celsius and kelvin equivalence
- Celsius and kelvin equivalence temperature changes in degrees celsius equal identical numerical values on the thermodynamic kelvin scale.
Thermal energy calculation
- Thermal energy calculation total energy transferred to a heating material equals mass multiplied by specific heat capacity and temperature rise.
Section 10 of 11Key terms
| Term | Meaning |
|---|---|
| Thermal equilibrium | no net transfer of thermal energy between bodies in contact, because they are at the same temperature |
| Absolute zero | the temperature at which a system has minimum internal energy; 0 K |
| Thermodynamic scale | a temperature scale that does not depend on the property of any particular substance |
| Specific heat capacity | the energy required per unit mass to raise the temperature by one kelvin |
| Specific latent heat of fusion | the energy per unit mass to change a substance from solid to liquid at constant temperature |
| Specific latent heat of vaporisation | the energy per unit mass to change a substance from liquid to gas at constant temperature |
| Joule | Unit of energy, 1 kg·m2/s2. |
Section 11 of 11Exam technique
1
Saying thermal equilibrium means the particles stop moving — there is no net transfer, but the particles keep colliding.
2
Adding 273.15 the wrong way round — T/K = θ/°C + 273.15 because 0 °C is 273.15 K.
3
Saying temperature rises during melting or boiling — it stays constant while the state changes.
4
Using Lf for boiling — vaporisation uses Lv, which is larger than Lf.