Temperature, heat and thermal energyIB MYP Physics: Revision notes
Section 1
Temperature and thermal energy
Temperature is a measure of the average kinetic energy of the particles in a substance. It is measured in degrees Celsius (°C) or kelvin (K). Thermal energy is the total energy of all the particles in a substance, measured in joules (J).
Thermal energy depends on the temperature, the mass and the type of substance. A swimming pool at 28 °C holds far more thermal energy than a cup of tea at 80 °C, because it has far more particles, even though the tea has a higher temperature.
To change from °C to K, add 273: 25 °C = 298 K. A rise of 1 °C is the same as a rise of 1 K.
Temperature and thermal energy are not the same thing. Temperature is not measured in joules, and a hotter object does not always have more thermal energy.
Section 2
Thermometers and temperature scales
A liquid-in-glass thermometer has a liquid, usually alcohol or mercury, in a thin tube. The liquid expands as it warms and rises up the scale. Digital thermometers use an electronic sensor.
The Celsius scale is based on two fixed points: ice melting at 0 °C and water boiling at 100 °C, both at normal pressure. The Kelvin scale starts at absolute zero, 0 K or −273 °C, where particles have the least possible energy.
Section 3
Thermal energy flows from hot to cold
Thermal energy always flows from a region of higher temperature to a region of lower temperature.
In particle terms, the faster particles in the hot object collide with the slower particles in the cold object and transfer energy to them. The hot object cools and the cold object warms.
This continues until both are at the same temperature. This is thermal equilibrium, and there is no longer any net flow of thermal energy.
Example: a hot spoon at 90 °C in cold water at 20 °C: energy flows from the spoon to the water until both reach the same temperature.
Section 4
Thermal expansion
Most substances expand when heated. The particles gain kinetic energy and vibrate or move more, so they move slightly further apart on average. The particles themselves do not get bigger.
- Solids expand a little, as the particles are held in fixed positions.
- Liquids expand more than solids.
- Gases expand most, because their particles are far apart and free to move.
When the substance cools, it contracts.
Say that the particles move further apart, not that the particles expand.
Section 5
Uses and problems of expansion
Uses:
- Liquid thermometers: the liquid expands up the narrow tube.
- Bimetallic strips: two metals, such as brass and steel, are joined together. When heated, both expand but by different amounts, so the strip bends, with the metal that expands more on the outside of the curve. This is used in thermostats and fire alarms to switch a circuit on or off.
Problems:
- Steel bridges and railway lines can buckle on hot days, so engineers leave expansion gaps or joints.
- Overhead cables sag in hot weather.
- Glass can crack if heated unevenly.
Brass expands more than steel, so a brass-steel strip bends with the brass on the outside of the curve.
Must know
- Temperature = average kinetic energy of particles (°C or K); thermal energy = total energy (J)
- Thermal energy flows from hot to cold until thermal equilibrium
- Heating makes particles move further apart, so solids, liquids and gases expand (gases most)
- Uses: thermometers, bimetallic strips; problems: bridges and rails need expansion gaps
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Temperature, heat and thermal energy
- A student places a metal spoon heated to 90 °C into a beaker of cold water at 20 °C and records the temperature of the water every minute until the readings stop changing.Explain, in terms of particles, why thermal energy is transferred from the spoon to the water.2 marks
- A swimming pool at 28 °C holds about 500 000 litres of water. A cup of tea at 80 °C holds about 0.25 litres of water.The tea is poured into the pool. Explain why energy flows from the tea to the pool even though the pool has more thermal energy.2 marks
- A student investigates how the length of an aluminium rod changes when it is heated. The rod is 1000 mm long at 20 °C. The increase in length was 0.9 mm for a 40 °C rise in temperature, 1.8 mm for an 80 °C rise, 2.8 mm for a 120 °C rise and 3.7 mm for a 160 °C rise.State a testable hypothesis for this investigation, giving a scientific reason, and name one variable that must be controlled.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).