Lesson summary
14.1.1 Thermal Energy Transfer — AS PHYSICS 9702 notes: Explain that thermal energy flows from higher to lower temperature and describe conduction, convection and radiation.Temperature — it tells us how hot an object is and determines the direction in which thermal energy flowsTh…
- Energy is conserved
- Hot to cold
- Until equal
- Thermometer in boiling water
- Ice in water
- Particle motion
Section 1 of 9Heat and temperature
Temperature
- Temperature it tells us how hot an object is and determines the direction in which thermal energy flows
Thermal energy
- Thermal energy energy, in joules, that moves between objects because of a temperature difference, often called heat when in transit
- A cup of hot tea has a high temperature but a swimming pool can store far more thermal energy because it holds much more water
Energy is conserved
- Energy is conserved thermal energy is transferred, never created or destroyed
Section 2 of 9Direction of transfer
1Hot to cold
- Hot to cold thermal energy flows spontaneously from a region of higher temperature to one of lower temperature
- It flows the other way only if work is done, as in a refrigerator
2Until equal
- Until equal the flow continues until both regions reach the same temperature
3Thermometer in boiling water
- Thermometer in boiling water its reading rises because energy passes from the water to the cooler thermometer
4Ice in water
- Ice in water the water transfers energy to the ice, which melts, rather than the ice passing cold to the water
Section 3 of 9The microscopic picture
The microscopic picture
Particle motion
- Particle motion particles vibrate in a solid and move randomly in a liquid or gas
Temperature
- Temperature a measure of the average random kinetic energy of the particles
Transfer at a boundary
- Transfer at a boundary fast particles collide with slower ones and pass on some energy
- Energy spreads until the average energy per particle is the same on both sides
Section 4 of 9Conduction
Conduction
- Conduction transfer of thermal energy through a substance without bulk movement of the material
In solids
- In solids vibrating atoms pass energy to neighbouring atoms through their bonds
In metals
- In metals free (delocalised) electrons gain kinetic energy at the hot end and transfer it by collisions with ions and other electrons
- This is why metals are much better conductors than wood or plastic
Gases
- Gases poor conductors because the particles are far apart and collide rarely
- Trapped air in clothing, double glazing and polystyrene is used for insulation
Section 5 of 9Convection and radiation
Convection
- Convection occurs only in fluids, when heated fluid expands, becomes less dense and rises while cooler, denser fluid sinks
- Examples are sea breezes, boiling water and radiators
No convection
- No convection it cannot occur in a solid or a vacuum
Radiation
- Radiation transfer by electromagnetic waves, mostly infrared at everyday temperatures
- It needs no medium, so energy reaches us from the Sun across empty space
Emission and absorption
- Emission and absorption all objects emit radiation and hotter objects radiate more power
- Dark matt surfaces are good emitters and absorbers, and shiny light surfaces are poor at both
Section 6 of 9Rate of transfer
Conduction rate
- Conduction rate increases with larger temperature difference, larger cross-sectional area, shorter path and a more conductive material
Radiation rate
- Radiation rate depends strongly on temperature and surface area, and is a net effect because the object also absorbs radiation
Cooling graph
- Cooling graph temperature falls steeply at first and then flattens towards the surroundings' temperature
- The rate of loss is proportional to the temperature difference
Net transfer stops
- Net transfer stops when the temperatures are equal, which is thermal equilibrium
Section 7 of 9Applications
1Saucepan
- Saucepan metal base conducts well and plastic handle conducts poorly
2Room heater
- Room heater placed low to set up a convection current
3Vacuum flask
- Vacuum flask the vacuum prevents conduction and convection and silvered surfaces reflect radiation
4Double glazing and loft insulation
- Double glazing and loft insulation trapped air or gas conducts poorly and restricts convection
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Thermal energy | energy transferred between regions because of a temperature difference |
| Conduction | transfer through a material by particle vibrations and collisions without bulk flow |
| Convection | transfer by movement of a heated fluid |
| Radiation | transfer by electromagnetic waves, requiring no medium |
| Temperature | a measure of the average random kinetic energy of particles, determining the direction of heat flow |
Section 9 of 9Exam technique
1
Saying heat and temperature are the same — temperature determines the direction of flow, heat is energy in transit.
2
Saying cold flows into a hot object — thermal energy flows from hot to cold.
3
Believing convection can occur in a solid or vacuum — it needs a fluid.
4
Saying radiation needs a medium — infrared crosses a vacuum.