Heat

Temperature and thermal energy
- Temperature is a measure of how hot or cold an object is
- SI unit : Kelvin (K)
- Thermometers and thermocouples are instruments that can be used to measure
- Thermal energy is the energy is the energy stored in the body due to its temperature

Transfer of Heat
- Heat energy always transfers from a region of higher temperature to lower temperature
- Net transfer of heat only occurs when there is a difference in temperature
- When two or more objects have the same temperature, they are in thermal equilibrium ( both objects same temperature ) and there is no net transfer of thermal energy between them

Effects of Heat
- When a substance is heated, its volume increases : → Spacing between particles increase
→ Size of particles remain unchanged
→ Substance expands
→ The process is called expansion

- When a substance is cooled, its volume decreases : → Spacing between particles decreases
→ Size of particles remain unchanged
→ Substance contracts
→ The process is called contraction

Uses of expansion and contraction

- Bimetallic strip → thin, flat piece of material composed of two different metals bonded together
→ two metals expand and contract at different rates causing the strip to bend when heated or cooled
→ thermostats and bimetallic thermometers use bimetallic strips

-Thermostat → keeps temperature constant. Usually found in ovens and refrigerators to keep the temperature hot and cold respectively
→ E.g. oven, when temperature gets too hot, the bimetallic strip bends away to break contact

- Brass expands more than steel when heated. Thus, the strip bends with brass on the outside of the curve
- Brass contracts more than steel when cooled. Thus, the strip bends with brass on the inside of the curve

Conduction
- Conduction is the process where energy is transferred through the passing of vibrational motion from one particle to another
- Particles at the heated end vibrate vigorously. They collide with neighbouring particle and transfer their energy. Eventually, the particles at the cooler end also vibrate vigorously
- Occurs without the flow of medium
- Factors affecting : good/poor conductor of heat
- No movement of particles in medium

Conduction (in solids)
- Metals conduct energy more quickly due to the presence of free electrons
- In non-metals, energy is transferred through the vibration and collision of particles

Conduction (in liquids)
- Water is a poor conductor of heat

Convection
- Convection is the transfer of energy by means of convection currents in liquid / gas, due to difference in density
- Unlike Conduction, energy is transferred through the actual movement of the heated particles in the medium from the warmer to cooler parts of the liquid / gas
- Factors affecting: Volume and density changes

Convection (in liquids)
- When water at the bottom of the flask is heated, it expands and the volume increases
- The heated water is less dense than the surrounding water, so it rises
- The cooler, denser water at the top sinks to the bottom
- The difference in densities of water in the two regions create a convection current

Convection (in gas)
- When air above candle is heated, it expands
- The heated air is less dense than the surrounding air, so it rises out of chimney B
- The cooler, denser, surrounding air sinks into chimney A
- The difference in densities of air at the two chimneys create a convection current

Radiation
- Radiation is the transfer of energy in the form of electromagnetic waves such as infrared radiation without the need of a medium
- All bodies emit infrared radiation
- Infrared radiation does not require a medium to be transmitted
→ ( temperature of body must be greater than 0K )

Effect on colour and texture on infrared radiation
- Emission rate : Black, rough, and dull surfaces emit infrared radiation at a faster rate than shiny, smooth, and silver surfaces
- Absorption rate : Black and dull surfaces absorb infrared radiation at a faster rate than silver and shiny surface

- S.A. on emission / absorption : Object with a larger S.A. will emit / absorb infrared radiation at a higher rate
- Surface temperature on emission : The higher the temperature of an object relative to its surrounding temperature, the higher the rate of emission of infrared radiation