9.1 Relationship between Temperature and Heat
Heat is a form of energy that flows from a region of higher temperature to a region of lower temperature.
- SI Unit of Heat: Joule (J)
- Temperature: The degree of hotness or coldness of an object. Measured in Degree Celsius (°C) or Kelvin (K) using a thermometer.
Differences Between Heat and Temperature
| Parameter |
Heat |
Temperature |
| Definition |
A form of energy transferred between objects. |
Degree of hotness or coldness of an object. |
| SI Unit |
Joule (J) |
Kelvin (K) / Degree Celsius (°C) |
| Measuring Tool |
Calorimeter |
Thermometer |
| Dependence |
Depends on type of material, total mass, and temperature. |
Depends on average kinetic energy of particles inside an object. |
9.2 Heat Flow and Thermal Equilibrium
Methods of Heat Flow
- Conduction: Heat transfer through solid mediums by particle vibration without actual particle movement. (Fastest in solids, especially metals).
- Convection: Heat transfer through fluids (liquids and gases) via moving convection currents (hotter, less dense fluid rises; cooler, denser fluid sinks).
- Radiation: Heat transfer through electromagnetic waves without requiring any physical medium (e.g., heat from the Sun reaching Earth through vacuum).
Heat Flow in Natural Phenomena
- Sea Breeze (Daytime): Land heats up faster than the sea. Warm air above land rises, and cooler, denser air from the sea blows in to replace it.
- Land Breeze (Nighttime): Land cools down faster than the sea. Warm air above the sea rises, and cooler air from the land blows towards the sea.
Thermal Conductors and Insulators
- Thermal Conductors: Materials that allow heat to pass through them quickly (e.g., copper, iron, aluminum). Used in cooking pans and kettle bases.
- Thermal Insulators: Materials that prevent or slow down heat transfer (e.g., wood, plastic, styrofoam, air). Used in pot handles and winter jackets.
Thermal Equilibrium
When two objects of different temperatures are brought into contact, heat energy transfers from the hotter object to the colder object until both reach the same temperature. At this point, net heat transfer is zero, and they are in thermal equilibrium.
9.3 Effect of Heat on Matter
Expansion and Contraction of Matter
Matter expands when heated and contracts when cooled across all three states:
$$\text{Rate of Expansion: } \text{Gas} > \text{Liquid} > \text{Solid}$$
- Solids: Particles gain kinetic energy, vibrate faster, and push slightly further apart.
- Liquids & Gases: Particles move faster and push significantly further apart.
Applications of Expansion and Contraction in Daily Life
- Bimetallic Strip: Made of two different metals (e.g., brass and iron) that expand at different rates when heated. Used in thermostats (electric irons, fire alarms).
- Mercury Thermometer: Liquid mercury inside the bulb expands along the fine capillary tube when temperature rises.
- Expansion Gaps in Railway Tracks & Bridges: Gaps prevent tracks from buckling or bending during hot weather.
- Rollers under Steel Bridges: Allow metal bridge girders to expand safely on hot days without damaging structural pillars.
9.4 Absorbing and Radiating Heat
- Dark, Dull/Rough Surfaces: Excellent heat absorbers and excellent heat radiators.
- White, Shiny/Smooth Surfaces: Poor heat absorbers (reflect heat) and poor heat radiators.
Applications in Daily Life
- Fuel storage tanks and houses in tropical countries are painted white or light colors to reflect solar radiation and stay cool.
- Solar collector panels are painted dull black to absorb maximum heat energy from sunlight.