P4 Science - Heat

Sources of Heat

Heat is a form of energy.
It can be felt but cannot be seen.
Heat makes things hot.

Natural Sources:

The Sun — the main source of heat on Earth. It heats Earth by radiation.
Volcanoes and geysers

Man-Made Sources:

Burning fuels such as firewood, charcoal, gas, electricity

Temperature

Temperature is a measure of the degree of hotness of an object.

Unit: degree Celsius (°C)
Measured using: thermometer (e.g., clinical thermometer, laboratory thermometer)

Key Points:
Temperature = measure of hotness (°C)
Heat energy = amount of energy an object has

Temperature vs. Heat Energy

	Heat Energy	Temperature
Definition	The amount of energy an object has	A measure of the degree of hotness of an object
Measurable?	Cannot be measured directly	Can be measured (using thermometer)

The amount of heat energy an object has depends on its:

Temperature
Mass or volume

The higher the temperature, the greater the amount of heat energy it contains.
Example: Two identical glasses of water — one at 10°C and one at 80°C. The 80°C water has more heat energy because of its higher temperature.

The greater the mass/volume, the greater the amount of heat energy it contains.
Example: Two glasses at the same temperature — one with 10 cm³ and one with 15 cm³. The larger volume has more heat energy because there is more matter to hold the energy.

Property of Heat

Heat flows from regions of higher temperature to regions of lower temperature, until they reach the same temperature.

Gaining and Losing Heat:

When heat flows to an object, the object has gained heat, and its temperature will increase.
When heat flows away from an object, the object has lost heat, causing its temperature to decrease.

Thermal Equilibrium:

A hot object (e.g., hot water) will lose heat and cool down until it reaches room temperature.
A cold object (e.g., ice cubes) will gain heat and warm up until it reaches room temperature.

Transfer of Heat

The temperature of hot water decreases after some time as the hot water loses heat to the surroundings.
The temperature of cold water increases after some time as the cold water gains heat from the surroundings.

Remember:
Hot objects → Lose heat to surroundings (temperature drops)
Cold objects ← Gain heat from surroundings (temperature rises)

Effects of Heat

1. Change of State:

Heat gain can cause a substance to change from solid to liquid — melting
Heat gain can cause a substance to change from liquid to gas — evaporation or boiling

States of Matter Cycle:

Process	Direction	Type
Solid → Liquid (Melting)	→	Heat gain
Liquid → Gas (Boiling)	→	Heat gain
Liquid → Solid (Freezing)	←	Heat loss
Gas → Liquid (Condensation)	←	Heat loss

2. Expansion and Contraction:

The gain or loss of heat can cause matter to expand or contract.
• When matter gains heat, it expands and occupies more space.
• When matter loses heat, it contracts and occupies less space.
• Mass will always stay the same!

	Expansion	Contraction
Cause	Object gains heat from a heat source	Object loses heat to surroundings
Effect	Expands — occupies more space (becomes bigger/longer)	Contracts — occupies less space (becomes smaller/shorter)

Thermal Expansion Experiment:

A flask containing air is placed in hot water.
The ink drop will move upwards.
The air in the flask gains heat from the hot water, expands and occupies more space, pushing the ink drop upwards.

Thermal Contraction Experiment:

A flask containing air is placed in ice water.
The ink drop will move downwards.
The air in the flask loses heat to the ice water, contracts and occupies less space, causing the ink drop to move downwards.

Conduction of Heat

Materials differ in their ability to conduct heat.

Good Conductors of Heat:

Metals are good conductors (e.g., iron, steel, copper)
Conduct heat from a warmer region to a cooler region quickly
Gain heat quickly → temperature increases quickly → cold substances become warmer faster
Lose heat quickly → temperature decreases quickly → hot substances become cooler faster

Poor Conductors of Heat:

Most non-metals (wood, rubber, plastic) and air are poor conductors
Conduct heat from a warmer region to a cooler region slowly
Gain heat slowly → temperature increases slowly → cold substances stay cool for a longer time
Lose heat slowly → temperature decreases slowly → hot substances stay warm for a longer time

Summary:
Good conductors (metals) = fast heat transfer
Poor conductors (wood, Styrofoam, air) = slow heat transfer — keep things hot/cold longer

Surface Area and Heat Transfer

As the surface area in contact increases, the rate of heat gain / heat loss increases.

Example: A wide beaker (X) heats up faster than a tall narrow test tube (Y) containing the same amount of water on the same hot plate — because X has more surface area in contact with the heat source.

⚠ Remember: Bigger contact area = faster heat transfer. Smaller contact area = slower heat transfer.

Key Takeaways

Heat is a form of energy — the Sun is the main source of heat on Earth.
Temperature measures hotness (°C); heat energy depends on temperature and mass/volume.
Heat flows from hot to cold until both reach the same temperature (thermal equilibrium).
Gaining heat → temperature rises; losing heat → temperature drops.
Effects of heat: change of state (melting, boiling) and expansion/contraction (mass stays the same).
Good conductors (metals) transfer heat quickly; poor conductors (wood, air) transfer heat slowly.
Larger surface area in contact = faster rate of heat gain/loss.