Principles of Thermal Energy and Heat Transfer

Understanding Heat and Temperature

Heat is the transfer of energy from hot bodies to cold bodies. It is also called thermal energy. A heat source is an object hot enough to transfer heat to another object that receives it.

Temperature is an indicator of the amount of heat a body has, which is related to the mobility of its particles. The more energy a body stores, the more internal energy its particles will have.

Thermal Equilibrium occurs if a hot body and a cold body are put in contact. The hot body will transmit heat to the cold one until both reach the same temperature.

Primary Sources of Thermal Energy

The main sources of heat include:

• The Sun: The sun provides heat naturally. It is a large ball of fire that gives light and heat to the Earth.
• Fire: This is an artificial source of heat. Thermal energy is produced from the combustion that takes place when something is burned.
• Electrical Resistance: Heat is produced due to the resistance that the particles forming conductive cables oppose to the flow of electricity.

How Heat and Temperature Are Measured

The Thermometer

The thermometer is an instrument used to accurately measure the temperature of bodies. To measure temperature, the thermometer is put in contact with the object (solid, liquid, or gas) until thermal equilibrium is reached. The lowest temperature that can theoretically be reached is -273°C.

Mass and Specific Heat

The greater the mass to be heated, the greater the number of particles that need to increase their mobility, and therefore, more heat must be supplied.

Each substance has a characteristic called specific heat, which is the amount of energy that must be supplied to a unit of mass to increase its temperature by one degree. The higher this value, the more heat the substance needs to raise its temperature by the same amount.

Physical Effects of Thermal Energy

Dilation and Contraction

Dilation is the phenomenon by which almost all substances increase their volume when heated, which causes their density to decrease. The opposite effect is called contraction, which occurs when substances are cooled and their volume decreases.

The increase in a substance's volume is more pronounced the greater the mobility of its particles. This is why the effect is greatest in gases, then in liquids, and to a lesser extent in solids.

The Anomalous Dilation of Water

While most substances expand when heated and contract when cooled, the opposite occurs with water between 0°C and 4°C. This has important implications, such as icebergs floating on water because their density is lower.

Changes of State

If we heat a solid until it reaches its melting temperature (or fusion point), the thermal energy supplied causes its particles to transition from a solid to a liquid state. The transition from solid to liquid requires the substance to absorb heat. Solidification is the reverse process, from a liquid to a solid state, where the substance must release heat to be cooled.

How Heat is Transferred

Conduction

Conduction is the process by which heat is transmitted through solid substances without the movement of matter itself.

Convection

Convection is the transfer of heat through the movement of matter. It is the primary way heat spreads in liquids and gases.

Conductors and Insulators

Materials that allow or prevent the passage of heat can be classified as conductors or insulators, which are selected for different uses based on these properties.