# Heat Transfer: Conduction, Convection and Radiation

This topic is part of the HSC Physics course under the section Thermodynamics.

### HSC Physics Syllabus

• investigate energy transfer by the process of:
– conduction
– convection

### What is Heat?

Heat is the transfer of energy from a system with higher temperature to one with lower temperature. Heat transfer occurs in one of three ways: conduction, convection and radiation.

### Conduction

Thermal conduction is the transfer of heat through a material due to the vibration of atoms and molecules in a given material, without the material itself moving. The transformation of heat into a particle's vibration kinetic energy causes it to collide more frequently with neighbouring particles. The collision between particles allows energy to be transferred through the material in thermal conduction.

Thermal conduction occurs mostly in solids. In a solid, particles are closely packed together. When one particle vibrates because of an increase in energy (temperature), it bumps into its neighbours, transferring some of its energy to them. This chain reaction continues throughout the material until energy has been spread evenly.

Liquids can also conduct heat, but because their particles are not fixed in a lattice, the process is less efficient than in solids. In liquids, particles are free to move about and collide with each other, allowing energy to be transferred through these collisions. However, convection is often a more dominant form of heat transfer in liquids.

Thermal conduction in gases is the least efficient compared to solids and liquids because the particles in a gas are much further apart. As in liquids, the energy is transferred through collisions between particles, but these collisions are less frequent due to the greater distances between particles in a gas. Here too, convection is typically a more significant mechanism for heat transfer.

How well a material transfers energy via conduction is described by its thermal conductivity. The concept of thermal conduction is discussed in greater detail here.

Examples of conduction in everyday life:

• Metal spoons get hot when left in a hot drink.
• Ironing clothes. The heat from the iron plate is conducted to the clothes, making them warm.

### Convection

Convection involves the movement of fluids (liquids and gases).

• Like conduction, when fluids are heated, the particles that make up the fluid gain kinetic energy.
• Unlike conduction, convection involves the movement of the fluids as well.

Hotter fluids generally have lower density than cooler fluids because of thermal expansion. When a fluid (which could be a liquid or a gas) is heated, its particles gain kinetic energy and begin to move more rapidly. This increased motion causes the particles to occupy more volume on average—in other words, the fluid expands.

As a fluid becomes hotter, it becomes less dense and rises. Conversely, cooler fluid is denser and sinks. This is why hot air rises and cool air sinks.

What is a convection current?

Hotter regions of a fluid rise because they are less dense, and cooler regions sink because they are denser. This cycle creates a convection current which transfers heat from the bottom of a fluid to its top.

For example, consider a pot of water being heated on a stove. The water at the bottom of the pot, in direct contact with the heat source, warms up first. As this layer of water heats, it expands slightly and becomes less dense, causing it to rise. As it moves upwards, the cooler water above sinks down to take its place and is subsequently heated. This cyclic process of rising warm water and sinking cooler water creates convection currents that effectively distribute the heat throughout the pot.

Examples of convection in everyday life:

• Boiling water in a pot. The hot water at the bottom rises to the top, and the cooler water at the top sinks to the bottom.
• Sea breezes. During the day, land heats up faster than the sea. The hot air above the land rises, and the cooler air from the sea rushes in to take its place.

Radiation involves the transfer of energy through electromagnetic waves, specifically infrared radiation in the context of heat transfer. Unlike conduction and convection, radiation does not require a medium; it can occur in a vacuum without the presence of any particles.

The following radiation curve shows the intensity of various types of electromagnetic waves emitted by objects at different temperatures.

All objects emit electromagnetic radiation in the form of infrared rays. Hotter objects emit more radiation than cooler ones. As the temperature of the object increases, it emits more radiation, and also begins to emit other types of electromagnetic waves such as visible light and ultraviolet rays (shown above).

#### Features of the Radiation Curve

• Peak Wavelength: The peak of the radiation curve corresponds to the wavelength at which the intensity of radiation is highest. This peak shifts to shorter wavelengths as the temperature increases (Wien's displacement law).

• Temperature Dependence: Higher temperatures result in higher intensities at every wavelength and a shift of the peak to shorter wavelengths. Conversely, cooler objects emit less intense radiation and peak at longer wavelengths.

• Colour of Hot Objects: The shift in peak wavelength with temperature is why hot objects change colour as they get hotter. For example, a heating iron may glow red, then orange, and finally white as its temperature increases.

When looking at the Sun's radiation curve, its surface temperature causes it to emit a spectrum that peaks in the visible range, which is why sunlight appears bright to our eyes. In contrast, a household radiator at room temperature emits a peak wavelength in the infrared, which our eyes cannot see, but we can feel as heat.

Examples of radiation in everyday life:

• Feeling warmth from the Sun, even in space where there's a vacuum.
• Feeling the heat from a campfire, even if you are standing far away from it.