Posted in Physics Revision

Unit P11: Energy; How It’s Transferred

P1 1.1, .3, .4

The three ways energy can be transferred are conduction, convection (both require particles) and infrared radiation (does not require particles).

Infrared Radiation

Definition: “Electromagnetic radiation that we can feel as heat. IR has a longer wavelength than visible light, but a shorter wavelength than microwaves.”

How it works: thermal energy is transferred from one place to another as an electromagnetic wave (more on that in P15). These are emitted and absorbed by everything, but some materials are better at absorbing or emitting than others. This radiation can occur in a vacuum, and so does not rely on particles to transfer heat energy.

Light-coloured shiny surfaces → good at reflecting radiation → poor at absorbing it.

Dark-coloured matte surfaces → poor at reflecting radiation → good at absorbing it.

Infrared can be used on thermal imaging cameras to detect people in the dark. It is how energy from the sun reaches earth → it is often used to heat water in less developed countries.

Conduction

Definition: “The transmission of temperature changes through a material (usually a solid) by transfer of vibration from particles to their neighbours”.

How it works: If a heat source is applied to one end of a material (e.g. a metal rod), those particles will gain kinetic energy → they will start to vibrate more vigorously, but remain in a fixed position. These then bump into their neighbouring particles, which gain kinetic energy, and so on, until the whole material is heated.

Conductor → material which is good at transferring energy by conduction. Metals are good conductors as they have free electrons which can move through the material and pass on the energy → they move faster when heated as they have more kinetic energy. The same principle applies with electricity.

Insulator → material which is poor at transferring energy by conduction → often used to reduce unwanted heat loss (e.g. in the walls of houses).

Convection

Definition: “Movement of particles in a fluid (a gas or a liquid) depending on their temperature. Hotter, less dense regions float, and cooler, denser regions sink.”

How it works: when a fluid is heated, the particles that form it gain kinetic energy. This means they move around more, and so move further apart from each other. This results in them taking up more space, so the fluid becomes less dense. It’s really important to remember that the fluid is becoming less dense, not the particles which make up the fluid. The less dense fluid is then able to rise up above the cooler fluid around it. The cooler fluid is denser than the hotter fluid, so sinks down below the hotter fluid. Then the hotter fluid cools and sinks down again → it’s a cycle.

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