SCIENCE 10.1 REVISION

What is a convection current?

A convection current is the cyclic movement of particles of a fluid (i.e. a liquid or gas) as a result of heating and cooling through the medium. In this process, when a fluid is heated, the particles near the source of heat gain energy, move faster and spread out. The particles of the cooler fluid nearby are moving more slowly and are less spread out. This cooler fluid is denser and sinks, forcing the warmer, less dense fluid upwards. As the warmer fluid rises, it cools and becomes denser and eventually sinks, replacing the newly heated fluid.

Why is air a poor conductor of heat?

Air is a poor conductor of heat as the particles are spread far apart.

Why do many sportspeople wear white clothing when competing on hot summer days?

Clothes that are white in colour reflect heat rather than absorbing it, minimising the transfer of heat to the wearer.

To which form of heat transfer do the following statements apply? This form of heat transfer is best explained by a wave model.

Conduction

To which form of heat transfer do the following statements apply? Particles move from one place to another.

Convection

Use the particle model of matter to explain why heat can't travel through metals and other solids by convection.

Convection can only occur in fluids, which have particles that are free to move around. In liquids and gases, heat can be transferred from one region to another by the actual movement of particles. In solids, the particles vibrate about a fixed position and convection does not occur.

Explain, with the aid of a diagram, how a coastal sea breeze results from convection currents.

During hot summer days, radiant energy from the sun heats the land and the sea. However, as a result of the different properties of the land and water, after a few hours the land has a higher temperature than the sea. The air near the land becomes hot as a result of conduction. As this air gets hot it expands, becoming less dense than the cooler, denser air over the sea. The air over the sea rushes in towards the land, replacing the rising warm air, causing a sea breeze.

Which form of electromagnetic radiation from the sun is responsible for most of the radiant heat reaching the Earth?

Infra-red

Advertisements in the media often describe all products designed to reduce heat transfer to keep homes warm in winter or cool in summer as insulators. Explain why it is incorrect to describe reflective metal foil as insulation.

Insulators are materials that prevent the transfer of heat through conduction or convection. Reflective foils prevent radiant heat being absorbed.

List three things that can happen to radiant heat when it arrives at any surface.

It can be transmitted, reflected or absorbed.

Which form of energy do particles transfer to each other as heat flows through a conductor?

Kinetic energy

Explain why solids such as polystyrene foam and wool are poor conductors of heat.

Materials such as polystyrene, foam, wool and fiberglass bats are effective insulators because they contain pockets of still air. Air is a very poor conductor of heat as the particles are spread far apart and therefore it is harder for kinetic energy to be transferred between them.

Identify the features of a vacuum flask that reduce heat transfer by: conduction

Partial vacuum between the double glass walls of the bottle; plastic or cork stopper

Identify the features of a vacuum flask that reduce heat transfer by: convection

Partial vacuum; plastic or cork stopper

Suggest why metal saucepans usually have plastic or wooden handles.

Plastic and wood are poor conductors of heat. As a result, they will remain cool to the touch when the metal sections of the saucepan become very hot.

To which form of heat transfer do the following statements apply? Energy is transferred at the speed of light.

Radiation

To which form of heat transfer do the following statements apply? No particles are required for energy transfer.

Radiation

Identify the features of a vacuum flask that reduce heat transfer by: radiation

Silvered facing surfaces of the glass bottle

When you heat water in a saucepan on a gas or electric hotplate, all of the water gets hot and not just the lower part near the heat source. How does this happen, given that water is a poor conductor of heat?

Water may not transfer heat very well by conduction, but it does transfer heat very easily by the process of convection.

Explain why air near a wall furnace rises when it gets warmer.

When air is heated, the particles near the source of heat gain energy, move faster and spread out. This means that the warm air now has a lower density than the cooler air around it, so it rises.