Conduction of heat Target To know how heat flows by conduction in substances. Thursday 5 November 2009

Target

To know how heat flows by conduction in substances.

Thinking task Why does a piece of metal feel cold but a piece of wood at the same temperature feels warm? Answer: The metal conducts heat away from the hands but wood stops heat flowing from the hand.

Why does a piece of metal feel cold but a piece of wood at the same temperature feels warm?

Experiment to show conduction of heat Attach some drawing pins to a metal rod using Vaseline. Heat the rod with a Bunsen flame – when the heat reaches the pins the Vaseline melt and the pins fall off.

Attach some drawing pins to a metal rod using Vaseline.

Heat the rod with a Bunsen flame – when the heat reaches the pins the Vaseline melt and the pins fall off.

Showing conduction The experiment showed how heat was conducted through a metal rod. There are moving particles (electrons) inside the metal – the faster they move the higher the temperature – they bump into each other to allow the heat to flow through the metal.

The experiment showed how heat was conducted through a metal rod.

Liquids are poor conductors of heat [ insulators ]. WEAR SAFETY GLASSES What about gases? Carefully put your finger 1cm away from a Bunsen burner flame. If gases were good conductors, you’d burn your finger - you don’t. Gases are good insulators. Conduction

Copy this summary table into your book : Conduction

Bad points about conduction Allows heat to flow out of buildings in winter. Can cause burns when touching something hot (pan handle).

Allows heat to flow out of buildings in winter.

Can cause burns when touching something hot (pan handle).

Good points about conduction Allows heat to flow through the bottom of a cooking pan.

Allows heat to flow through the bottom of a cooking pan.

Using insulators We put gloves and jumpers on to keep warm in winter. We use place mats to protect tables from damage from hot plates. We reduce heat loss from houses with loft insulation.

We put gloves and jumpers on to keep warm in winter.

We use place mats to protect tables from damage from hot plates.

We reduce heat loss from houses with loft insulation.

A model for conduction In metals ‘free’ electrons easily carry the kinetic energy from one side to the other. In non metals inter atomic collisions are needed to carry the kinetic energy from one end of the material to the other. Questions: Hodder B Pages 28 and 29

In metals ‘free’ electrons easily carry the kinetic energy from one side to the other.

In non metals inter atomic collisions are needed to carry the kinetic energy from one end of the material to the other.