ConvectionConvection

TargetTarget

To be able to explain how convection To be able to explain how convection takes place in liquids and gases.takes place in liquids and gases.

Friday 7 April 2023Friday 7 April 2023

Experiment -What is convection?Experiment -What is convection?

Observe the movement of the pink colour Observe the movement of the pink colour when water containing potassium when water containing potassium permanganate is heated. How does the permanganate is heated. How does the water move? Explain the movement.water move? Explain the movement.

The purple colour The purple colour from the from the dissolving dissolving potassium potassium permanganate permanganate rises showing the rises showing the flow of hot water flow of hot water in the convection in the convection current.current.

Ice and boiling water in the same Ice and boiling water in the same boiling tube!boiling tube!

Copper wire holds the ice Copper wire holds the ice at the bottom of the at the bottom of the boiling tube, the top is boiling tube, the top is heated.heated.The heat from the boiling The heat from the boiling water at the top of the water at the top of the tube does not reach the tube does not reach the ice at the bottom because ice at the bottom because a convection flow of heat a convection flow of heat cannot occur.cannot occur.

ConvectionConvection• Convection takes Convection takes

place when a hot place when a hot liquid or gas at a liquid or gas at a low level rises and low level rises and cold liquid or gas cold liquid or gas falls.falls.

• Convection does Convection does not take place in not take place in solids because the solids because the particles in a solid particles in a solid cannot move.cannot move.

Showing convection in a gas.Showing convection in a gas.

Smoke enters the Smoke enters the metal box, it falls metal box, it falls in the cool air and in the cool air and then rises once then rises once the air is heated the air is heated by the candle by the candle flame.flame.

Hot air balloonsHot air balloonsA hot air balloon works because the hot air inside the A hot air balloon works because the hot air inside the balloon has a lower density than the air surrounding it.balloon has a lower density than the air surrounding it.We can make a simple one using a plastic bag.We can make a simple one using a plastic bag.A real one can lift a lot of people in a basket.A real one can lift a lot of people in a basket.

ExplanationExplanation

• Hot gases or liquids Hot gases or liquids have a lower have a lower density than cold density than cold gases or liquids so gases or liquids so they rise.they rise.

• Cold gases or Cold gases or liquids have a liquids have a higher density so higher density so they fall.they fall.

Convection versus conductionConvection versus conduction

In CONDUCTION the heat was passed on by VIBRATIONS in a SOLID

In CONVECTION the heat is passed on by the FLUID expanding, rising and TAKING THE HEAT with it

Some questions on Some questions on convection…convection…

1) Freezers in supermarkets are often left open to the air. Explain why the food does not melt easily.

2) Explain why a hot air balloon rises in the air.

3) Explain why an ice cube floating at the top of a drink will cool all of the drink.

Question 4Question 4

Draw a diagram of a Draw a diagram of a candle like the one candle like the one shown. Add an shown. Add an arrow to show how arrow to show how heat from the flame heat from the flame will start a will start a convection current. convection current.

Question 5Question 5

Ice floats at the top Ice floats at the top of a drink. Explain of a drink. Explain how this cools the how this cools the drink at the bottom drink at the bottom of the glass. of the glass.

Question 6Question 6

• Take-away containers for hot drinks Take-away containers for hot drinks usually have a lid on them. Explain why usually have a lid on them. Explain why this is a good idea.this is a good idea.

Question 7Question 7• Copy the diagram of the hot Copy the diagram of the hot

water tank.water tank.• aa Draw an arrow to show Draw an arrow to show

which way the heated water which way the heated water travels away from the element. travels away from the element. Label it Label it ‘‘heated waterheated water’’..

• bb Draw an arrow to show Draw an arrow to show the flow of colder water within the the flow of colder water within the tank. Label it tank. Label it ‘‘colder watercolder water’’..

• cc The heating element is The heating element is quite close to the bottom of the quite close to the bottom of the tank. Why is the hot water outlet tank. Why is the hot water outlet not at the bottom of the tank?not at the bottom of the tank?