Convection

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Convection

Objectives

• Carry out a practical task to investigate convection.

• Describe the process of convection in terms of the movement and changing density of regions of fluid undergoing heating or cooling.

Task 1

We are going to investigate the cooling of water by convection. We will monitor the temperature, both at the top and the bottom of the beaker, every minute for 10 minutes.

Higher tier - we will also monitor a second beaker, with a lid, to see the difference in the rate of cooling.

Your teacher will give you a worksheet with a copy of the diagram and a table to record your results. Write the title “Investigating Convection” in your exercise book and stick in the worksheet.

(Note - higher tier will need this worksheet instead)

Task 2

We want to predict whether the water will be hotter at the top or the bottom of the beaker. Write the subtitle “Prediction” at the left hand side of the page and copy and complete the sentence below:

"I think the water will be hotter at the ______ of the beaker because..."

(Note to higher tier groups: forget about the second beaker for now - you only need to worry about the open beaker! )

Try to give a reason, using technical scientific language, to explain your prediction.

Task 3

Carry out the experiment according to the instructions on the worksheet. Record your results in the table provided.

Task 4

Your teacher will give you a copy of the text below. Fill in the gaps with the words provided.

(Higher tier will need this sheet instead)

Convection is taking place in the water. The water at the top of the beaker cools __________ rapidly than the water at the ______________, because it is in contact with the air which is at a lower temperature. These cooler regions of fluid _______________ so their density ________________ and they sink. Regions of warmer fluid, which are _________ dense, rise up to replace them. This means the water is warmest at the _________ of the beaker, and coolest at the _____________ of the beaker.

Convection is also taking place in the air above the water. The air at the __________________ of the water is warmed and so ______________ and decreases in density. It rises, carrying _________ energy away with it. _________ air sinks to replace it. If we add a lid, it stops the convection current in the air by preventing the warm air escaping, which slows down the rate of cooling.

Extension / MAT task

Plot a graph of the results for the open beaker. The points should form a smooth curve. Calculate the gradient of the curve at 1 minute and at 9 minutes.

· What does this gradient represent? What are the units?

· which of the two gradients is larger? Can you explain why?