Electrical Circuit
Electricity and Magnetism

Calculating for understanding

Classroom Activity for 14-16 Supporting Physics Teaching

What the Activity is for

Exemplifying good practice – students have something to copy.

There are many calculations to do in making predictions for electric circuits. These involve quite a few quantities.

What to Prepare

  • a well practised hand on the board, together with some calculations that you have run through beforehand – it's not easy doing these live in front of a class
  • a number of relationships, carefully laid out with their units (These are the toolkit, each tool to be selected and used as needed. Don't be caught out using a hammer where a screwdriver is appropriate.)
  • support sheet (see below)

What Happens During this Activity

Select things to be calculated where you are confident that you can explain the strategy – linking each line of calculation to the final goal. That means tuning what you do to your own confidence and to the abilities and aptitudes of the class. Do try to avoid just listing the quantities and then guessing which tool is appropriate. The calculations should be a part of an explanatory story that's being woven about the circuit.

Some rules of thumb to help this to come true:

  • Don't hurry.
  • Always refer to a diagram of the circuit where the loop in which you're working can be identified.
  • If dealing with only a part of the loop, redraw only that part and mark in the quantities explicitly.
  • Always write down the relationship, then substitute in values with units, then evaluate.
  • Never use raw numbers: it's physical quantities that you're dealing with and they consist of number and unit. (This is using quantity algebra, and it is strongly recommended as a means of clarifying and checking thinking throughout.)
  • Base everything on only a few relationships.
  • It bears repeating: don't be tempted to hurry.


Download the support sheet / student worksheet for this activity.

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