Measuring electric currents(Activity)



Part 1: measuring what?


What the activity is for

Having introduced the basic elements of the electric circuit model, attention is now turned to measuring electric currents. It is important that pupils have an understanding of what it is that they are measuring when they measure the size of electric currents.

What the activity is for

  • 12 volt DC power supply
  • 12 volt, 24 watt bulb in holder
  • some connecting leads
  • a demonstration rope loop, 3–5 metre long and 5–9 millimetre in diameter

  • What happens during this activity

    To help the pupils visualise what it is that is being measured, it is a good idea to start with a teaching model. Using the rope loop, draw your pupils' attention to the following points:

  • As the rope moves round, all parts of the loop move at the same speed.
  • The same amount (or length) of rope passes each point in the circuit in a given time.
  • If a bigger push/pull is provided by the battery, more rope passes each point in a given time (as the rope all around the circuit moves more quickly).
  • If a smaller push/pull is provided by the battery, less rope passes each point in a given time (as the rope all around the circuit moves less quickly).
  • You should make the link between:

    Teacher: The amount of rope passing in a given time and the charge passing per second (current).

    Part 2: measuring how?


    What the activity is for

    The purpose of this part of the activity is to demonstrate how to use an ammeter to measure electric currents.

    Teacher Tip: For this and other demonstrations we recommend using mounted 12 volt, 24 watt bulbs (car headlamp bulbs) with a 12 volt DC power supply and a demonstration ammeter.


    What happens during this activity

    Demonstrate how to connect the ammeter by talking through and demonstrating the following sequence with the pupils:

    NumberList

  • Make the complete battery/bulb circuit.
  • Teacher: So we make the connection from the positive side of the supply to the bulb and then from the bulb back to the negative terminal. Switch on and… hola!… the bulb lights.

  • Make a gap and connect in the ammeter.
  • Teacher: OK, switch off again. Now then! Where do you think the ammeter might go? Does it matter where in the circuit you connect it? Why? Now let's make a gap in the circuit where the meter is to go. So remove this lead and place the ammeter here.

  • Connect the ammeter the right way round.
  • Teacher: We then need to connect the ammeter the right way around. The ammeter has a positive terminal and a negative terminal. The positive terminal of the ammeter must be connected to the positive side of the supply.

    EndList

    The ammeter goes in the circuit

    Teacher: Switch it on, and the ammeter gives us the current reading. Every time you use an ammeter I want you to follow this same procedure. Make the circuit; make a gap in the circuit; and be careful to connect the ammeter the right way around.

    Emphasising the point of making a gap in the circuit and placing the ammeter in the gap helps to promote an understanding of electric current measurement. All of the charged particles flowing around the circuit must pass through the ammeter. All of the passing charged particles are counted in the ammeter.



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