Measuring Current and Voltage - Activity

Introduction

A kettle heats water because of an electric current passing through the kettle element. The designer must design it to boil water as quickly as possible. A big current and voltage means more energy gets into the water faster. In the UK, a kettle can draw no more than 13 A with a rated mains supply voltage of 230 V. The designer uses an ammeter to check her kettle current, and a voltmeter to check the voltage.

Task 1: Measuring voltage

1. Open the Yenka Model 1.
2. Add a voltmeter to the simulation from the Lab Equipment > Measurement folder.
3. Drag a wire from each end of the meter. Place one wire at A and one at B.
4. Read the voltage and make a note of it. If the reading is negative, you have connected it the wrong way round. Either turn the voltmeter round, or cross the connecting wires over.
5. Draw a circuit diagram for a circuit to measure the voltage across L2. Use circuit symbols.
   Answer

   
   Answer
6. How many extra wires do you need to bring to the circuit?
   Answer

   Two.
7. Do you need to break into the circuit to use a voltmeter?
   Answer

   No.
8. How could a poorly designed voltmeter affect the current in the circuit?
   Answer

   Low resistance pathway would allow current to by-pass bulb.

Summary

A voltmeter must be connected in parallel with a component - we often describe it as 'connecting across' the component. It should have as high a resistance as possible to stop it affecting the circuit.

Task 2: Measuring current

1. Keep Yenka Model 1 open.
2. Disconnect the wire between B and C.
3. Place an ammeter in the gap between B and C. Connect its terminals to B and C with new wires.
4. If the reading on the ammeter is negative, you have connected it the wrong way round. Either turn the ammeter round, or cross the connecting wires over.
5. Draw the circuit diagram for the circuit you have just made. Use circuit symbols.
   Answer

   
   Answer
6. Do you have to break into the circuit to make it work?
   Answer

   Yes.
7. How many extra wires do you need to bring to the circuit?
   Answer

   One.
8. How could a badly designed ammeter affect the current in the circuit?
   Answer

   High resistance reduces current in circuit.

Summary

An ammeter must be connected in series with other components in a circuit. This means you add an ammeter in much the same way that you add a new daisy into a closed daisy chain. An ammeter should have as low a resistance as possible to stop it affecting the circuit.