Measuring the flow of current
In this topic you will:
- learn the unit for measuring current
- learn how to measure current
- learn a rule about current in series circuits.
Key Words
- amps
- in series
Measuring current
In Topic 9.1 you saw that electrons move when current flows.
When electrons move more quickly, the current increases.
When electrons move more slowly, the current decreases.
In Topic 9.2 you saw that you can use an ammeter to measure current.
Current is measured in units called amps. Amps have the symbol A.
The picture shows two different ammeters.
Look carefully at the ammeters in the picture. They both have two terminals, for wires to be attached. One terminal is red and the other terminal is black.
In a circuit, the red terminal of the ammeter must be connected to the positive terminal of the power supply. The red terminal may be connected directly, or through other components, to the positive terminal of the power supply.
Both ends of the ammeter are the same in a circuit diagram. You do not need to show the red and black terminals on the circuit symbol for an ammeter.
Ammeter are always connected in series with other components in a circuit. If the components are connected in series they are all connected end-to-end, one after another, and there are no branches in the circuit.
The drawing shows an ammeter connected in series with a battery, a switch and a lamp. The circuit diagram shows the same circuit.
Common Mistake
Don't forget: an ammeter must be connected in series with other components, not in parallel. Connecting it incorrectly could cause errors or damage.
Questions
Show Answer
Amps (A)
Write down the current shown on each ammeter. Include the unit in your answer.
Show Answer
A: 2.50 A
B: 5 A
C: 6 A
D: 1500 A
Each word may be used once, more than once or not at all.
Show Answer
faster, electrons
He wants to measure the current in the circuit.
Which is the correct way to connect the ammeter in this circuit.
Show Answer
Diagram B
Think Like a Scientist
In this task, you will make and test predictions about current in a series circuit using an ammeter at different points.
• Cell or cells that can power two lamps in series
• Switch, lamps, wires, connectors
• Ammeter
- Connect the circuit with two lamps and a switch in series. Do not include the ammeter yet.
- Copy the circuit diagram.
- Add arrows to your diagram to show the direction of electron flow.
-
You will measure current at positions X, Y, and Z using the ammeter.
- Predict what will happen to the current. Choose one:
- C Current increases as it goes around
- B Current decreases as it goes around
- A Current stays the same throughout the circuit
- Explain your prediction.
- Connect the ammeter at position X.
- Close the switch and record the reading.
- Repeat for positions Y and Z, opening the switch between changes.
- Copy all three diagrams with ammeter positions.
- Write the ammeter readings next to each diagram.
- Describe the trend in current readings.
- Was your earlier prediction correct?
- Explain your results using ideas about electron movement in wires.