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Measuring Current
Learning Objectives:
• Describe current as a flow of electrons.
• Link amount of cells in a circuit to the size of the current.
Measuring electric currentWe can measure the amount of electric current flowing through a circuit using a device called an “ammeter”.
Current is measured in Amperes.
A
What is the trend here?
2 electrons per second = 1A
8 electrons per second = 4A
4 electrons per second = 2A
12 electrons per second = 6A
Rich questions:
• Which way do electrons flow?
• Do electrons always flow the same way around a circuit?
Measuring Current
• How much the current is flowing is the number of electrons passing a point every second.
• Therefore current depends on how many electrons are moving and how fast they are moving.
• Current & Voltage in a circuit
The unit of measure for current is the amp, which has the symbol A.
A
Measuring current
AA
Current is measured using a device called an ammeter. In a circuit diagram, an ammeter is shown by the symbol .
When measuring the current through a component, the ammeter is always connected in series (in the same loop) with that component.
1
2
1. Set up the circuit as shown above.
R1
A
A
Experiment 1: Current in series circuitCircuit 1
2. Measure the current using the ammeter at positions 1 and 2.
3
1. Add another lamp into the circuit (R2) and another ammeter after it.
R1
A
A
A
R2
Experiment 1: Current in a series circuitCircuit 2
1
2
2. Now measure the current using the ammeter at positions 1, 2 and 3.
Circuit 2 results:
Current at position 1 =
Current at position 2 =
Current at position 3 =
Circuit 1 results:
Current at position 1 =
Current at position 2 =
Experiment 1: Current in a series circuit – results
R1A
A
Circuit 1 Circuit 2
R1 R2A
AA1
2
1
2
3
Draw each circuit and trace the path of the electrons through the circuit
Experiment 1: Current in a series circuit – summary
Circuit 1The current at different positions in the circuit, before and after the lamps, was the _______.
Current is _____ used up by the components in the circuit.
Circuit 2Increasing the number of components in the circuit ________ the current.
The current at all points in a series circuit is the _______.
same / same / decreased / not
Circuit 1 Circuit 2
R1A
A R1 R2A
AA
Current in a series circuit
If the current here is 2 amps…
The current here will be…
The current here will be…
And the current here will be…
In other words, the current in a series circuit is THE SAME at any
point
Compare and contrast
• Do not attempt to answer the question initially• Discuss with neighbour which of two questions
do you think is highest level and why?Level• Current is a flow of electrons• Would there be more or less electrons flowing if
the current was large?Level • Current is a flow of electrons• Which of the three currents (2A, 5A or 10A)
would have the largest flow of electrons?
Next lesson
• What do you think we should be learning next lesson?
Measuring Current
Learning Objectives:
• Describe current as a flow of electrons.
• Link amount of cells in a circuit to the size of the current.