When the bulb gives out light, is some of the electricity used up?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
Explain current as a flow of charge round a circuit
Explain that current is measured in amps with an ammeter
Press the switch of a torch and the bulb lights at once. The cell, the wires and the bulb make one loop. Something moves round that loop. We call it the current.
A metal wire is full of tiny particles called electrons. Each electron carries a tiny amount of charge. The cell pushes all the electrons round the loop together. A flow of charge is a current.
A model. The beads are charges in a loop of wire. The cell pushes them all round at once.
Think of a ring of beads in a tube. Push one bead and all the beads move. No bead is lost. The same number of beads pass every spot each second. A current is like that.
Draw it first
Draw the current round a loop
1 Draw a loop with a cell, a bulb and a closed switch. The bulb is lit.
2 Draw arrows inside the loop. They leave the cell by the long plate and go round the loop. Write current.
3 Write + beside the long plate and − beside the short plate. The arrows run from + round to −.
In a drawing, the current runs from the + end of the cell, round the loop, to the − end. That is the rule.
The electrons in the wire really drift the other way round. The sums come out the same either way, so we keep to the drawing rule.
Electrons are the moving charges in a metal wire. They carry a negative charge. They drift from the − end of the cell to the + end. We draw the current the other way round, from + to −. Keep the two ideas apart in your mind.
We measure current in amps. We write A. A torch bulb might take 0.3 A. An ammeter is the meter that reads the current. It must be part of the loop, so the whole current goes through it.
See it three ways
As a picture
In words
The ammeter sits in the loop, so the current goes through it. It shows how much charge flows past each second. A bigger reading means a bigger current.
In symbols
The reading is 0.3 A. That is 0.3 amps. The symbol for amps is A.
Now put an ammeter on each side of the bulb. If the bulb used up current, the second ammeter would read less. Look.
Both ammeters read 0.3 A. The current is the same on each side of the bulb.
The current is the same all round the loop. The bulb does not use it up. The bulb changes energy from the cell into light and heat. The charges keep going round.
Worked example
In the bead model, 30 beads pass spot A in 10 seconds. How many pass each second? How many pass spot B each second?
The current is how many beads pass a spot each second.
Spot A: 30 beads in 10 s. 30 ÷ 10 = 3 beads each second.
The beads are all in one loop and none is lost. The same number pass spot B each second.
Spot B: 3 beads each second.
Answer: 3 beads each second, at both spots
Try it
Count beads round a loop
You need: a string loop about 1 metre round, 12 beads, seeds or knots on the string, evenly spaced, two stones to mark spot A and spot B, a clock, or a friend who counts seconds.
Be safe: Keep the string loop off your neck and away from small children.
Fix the beads on the string loop at equal gaps. Lay the loop in a ring on the ground.
Put a stone beside the loop at spot A and another at spot B, on the other side.
Pull the string round with your fingers at a steady pace. You are the cell.
Count the beads that pass spot A in 10 seconds. Then count at spot B.
What you will see: The two counts are the same, or off by one if you lose count. Ours were 14 and 14. The string is one loop, so the same number of beads pass every spot. Your counts will differ from ours.
If you cannot try it, this is what happens.
A wrong idea many people have
The bulb uses up the current, so less current comes back to the cell.
Why it feels right: The bulb gives out light and heat, so it feels as if something has been used up.
The test that breaks it: Put an ammeter on each side of the bulb. The first reads 0.3 A. The second also reads 0.3 A.
What is true: The bulb changes energy into light and heat. The charges are not used up. The same current goes all round the loop.
Key words
current
a flow of charge round a complete circuit.
charge
a tiny amount of electricity carried by a particle.
electrons
tiny particles that carry charge. They move in a metal wire.
amps
the unit of current. We write A.
ammeter
a meter that reads the current. It goes in the loop.
Back to your guess
The answer is A: No. The same current goes all the way round the loop.
The same current goes all the way round the loop. The bulb changes energy into light and heat, but the charges are not used up. They keep going round.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
What makes up the current in a metal wire?
See the answer
Electrons moving round the loop, each carrying a little charge.
A current is a flow of charge. In a metal wire the charges are electrons, and they keep moving round the whole loop.
If you chose A: The bulb does not use up electrons. It changes energy into light and heat while the electrons keep going round. Go to S3.10.1.
If you chose C: The light is made in the bulb. The current is charge moving in the wire. Go to S3.10.1.
Ammeter A1 reads 0.2 A. What does ammeter A2 read, on the other side of the bulb?
See the answer
0.2 A
The current is the same all round a series loop. The bulb does not use it up, so A2 reads 0.2 A.
If you chose A: You may think the bulb uses up half the current. It does not. The same current goes all round the loop. Go to S3.10.1.
If you chose B: If the bulb used it all up, no current would come back. But the same current goes all round the loop. Go to S3.10.1.
Where must you put an ammeter to measure the current through a bulb?
See the answer
In the loop, so the current goes through the ammeter.
An ammeter measures the current that goes through it. So it must be part of the loop.
If you chose B: That is how a voltmeter is joined. An ammeter must be in the loop, so the current goes through it. Go to S3.10.1.
If you chose C: The current is the same all round the loop, so the ammeter can go anywhere in it. It must be part of the loop. Go to S3.10.1.
Ravi reads 0.4 on his ammeter. Which unit does he write after the 0.4?
See the answer
A, for amps.
Current is measured in amps, written A. An ammeter reads in amps.
If you chose A: Volts measure the push of a cell, which is the next chapter. An ammeter reads current. Go to S3.10.2.
If you chose C: Metres per second measure speed. Current is measured in amps. Go to S3.7.1.
A cell has a long plate (+) and a short plate (−). In a circuit drawing, which way do we draw the current?
See the answer
From the + end of the cell round the loop to the − end.
We draw the current from + round to −. The electrons drift the other way, but the sums come out the same.
If you chose A: Electrons do drift that way, but the drawing rule is the other way round, from + to −. Go to S3.10.1.
If you chose B: A cell pushes the current one way round the loop. It does not leave by both ends. Go to S3.10.1.
Joseph's first ammeter reads 0.25 A. What does the ammeter on the other side of the bulb read?
See the answer
0.25 A
The current is the same on each side of the bulb, so it also reads 0.25 A.
Keep it
Copy this into your exercise book.
Current is a flow of charge round a loop. We measure it in amps with an ammeter in the loop. It is the same all round and is not used up.
Draw: A loop with a cell, a bulb and two ammeters, both reading 0.3 A. Draw arrows from + round to −.
