Electric circuits · Model
Current and circuits
Put an ammeter in front of the bulb and behind it. Both read exactly the same. Whatever the bulb is doing, it is not using up the thing that flows.
Start here
Snip the wire anywhere. The bulb goes out.
A torch has a cell, a bulb and two strips of metal joining them into a ring. Cut the ring on the way to the bulb and it goes dark. Cut it on the way back from the bulb and it goes dark just the same.
Why does a gap on the far side of the bulb matter, when the electricity has already been past it?
A current is a flow round a ring, not a delivery from one place to another. The wire is already full of free electrons before you switch anything on, and the cell pushes on all of them together. Open a gap and there is nowhere for the charge to go and nothing to take its place, so the whole ring stops — both sides of the bulb, instantly.
Every metal is full of electrons that are free to move. They are already there, spread all the way round the wire, before anything is switched on. A cell does not fill the wire with them; it pushes on the ones already in it. When they all shuffle along together in the same direction you have an electric current: a flow of charge.
Because the flow has to be a flow all the way round, a circuit must be a complete loop. Break it anywhere and everything stops everywhere, instantly — there is nowhere for the charge to go and nothing to take its place. That is why a gap behind the bulb is as fatal as a gap in front of it.
Current is measured in amperes, shortened to amps and written A. You measure it with an ammeter, which goes in the loop so the current runs through it. One amp is a big current for a classroom circuit; a torch bulb usually draws a few tenths of an amp, so readings like 0.30 A are normal.
At the bench · one loop, one meter, three places to put it
Move the meter. Watch the reading.
Change a control to begin
One cell holder, one bulb, one switch and one ammeter. The meter can go in three places round the loop. Add cells, open and close the switch, and move the meter.
Commit first. The meter sits between the switch and the bulb and reads 0.30 A. You move it to the far side of the bulb, on the way back to the cell. What does it read there?
Cells in the holder
The switch
Where the ammeter goes
The cells push with
—
—
The loop is
—
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The ammeter reads
—
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The bulb is
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The figure
The symbols a circuit diagram is written in
A circuit diagram is not a drawing of the apparatus. It is a set of agreed symbols joined by straight lines, so that anyone anywhere can build the same circuit from the same picture. These eight cover almost everything in this unit.
Cell
long line is +
Battery
two or more cells
Lamp
a bulb
Switch
drawn open
Ammeter
goes in the loop
Voltmeter
goes across a part
Resistor
a fixed value
Variable resistor
you can turn it up
Two of these already tell you something. An ammeter is drawn in the line, so the current goes through it. A voltmeter is drawn as a loop off to one side, across a component, because it is not measuring a flow at all.
Key fact
An electric current is a flow of charge — electrons already in the metal, all drifting the same way. It needs a complete loop, and in a single loop it is the same size everywhere. It is measured in amperes (A) on an ammeter, which is placed in the loop.
Think again
“The bulb uses up the current, so there is less of it coming back than going in.”
Two ammeters, one either side of the bulb, read the same to the last digit. Nothing is consumed. What the bulb takes is energy, which is carried by the moving charge and left behind as light and heat, and energy is not the same thing as the charge doing the carrying. Think of a bicycle chain: the chain is not used up by the back wheel, but it does deliver something on every turn.
“The electricity has to get from the cell to the bulb, which is why there is a tiny delay when you flick the switch.”
The electrons were already there, all the way round, standing in the wire like water in a full pipe. Closing the switch pushes on all of them at once, so the far end starts moving almost the instant the near end does. The individual electrons drift astonishingly slowly — well under a millimetre a second in a lamp wire — while the push that sets them going travels at close to the speed of light.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Read the meters
A single loop holds a cell, a switch, a bulb and two ammeters, one either side of the bulb. The first reads 0.24 A. What does the second read?
Rung 2 · The one that catches people
A student wires a lamp on a very long cable, then says the light will come on a moment late because the electricity has to travel down the wire first. What is wrong with that?
Rung 3 · Explain
A torch bulb goes out when the metal strip behind it is broken, even though the current has already been through the bulb. Explain why, using the words current, charge and loop.
Rung 4 · Take it somewhere new
A string of forty fairy lights is wired as one loop. One bulb fails and the whole string goes dark, and you cannot tell which bulb it was. Explain why the string fails like this, and describe how you would find the dead bulb with an ammeter, a cell and some wire.
Key note
An electric current is a flow of charge: the free electrons already present in a metal, all drifting the same way when a cell pushes on them. A current only flows in a complete loop, so a gap anywhere stops it everywhere. In a single loop the current is the same size at every point — an ammeter reads the same before and after a bulb — because charge is not used up. Energy is. Current is measured in amperes (A), and an ammeter is placed in the loop so the current runs through it.
Going further
The unit is named after André-Marie Ampère, and one amp is defined as one coulomb of charge going past each second. A coulomb is a strange quantity to picture: it is the charge on about six million million million electrons. So an ordinary 0.30 A torch bulb has roughly two million million million electrons going past the filament every second — which is why nobody counts them and everybody uses amps.
The electrons themselves crawl. In a lamp flex carrying an ordinary current they drift at well under a millimetre per second, so an electron leaving the plug would take hours to reach the bulb. Nothing waits for it. The wire is already full, and the push travels through the standing electrons at a large fraction of the speed of light, which is why a light comes on the instant the switch closes even on a long cable.
Before this lesson
- Nothing — this is where the unit starts.
Connects to
At GCSE this becomes
- Charge flow = current × time, the coulomb as a unit, and the difference between conventional current and the direction the electrons actually move.
Where to next
Ask Mr Badmus AI
Not sure why an ammeter goes in the loop and a voltmeter goes across?
The bench is a teaching model. Each cell is taken as 1.5 V and the bulb's current is drawn as a straight multiple of the number of cells; in a real filament lamp the resistance rises as it heats, so doubling the cells gives somewhat less than double the current. The readings are rounded to two decimal places, and the meter itself is treated as having no resistance of its own. Conventional current is drawn from + to − by long-standing convention; the electrons in a metal drift the other way.
Lesson content © MrBadmusAI.