Electric Circuits · Grade 10
If electrons crawl, why does the lamp light up instantly?
Because the electrons are already spread along the whole wire before the circuit is closed. Closing the switch does not send an electron from the battery to the lamp — it pushes every electron at once, like water in a full pipe. The push travels at close to the speed of light; the electrons themselves drift at millimetres per second.
Learning objectives
- Calculate the current from the amount of charge that passes a point in a given time
- Explain why a circuit has to be closed before any current can flow
- Draw a simple circuit using the standard symbols for a cell, a lamp, a switch and an ammeter
- Explain why an ammeter is connected in series and what would happen if it were connected in parallel
What is actually being measured
Current is the amount of charge passing a point each second. One ampere means one coulomb of charge every second, and that is the whole definition.
From that comes the calculation: current equals charge divided by time. If 60 coulombs pass in a minute, that is 60 seconds, so the current is one ampere.
Note that current is not 'how much electricity there is' but a rate. The same charge passing in a shorter time is a larger current, and that is exactly what makes a short circuit dangerous.
Why the loop must be closed
Charge is neither created nor destroyed. If it leaves one terminal of the battery, exactly the same amount must return to the other, or charge would pile up at the end of the wire until the push stopped.
So an open switch stops the current everywhere in the circuit at once, not only where it sits. There is no half circuit and no current that runs up to the break and waits there.
And so current is not 'used up' in the lamp. The same current that enters leaves. What is used up is energy, which is the next lesson.
Measuring without changing
An ammeter goes in series, meaning the current has to pass through it. So that it does not alter what it measures, its resistance is made as low as possible.
Connecting an ammeter in parallel with a component is a dangerous mistake: it opens a nearly resistance-free path beside the component, an enormous current passes through it, and the instrument burns out. It is a short circuit created deliberately without meaning to.
The standard symbols save words: a long line and a short line for a cell, a circle with a cross for a lamp, a gap with a lever for a switch, and a circle marked A for an ammeter.
Worked examples
24 coulombs pass a point in 8 seconds. What is the current?
- Current equals charge divided by time
- 24 divided by 8
Answer: 3 amperes
A current of 0.5 A flows for 2 minutes. How much charge has passed?
- Two minutes is 120 seconds
- Charge equals current times time
- 0.5 times 120
Answer: 60 coulombs
A student connects an ammeter in parallel with a lamp. What happens?
- An ammeter has very low resistance
- It offers an almost free path beside the lamp
- Nearly all the current goes through it
Answer: A very large current passes through the ammeter and damages it. That is a short circuit
Common mistakes
- Thinking current is used up in the lamp
- The same current enters and leaves. What the lamp takes is energy, not charge, and an ammeter on either side reads the same number.
- Picturing electrons racing from the battery to the lamp
- They are already there, along the whole wire, drifting at millimetres per second. What travels fast is the push, which is why the lamp lights at once.
- Connecting an ammeter in parallel
- Its low resistance turns the connection into a short. An ammeter always goes in series, and a voltmeter — its opposite — always in parallel.
What to remember
- Current equals charge over time. An ampere is a coulomb per second.
- The loop must be closed, or nothing flows anywhere in it.
- Current is not consumed; energy is.
- An ammeter goes in series and has low resistance.