Electric current and the electric circuit

What is electricity?

You press a switch, and a bulb lights. Nothing seems to move. But inside the wire, something does move. It is electric charge, and the flow of charge is called electric current.

Electric charge

All matter has charge. There are two kinds, called positive and negative. The electron has a negative charge. The smallest charge that we can find on its own is the charge of an electron, $e = 1.6 \times 10^{-19}\ \text{C}$.

The unit of charge is the coulomb (C). One coulomb is a very large charge. It is the charge of about $6.25 \times 10^{18}$ electrons.

In a metal wire, the electrons that are loosely held by the atoms can move. We call them free electrons. In an insulator, such as rubber or glass, there are almost no free electrons, so charge cannot flow.

Electric current

You should find that a bigger current means more electrons passing every second: about $6.25 \times 10^{18}$ for each ampere.

The electric current $I$ is the amount of charge $Q$ that passes through a point of the conductor in one second:

The unit of current is the ampere (A). A current of one ampere means that one coulomb of charge passes in one second: $1\ \text{A} = 1\ \text{C/s}$. Smaller currents use the milliampere ($1\ \text{mA} = 10^{-3}\ \text{A}$) and the microampere ($1\ \mu\text{A} = 10^{-6}\ \text{A}$).

When we want the charge, $Q = I \times t$.

The direction of the current

Before the electron was found, scientists agreed that the current goes from the positive terminal of a cell, through the outside circuit, to the negative terminal. We still use this rule, and it is called the conventional current.

In fact, the free electrons move the other way, from the negative terminal to the positive terminal. The two ways of looking at it give the same results in all the calculations of this chapter.

An electric circuit

A circuit is a closed path along which the current flows. It needs a source of energy (a cell or a battery), wires, and the devices that use the current, such as a bulb. A switch (plug key) opens or closes the path. When the key is open, the path is broken, and no current flows.

To draw a circuit, we use standard symbols.

Measuring the current: the ammeter

An ammeter measures the current. We connect it in series in the circuit: the current must pass through it. An ammeter has a very small resistance, so it does not change the current that it measures. The positive terminal of the ammeter goes toward the positive terminal of the cell.

A worked example

A charge of $12\ \text{C}$ passes through a wire in $4\ \text{s}$. What is the current?

We use $I = \dfrac{Q}{t} = \dfrac{12\ \text{C}}{4\ \text{s}} = 3\ \text{A}$.

Practise with new numbers

Try it in Python

The program finds the charge that passes in one second, and the number of electrons, for four currents.

Remember these

• Electric current is the flow of charge: $I = Q/t$. The unit is the ampere ($1\ \text{A} = 1\ \text{C/s}$).
• The conventional current goes from the positive terminal to the negative terminal. The electrons go the other way.
• A circuit is a closed path. An ammeter is connected in series.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. A charge of $42\ \text{C}$ passes through a wire in $12\ \text{s}$. What is the current?

2. How much charge passes through a wire in $3$ minutes when the current is $0.4\ \text{A}$?

3. Why does a bulb not light when the plug key is open?

4. In the wires outside a cell, which way do the free electrons drift?