Electrical basics · Lesson 2 of 13

Voltage and current

In the last lesson we learned the words. Here we go inside the two most important ones: voltage (volts), which pushes, and current (amps), which flows. You will run a circuit yourself, learn to convert units, and know where to put the meter.

‹ Lessons ← All courses

1Voltage: the push that moves the electrons

Voltage is the difference in electrical pressure between two points. Like a water tank on the roof: the higher it is, the harder the water comes down. A battery has one terminal "full" of electrons (−) and one "short" of them (+), and that difference pushes the electrons when we connect a wire between them.

Voltage is always between two points. That is why we say "220V between live and neutral". A bird standing on one wire doesn’t get a shock, because both its feet are at the same voltage, with no difference between them. But if you touch the wire and the ground at the same time, there is a voltage difference and current flows.

Where does voltage come from?

The battery

A chemical reaction inside separates the charges. It gives direct voltage (DC): an AA cell 1.5V, a car battery 12V.

The generator (dynamo)

A coil turns next to a magnet, and a voltage is produced. This is what is inside power stations, standby generators and a car’s alternator.

The wall socket

It comes from the power station through lines and transformers until it reaches the house as 220V alternating (AC).

The solar cell

Light moves the electrons and gives a small DC voltage, so many cells are connected one after another.

2Current: the electrons on the move

Current is the amount of charge passing through the wire every second. The unit is the ampere, A: 1 amp means about 6,240,000,000,000,000,000 electrons passing every second (that is 1 coulomb per second).

I = Q ÷ t

That is: current (amps) = charge (coulombs) ÷ time (seconds).

Electron flow: electrons are negative, so they leave the negative (−) terminal and go to the positive (+).

Conventional current: long ago, before the electron was discovered, scientists agreed that current flows from positive (+) to negative (−). That is still what every book and drawing uses today.

Both are right; they describe the same thing from opposite sides. In calculations it makes no difference, as long as you stick to one direction.

A strange fact: the electron itself moves very slowly (less than a millimetre per second!), yet the lamp lights instantly. Why? Because the wire is already full of electrons, like a pipe full of balls: push a ball in at one end and a ball comes out of the other end at the same moment.

3Closed and open circuits

Current needs a complete path: out of the source, through the appliance, and back to the source. If that path is broken anywhere (an open switch, a broken wire, a burnt-out lamp), all the current stops.

Be careful: an open switch stops the current, but it doesn’t remove the voltage. The terminal coming from the supply still has 220V on it. That is why we switch off the breaker before any work, not the light switch.

Try it yourself: add cells (voltage), open and close the switch, and swap between electron flow (blue dots) and conventional current (orange dashes). The lamp here has a resistance of 12Ω.

A battery and lamp circuit

4Units: milli, kilo and micro

Numbers in electricity are sometimes very small and sometimes very large, so we put prefixes in front of the unit:

Prefix	Meaning	Voltage	Current
kilo k	× 1000	kV: 11kV = 11000V	kA (a short circuit at a substation)
(none)	× 1	V	A
milli m	÷ 1000	mV: 500mV = 0.5V	mA: 30mA = 0.03A
micro µ	÷ 1,000,000	µV	µA: 500µA = 0.5mA

The rule: from a bigger unit to a smaller one (A to mA), multiply by 1000. From a smaller unit to a bigger one (mA to A), divide by 1000. So 0.25 A = 250 mA and 1500 mV = 1.5 V.

Unit conversion drill

5Numbers from everyday life

Source	Voltage
AA battery	1.5 V
9V (square) battery	9 V
Phone charger (output)	5 V
Car battery	12 V
Household mains	220 V
Three-phase in a factory	380 V
Medium-voltage cables	11 kV – 22 kV

Load (on 220V)	Current (approx.)
10W LED lamp	0.05 A = 50 mA
Phone charger	0.1 A
Fridge	0.7 A
Iron	5.5 A
2000W water heater	9 A
Current that can kill a person	from only 30 mA!

Notice: just 0.03 amps (30mA) through the heart can stop it. That is why an RCD trips at 30mA. High voltage is dangerous because it can push a large current through your body.

6How to measure voltage and current

The voltmeter goes in parallel (across the two ends), and the ammeter in series (the current flows through it).

Voltage (voltmeter): you put the two probes on the two points whose difference you want, without disconnecting anything. The meter’s resistance is very high, so it doesn’t affect the circuit.
Current (ammeter): you have to open the circuit and make the current flow through the meter. Its resistance is very low.
Clamp meter: measures current without opening the circuit, by clamping around one wire only (you will see it in lesson 9).

The most dangerous mistake: putting the ammeter (or the multimeter on its amps setting) across a socket or a battery. Its resistance is almost zero, so this is a short circuit: the fuse inside the meter blows, or the meter catches fire in your hand.

Answer these 3 questions correctly to make sure you know where the meter goes:

?Lesson questions

Get at least 5 out of 6 right to tick off the lesson.

The numbers are approximate, for common appliances. Conventional current (from + to −) is the direction used in drawings and books. Your progress is saved on this device only.
© 2026 LearnElectricals. All rights reserved. This site is not affiliated with any of the companies mentioned (such as Siemens, Allen-Bradley, Delta, Schneider and others). Names and trademarks belong to their owners and are used only to identify the devices being taught.