What is AC and DC? Is it harmful?

Introduction

Your phone charger plugs into the wall, but your phone runs on a battery. Are they the same kind of electricity? Not quite!

There are two kinds of electricity: AC and DC. In this lesson you'll find out how they're different, how to spot them on a charger, and which one can hurt you.

Working principle

DC: Direct Current

In direct current (DC), electricity flows in one direction only, out of the + end, round the circuit and back into the − end. The + and − ends never swap, so we say its polarity stays the same.

Batteries and solar cells give DC.
On a graph, DC is a flat straight line: the same push, all the time.

AC: Alternating Current

In alternating current (AC), electricity keeps changing direction. "Alternating" means taking turns. The + and − ends keep swapping, so the electricity flows one way, then the other.

The wall sockets in your home give AC.
In India, AC changes direction 50 times every second. This is called its frequency: 50 hertz (50 Hz).
On a graph, AC is a wave that goes up to +, down through zero to −, and back up again.

See them both in action. The blue dots are the electricity. Switch between DC and AC and watch the dots, the bulb and the graph.

DC flows one way only, like a river. The bulb stays steady, and the graph is a flat line.

Why do we have both?

AC is easy to make in power stations, and its voltage is easy to change with a machine called a transformer. That makes AC good for sending electricity a long way along power lines to our homes.

Most electronics, like phones, laptops and Arduino boards, need DC inside them. So their chargers turn the wall's AC into DC.

What you'll need

A phone or laptop charger, unplugged from the wall
Good light, or a magnifying glass, to read the tiny writing

Circuit diagram

Engineers have a symbol for each kind of electricity:

~ (a wavy line) means AC, like the wave on the graph.
⎓ (a straight line above a dashed line) means DC, like the flat line on the graph.
In circuit diagrams, AC is drawn as a circle with ~ inside, and a battery (DC) is drawn as a long line (+) and a short line (−).

Working in real life

Let's read a charger like an engineer.

Make sure the charger is not plugged into the wall.
Find the small writing printed on it.
Look for INPUT. You'll see something like 100–240 V ~ 50/60 Hz. The ~ tells you the charger takes AC from the wall.
Look for OUTPUT. You'll see something like 5 V ⎓ 2 A. The ⎓ tells you the charger gives DC to your phone.

So a charger is a little machine that turns the wall's strong AC into gentle DC.

Is it harmful?

Yes, the AC from the wall socket is dangerous. In India it pushes with about 230 V. The batteries we use in our projects push with only 1.5 V to 9 V, which is far too gentle to hurt you.

What actually hurts you in an electric shock is current: electricity flowing through your body. Voltage is the push that makes the current flow, so the bigger the voltage, the bigger the current.

Electricity always tries to flow from a higher point to a lower point, just like water flows from a full tank to an emptier one. The ground under your feet is the lowest point of all.

Danger! Water lets electricity flow easily, so a big current rushes through the body and out through the wet feet into the ground.

Look at the animation. The heater is faulty, so the person touching it becomes part of the circuit. Electricity flows in through their hand, down through their body, and out through their wet feet into the ground. Switch to dry rubber slippers and see how much less gets through. Water makes it much easier for electricity to flow, which is why you must never touch switches or plugs with wet hands or wet feet.

Engineers use a rule called Ohm's law to work out how much current flows:

Current = Voltage ÷ Resistance, or I = V ÷ R

Your body has some resistance: it slows electricity down. But 230 V is such a strong push that enough current gets through to be very dangerous. Wet skin has much less resistance, so even more current flows.

Try it

Find two chargers at home (unplugged!). Do they both say ~ for INPUT and ⎓ for OUTPUT?
Go round your home with a grown-up and list three things that run on AC (they plug into the wall) and three that run on DC (they use batteries).

Check what you learnt

Which kind of electricity comes from a battery?
A charger says INPUT 100–240 V ~. What does the ~ mean?
Why must you never touch a switch with wet hands?

Conclusion

DC flows one way only, like a river. Batteries and solar cells give DC.
AC keeps changing direction, like a saw going back and forth. Wall sockets give AC, 50 times a second in India.
The wall socket's 230 V AC can push a dangerous current through your body. Our battery projects are safe, but never play with the wall socket, and never touch switches with wet hands.

What next?

Now you know where electricity comes from. Next, meet the parts we build circuits with: What are the most basic electronic components?