Magnetic Effects of Electric Current

Why This Matters

In the year 1820, a teacher in Denmark named Hans Christian Oersted was setting up an experiment. He kept a compass near a wire. Every time he switched on the current in the wire, the compass needle moved a little. This looked small. But it was one of the biggest discoveries in all of physics.

What did it show? It showed that electricity and magnetism are connected. They are really two parts of the same thing. When current flows through a wire, the wire does not just get hot. It also creates a magnetic field all around it. (A magnetic field is the region where a magnet’s pull can be felt — more on this soon.)

This one idea runs the modern world. Think about the fan, mixer, and washing machine in your home. They all have electric motors. A motor spins because a current sitting in a magnetic field gets a push. Loudspeakers use this idea. So do MRI machines in hospitals, and the big magnets in cranes that lift scrap iron.

In this chapter you will see the link work both ways. First: how a current makes a magnetic field. Second: how a magnetic field pushes on a current — this is what spins an electric motor.

Then comes the best part of all. Turn that second idea around and you can make electricity out of nothing but movement. Move a magnet near a coil of wire and a current appears in the wire, from nowhere. This is called electromagnetic induction, and it is how every power station in the world makes the electricity in your plug socket — whether it burns coal, splits atoms, or catches the wind. Something spins, and current comes out.

By the end, even the wiring in your own house — the live, neutral, and earth wires, and the fuse — will make full sense.

The Big Idea

A current creates a magnetic field around it. A current placed in a magnetic field feels a force (a push) — that runs the motor. And a wire moved in a magnetic field has a current pushed into it — that runs the generator. Three simple hand rules give you the directions. The right-hand thumb rule gives the direction of the field. Fleming’s left-hand rule gives the force. Fleming’s right-hand rule gives the induced current.

Keep these ideas separate in your mind. First: a wire makes a field — use your right hand (thumb rule). Second: a wire sitting inside a field feels a force — use your left hand. Third: a wire moved inside a field gets a current pushed into it — use your right hand again (Fleming’s rule).

Notice that the second and third ideas are the same physics running in opposite directions. In a motor you put current in and get movement out. In a generator you put movement in and get current out. Almost everything in this chapter is one of these three ideas.

Let’s Break It Down

Magnetic field and field lines

Before we connect electricity to magnetism, let us quickly bring back what you already know about plain magnets and their poles.

Around every magnet there is a space where its pull can be felt. This space is called the magnetic field. For example, when you bring a pin near a magnet and it gets pulled, the pin was inside the magnet’s field.

We cannot see a field with our eyes. So we draw it using field lines. Field lines are imaginary lines that show the shape and direction of the field. If you place a small compass in the field, the compass needle turns and lies along these lines. The north pole of the compass points in the direction of the field.

Figure 12.1 below shows what these field lines look like around a simple bar magnet — notice how they curve from one pole to the other.

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Here are three rules about field lines that you should remember:

• Outside the magnet, the lines go from the north pole to the south pole. Inside the magnet, they go from south back to north. So each line is a full closed loop — it has no real start or end.
• Where the lines are close together, the field is stronger. The lines are most crowded near the poles, so the field is strongest there.
• Two field lines never cross each other. Think about why. If they crossed, a compass kept at that point would have to point in two different directions at the same time. That is impossible. So lines never cross.

But here is a quiet question that “crowded lines mean a stronger field” never answers: why should squeezing lines closer mean more strength? It is not a separate fact you must memorise — it is just what the spacing of the lines was measuring all along.

Think about how many lines we draw. The number of lines does not change as you move away from the pole — the same lines simply fan out and spread apart. Near the pole, where the magnet’s pull is strong, those lines are forced into a small space, so they sit packed tightly. Far away, where the pull has faded, the very same lines have room to spread, so they sit far apart. So the closeness of the lines is not a rule on top of the field — it is a picture of the field’s strength. Tight = strong, spread = weak.

Figure 12.2 below makes this clear: watch how one set of lines is packed near the pole and the same lines have drifted far apart further out.

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The field around a straight current-carrying wire

The hand rules ahead all depend on knowing which way the current points, so let us first nail down exactly what “direction of current” means.

When current flows through a straight wire, a magnetic field forms around it. This field is shaped like circles, one inside the other, all around the wire. (We call these “concentric circles” — circles that share the same centre. Here the centre is the wire.)

The circles further from the wire are bigger, and the field there is weaker. The circles close to the wire are smaller, and the field there is stronger. Also, if you make the current larger, the field becomes stronger.

See Figure 12.3 below: it shows these circles around the wire, with arrows giving the direction the field turns.

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How do you find which way the field circles around the wire? Use the right-hand thumb rule. Do this step by step. Take your right hand. Hold the wire as if you are gripping it in your fist. Point your thumb in the direction the current flows. Now look at your other four fingers — they curl around the wire. The way your curled fingers point is the direction of the field lines. That is all the rule says.

Let us try the rule on a real overhead power line and work out the field direction below it.

The field due to a circular loop and a solenoid

Now bend the same wire into a circular loop (a ring). Each small bit of the wire still makes its own circles of field. But at the centre of the loop, all these fields point the same way and add up. The result is a field that goes straight through the middle of the loop.

What if you make many loops instead of one? A coil with n turns gives n times the field of a single turn. This is because every turn adds its field in the same direction, so they pile up.

Now wind many such turns close together, like a spring, into a long tube shape. This is called a solenoid. A solenoid is just a long coil of many turns of wire. When current flows through it, the field of a solenoid looks exactly like the field of a bar magnet. One end of the solenoid acts as a north pole and the other end as a south pole. And inside the solenoid the field is uniform. “Uniform” means the field has the same strength and direction at every point inside — the lines there are parallel and equally spaced.

That sentence — “it looks exactly like a bar magnet” — sounds like a coincidence. It is not. It follows directly from what each single loop already does. Remember: every loop pushes its field straight through its own middle, and all the loops carry current the same way, so all their middle-fields point the same direction. Now stack the loops up in a line. Inside the tube, each loop’s field lands right on top of its neighbour’s, all pointing one way. They add up into one strong, straight, even line of field — that is why the inside is uniform. The field has to come out somewhere: it streams out of one end (that end becomes the north pole), curves around the outside, and dives back into the other end (the south pole). A strong straight field inside, looping out of one end and back into the other — that is the exact shape of a bar magnet’s field. So a solenoid is not like a bar magnet by chance; lining up the loops builds a bar-magnet field.

Figure 12.4 below shows the two steps: first separate loops each firing their field the same way, then the loops pushed together so those fields merge into one.

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Figure 12.5 below draws the finished solenoid on its own, so you can see the complete field pattern it makes.

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Now place a piece of soft iron inside the solenoid. The strong field of the solenoid turns the iron into a magnet too. This whole thing — a coil with an iron core inside — is called an electromagnet. The big difference from an ordinary magnet is this: an electromagnet works only while the current is on. You can switch it on and off whenever you want. You can also make it very strong. That is why cranes (to lift iron), electric bells, and many machines use electromagnets.

Force on a current-carrying conductor

So far we saw the first half: a current makes a field. Now let us look at the second half — the reverse idea. Oersted showed that a current makes a magnetic field. A scientist named Ampère then thought about it the other way round. If a current can push a magnet (the compass needle moved), then a magnet should also push back on a current. And it does. A wire carrying current, when placed in a magnetic field, feels a force. The field pushes the wire.

But “the field pushes the wire” hides the real reason. There is no invisible hand. The push comes from a simple fact you already know: the wire carries current, so it has its own circular field too. Now there are two fields in the same space — the magnet’s straight field, and the wire’s circular field — and they add together, point by point.

Here is the whole trick. On one side of the wire, both fields point the same way, so they reinforce and the lines crowd tightly — a strong region. On the other side they point opposite ways, so they partly cancel and the lines thin out — a weak region. The field is now lopsided: packed on one side of the wire, sparse on the other. And crowded field lines behave like a stretched rubber band that wants to straighten and spread out. So they shove the wire away from the crowded side, toward the empty side. That sideways shove is the force. This also explains the two facts above for free: flip the current (or flip the magnet) and the “crowded” and “empty” sides swap, so the push flips too.

Figure 12.6 below builds it in three steps: the magnet’s field, the wire’s own circular field, and the two added — crowded above, empty below, so the wire is pushed down.

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The rule that gives this force’s direction keeps talking about directions “at right angles”, so let us make sure that phrase is crystal clear first.

Two important points about this force. First, the force is largest when the current and the field are at right angles (90°) to each other. Second, the force flips to the opposite direction if you reverse either the current or the field.

Figure 12.7 below sets all three directions out clearly. It is the same set-up as Figure 12.6 — the field points across the page from N to S, and the current goes into the page — so, just as before, the rod is pushed straight down.

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How do you find which way this force points? Use Fleming’s left-hand rule. Here is how to do it, step by step:

Take your left hand. Stretch out your thumb, your first finger (forefinger), and your middle finger. Arrange them so all three point in three different directions, each at a right angle to the other two — like the corner of a box. Now match them up: your first finger points along the Field, your Middle finger points along the Current, and your Thumb then points along the Thrust (that is, the force, or the way the wire moves).

A small trick to remember which finger is which: Fore finger = Field, and Middle finger = … think of “Mi” in “Middle” for the Curr-ent. The thumb is left for Thrust. So it goes Field, Current, Thrust (F-C-T) on fore finger, middle finger, thumb.

Figure 12.8 below draws the hand position out, so you can copy it with your own left hand as you look.

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Now let us put Fleming’s rule to work — but with a twist, because here it is an electron that moves, not an ordinary current.

The electric motor

This force is the idea behind the electric motor — the machine inside your fan, your mixer, and your washing machine. It is also what moves the cone of a loudspeaker. In all of them, a current sitting in a magnetic field is made to move.

Let us build a motor step by step. A rectangular coil of wire sits between the two poles of a magnet, mounted on an axle so it is free to spin.

To see the forces clearly, look at the motor end-on — that is, stand at one end of the axle and look straight along it. Now the coil’s two long arms run directly away from you, into the page. Call them arm P and arm Q.

Here is the key point about the coil. It is a single loop, so the current goes in at one end, travels all the way round, and comes out at the other end. Going round a loop means that when the current runs one way along arm P, it must come back the opposite way along arm Q. So if arm P carries current out of the page (towards you), arm Q carries it into the page (away from you). The two arms always carry current in opposite directions — that is not a coincidence, it is just what a loop does.

Now apply Fleming’s left-hand rule to each arm in turn. The field points to the right (N to S).

• Arm P — field to the right, current out of the page. Your thumb points up. Arm P is pushed up.
• Arm Q — field to the right, current into the page. Your thumb points down. Arm Q is pushed down.

So at the same instant, one arm is shoved up and the other is shoved down — and they are on opposite sides of the axle. Think of a steering wheel: push the left side up and pull the right side down, and the wheel turns. That is exactly what happens here. The coil spins.

Figure 12.9 below shows this end-on view, with both forces and the hardware that feeds the current in.

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Why a motor needs a split ring

The coil is spinning. But now a problem appears, and it is worth seeing clearly rather than being told to memorise a part name.

Let the coil turn half a way round. The two arms have now swapped sides: arm P has travelled over to the right, and arm Q has come round to the left. But the battery is still pushing current the same way, so the current in each arm has not changed. Arm P still carries current out of the page — so it is still pushed up. But it is now on the right. And arm Q is still pushed down, but it is now on the left.

Left side pushed down, right side pushed up. That is the opposite twist! The coil would slow down, stop, and get shoved back the way it came. Instead of spinning, it would just rock back and forth. A motor that rocks is useless.

The fix is a clever little piece of hardware called the split-ring commutator. Take a metal ring and cut it into two half-rings, with a small insulating gap between them. Mount it on the axle, so it turns with the coil. Join each half-ring to one end of the coil. Then press two fixed carbon brushes against it from the sides — these carry the current in from the battery.

Now here is the trick. The ring spins, but the brushes stay still. So after every half turn, each half-ring has travelled round to meet the other brush. The connections swap over. That means the current through the coil reverses every half turn — at exactly the moment it needs to.

Follow it through. After half a turn, arm P is on the right, but the commutator has just reversed its current, so it now carries current into the page — and so it is pushed down. Arm Q is on the left, now carrying current out of the page, so it is pushed up. Left up, right down: the same twist as before. The coil keeps turning the same way, round and round, without stopping.

Figure 12.10 below lines up the three moments side by side, so you can see the problem and the fix at a glance.

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Electricity from Movement

So far, everything has flowed one way: we put a current in, and got a field or a force out. Now we turn the whole chapter around.

Electromagnetic induction — the motor run backwards

A motor takes current + field and gives you movement. In 1831, an English scientist named Michael Faraday asked the obvious question in reverse: could you take movement + field and get current out?

If he was right, you would be able to make electricity out of nothing but motion. No battery, no chemicals — just a magnet and a wire, and a push. It sounds too good to be true. It is true, and it is how every power station on Earth works.

Faraday’s experiment was simple enough to do on a table. He took a coil of wire and joined its two ends to a galvanometer.

A galvanometer is a very sensitive meter that detects even tiny currents. Its needle rests at zero in the middle of the dial. If a current flows through it one way, the needle kicks to one side. If the current flows the other way, the needle kicks to the other side. So it does not just tell you how much current there is — it also tells you which way it is flowing. That is exactly what we need here.

Then he took a bar magnet and tried three things.

1. He pushed the magnet towards the coil. The needle kicked to one side. A current was flowing!
2. He held the magnet perfectly still, right next to the coil. The needle dropped back to zero. No current at all — even though the magnet was still sitting there, its field still passing through the coil.
3. He pulled the magnet away. The needle kicked again — but this time to the other side. A current, flowing the opposite way.

Figure 12.11 below shows all three cases together. Look at the needle in each one.

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Panel (b) is the one that should stop you. The magnet is there. Its field is passing straight through the coil. And yet nothing happens. So the current is not caused by the magnetic field simply being present.

The current appears only while the magnetic field through the coil is changing. It is the change that makes the current, not the field itself. This is called electromagnetic induction, and the current it makes is called an induced current.

There is one more thing Faraday found, and it matters. It does not have to be the magnet that moves. You can hold the magnet still and move the coil towards it instead — and you get exactly the same current. Only the relative motion between the two counts. Either way, the field through the coil is changing, and that is the one thing that matters.

Why does a CHANGING field make a current?

Most books tell you that a changing field induces a current, and move straight on. But stop and ask the question a curious person would ask: why?

Why should a magnet that never even touches the wire be able to push a current into it? And why does a still magnet — with its field passing right through the coil — do absolutely nothing? The field is there in both cases. Something must depend on the movement itself.

The answer is beautiful, and you already have every piece of it. It is the same force you met a few pages ago.

Remember what makes a current-carrying wire feel a push. A wire is full of tiny charged particles that are free to move — they are what a current is made of. When those charges travel across a magnetic field, the field shoves them sideways. Add up that shove on all the charges and you get the force on the wire. That was Fleming’s left-hand rule.

Now run the whole thing backwards.

Take a wire with no current in it at all and simply drag the wire sideways across a magnetic field. The charges inside the wire are part of the wire, so they are dragged along with it. Which means those charges are now moving across the field — exactly the condition the field needs in order to shove them.

So the field shoves them. Which way? At right angles to both the motion and the field. And if you set the wire up across the field, that direction turns out to be straight along the wire. The field sweeps the charges along the length of the wire, from one end towards the other.

And charges flowing along a wire is a current. Nobody pushed them with a battery. The magnetic field did the pushing, and your muscles paid for it.

Now look at the still magnet again and it makes complete sense. If nothing moves, the charges in the wire are not being carried across the field. No motion across the field means no shove, which means no current. The magnet can be as strong as you like — a field that just sits there does nothing at all.

It is the motion, not the magnet, that makes the current.

Figure 12.12 below builds this in three steps, ending with a zoom right in on a single charge so you can see the shove happening.

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The induced current always fights back

Here is one more question worth asking, and its answer is one of the deepest ideas in physics.

Once a current has been induced in that moving wire, look at what you now have: a current-carrying wire sitting in a magnetic field. And you know exactly what happens to those — they feel a force! So there is now a force on the wire. Which way does it point?

Work it out with Fleming’s left hand (because now you have a current in a field, and you want the force). Do it and you find something striking: the force points backwards, against the direction you were pushing the wire. Figure 12.15 later on shows this clearly. The induced current always arranges itself so that the force on it opposes the very motion that created it.

Why must nature work this way? Because otherwise you would get energy for free.

Imagine for a moment that the force helped your push instead of resisting it. Then the wire would speed up all by itself. Faster motion means a bigger induced current, which means an even bigger helping force, which speeds it up more… You would get unlimited electricity out of one small shove, out of nothing. That would break the law of conservation of energy — the rule that energy can never be created out of nothing, only converted from one form to another. Nature does not allow it. So the force has to push back.

And this is why making electricity takes effort. Think of an old bicycle with a dynamo lamp. The moment the lamp lights up, the pedals get harder to push. That extra hardness is this opposing force. The electrical energy lighting the lamp is exactly the muscle energy you spent fighting it. In a power station, the same thing: steam or falling water or wind must keep working hard against this backward push, forever. Electricity is never created. It is only converted from something else.

What decides how big the induced current is

Because we now understand the mechanism, we can predict the answer instead of memorising it. The current comes from the field shoving the charges. So anything that makes that shove bigger will make the current bigger.

• Move it faster. A faster wire (or a faster magnet) means the charges are carried across the field more quickly. A faster-moving charge gets a bigger shove. So a bigger current.
• Use more turns in the coil. Every single turn of the coil gets its own shove, and the turns are joined end to end. So their pushes add up, one after another, like batteries in series. Double the turns, roughly double the current.
• Use a stronger magnet. A stronger field pushes each moving charge harder. Bigger shove, bigger current.

All three of these are really the same one rule: a bigger, faster CHANGE in the magnetic field gives a bigger induced current.

Figure 12.13 below shows each factor as a pair of galvanometer readings. Watch how far the needle swings.

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Fleming’s right-hand rule

We now need a quick way to find which way the induced current flows. There is a hand rule for it, and — annoyingly for students — it uses the other hand.

Take your right hand. Stretch out your thumb, first finger and middle finger so all three are at right angles to one another, like the corner of a box. Now: your First finger points along the Field, your thuMb points along the Motion of the wire, and your Middle finger then gives the direction of the induced Current.

This is Fleming’s right-hand rule, and it is the generator rule. Figure 12.14 below sets out the three directions.

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Left hand or right hand?

This is where most students slip, so let us kill the confusion now.

Notice first what the two rules have in common. In both of them, the Forefinger is the Field and the Middle finger is the Current. That part never changes. So the fingers are not the problem.

The difference is what you already know and what you are trying to find out.

So ask yourself one question before you pick a hand: am I looking for a force, or for a current?

• Looking for a force? The current already exists. Use the LEFT hand. (Motor.)
• Looking for a current? Something is being moved. Use the RIGHT hand. (Generator.)

Figure 12.15 below puts both rules on the exact same picture, so you can see that it is only the question that changes.

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Let us use the right-hand rule on a real generator arm.

The electric generator

Now we can build the machine that powers the world.

Take the electric motor from earlier and simply run it backwards. Instead of feeding current in to make the coil spin, we will spin the coil ourselves and collect the current that comes out. That machine is an electric generator.

The parts are almost the same: a coil of wire, mounted on an axle, free to spin between the poles of a magnet. But there is no battery. Instead, something turns the axle — falling water at a dam, high-pressure steam from a coal or nuclear boiler, or the blades of a wind turbine.

Look at it end-on again, and follow one full turn.

As the coil spins clockwise, arm P sweeps upwards through the field while arm Q sweeps downwards through it. Two arms, moving through the same field in opposite directions. Apply Fleming’s right-hand rule to each:

• Arm P is moving up, field to the right → the induced current in it flows into the page.
• Arm Q is moving down, field to the right → its current flows out of the page.

Opposite directions in the two arms — which is exactly right, because they are the two sides of one loop. So current flows all the way round the coil and out into the circuit.

Now keep spinning, and watch what happens after half a turn. The arms have swapped sides. Arm P, which was sweeping up, is now over on the other side sweeping down. And arm Q, which was going down, is now going up.

Each arm is now moving through the field the opposite way from before. So by the right-hand rule, the current induced in each arm reverses. The current in the coil flips its direction — all by itself, with nothing switching anything. It happens simply because a spinning arm must go up for half a turn and down for the other half.

This flip happens every single half turn, for as long as the coil spins. So the current keeps swinging back and forth: one way, then the other, one way, then the other.

Here is the design decision that follows. In a motor, we used a split ring precisely to cancel this reversal and keep the coil spinning one way. In a generator, we usually want the reversal — so we do the opposite. We use slip rings: two complete, unbroken rings, one joined to each end of the coil, each with its own brush pressing on it. Because each ring is a full circle, it never changes brushes. So the coil’s natural flip is passed straight out to the circuit, untouched.

A current that keeps reversing like this is called alternating current (AC), and this machine is an AC generator. Figure 12.16 below shows both halves of the turn and the slip rings.

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(If you instead fit a motor’s split ring to a generator, the split ring cancels the flip every half turn — and the output comes out always in the same direction. That gives you a DC generator. Same machine, different ring.)

AC and DC

We have just met a current that keeps reversing. Let us name the two kinds properly, because you use both every day.

• Direct current (DC) always flows in one direction only, and its size stays steady. This is what a cell or a battery gives you. The current in your torch, your TV remote, and your phone’s battery is DC.
• Alternating current (AC) keeps reversing its direction, over and over, many times a second. This is what comes out of the sockets in your wall.

Before we put a number on “many times a second”, let us make sure that unit is clear.

In India, the mains AC supply has a frequency of 50 Hz. That means the current completes 50 full cycles every second.

Now here is a point students often get wrong, so go slowly. In one full cycle, the current flows one way, then turns around and flows the other way, and then returns to the start. So it changes direction twice in every single cycle — not once.

That gives us: 50 cycles per second × 2 direction changes per cycle = 100 direction changes every second. The current in your fan is flipping its direction a hundred times a second. Your eyes can never see it, but it is.

Figure 12.17 below draws both currents as graphs, so the difference is impossible to miss.

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Why bother with AC at all?

This is the question the textbook rarely answers. If DC is simpler — always one direction, no flipping about — why does the whole country run on AC?

The answer is about not wasting energy on the journey.

A power station may be hundreds of kilometres from your town. The electricity has to travel all that way along thick metal cables. But every metre of cable has some resistance, and you know from Chapter 11 what a current does in a resistance: it heats it up. That heat is wasted energy, leaking away into the air the whole way.

So the trick is obvious: send the power with as small a current as possible.

But we still need to deliver the same amount of power. Recall that power is P = VI. If we must keep P the same but want I to be small, then V must be huge. High voltage, small current, same power delivered — and hardly any heat wasted on the way.

And here is where AC wins. Only AC can have its voltage stepped up and down easily, using a device called a transformer (a coil-and-magnet device that works on induction — the very idea in this section). So we do this:

1. At the power station, step the AC up to a very high voltage (hundreds of thousands of volts). The current is now tiny.
2. Send it across the country on the tall transmission towers you see beside highways. Very little energy is lost as heat.
3. Near your town, step it back down to a safe 220 V for your home.

DC cannot be stepped up and down anything like as easily, so it would have to travel at a low voltage — which means a large current, which means enormous heat losses along the way. That is why the grid runs on AC.

Electricity in Your Home

Domestic electric circuits

The electricity that comes into your home arrives on two main wires. One is the live wire (in India, its insulation is usually red, and it is at about 220 V). The other is the neutral wire (its insulation is usually black). There is also a third wire, the earth wire (usually green). The earth wire joins the metal body of an appliance to a metal plate buried deep in the ground.

• Appliances are joined in parallel across the live and neutral wires. Because of this, each appliance gets the full 220 V and can have its own switch.
• The earth wire is a safety wire. Suppose a fault makes the metal body of an appliance live (charged). Then the extra current runs away safely into the ground through the earth wire — instead of running through your body if you touch it.
• A fuse is joined in series with the circuit. It protects the circuit. If the current becomes too high, the thin fuse wire heats up (this heating is called Joule heating). It then melts and breaks the circuit, so the current stops.
• Overloading means too much current is being drawn. This can happen if too many appliances are plugged into one socket, or if the supply voltage suddenly jumps high. Short-circuiting means the live wire and the neutral wire touch each other directly — for example, when their covering (insulation) is damaged. This causes a sudden, very large current. In both cases the fuse melts and cuts off the supply.

Let us see how this plays out for a real appliance — a power-hungry oven on a circuit with a small current rating.

Common Mistakes

Quick Check

Practice Problems

Summary

• A wire carrying current makes a magnetic field around it (Oersted’s discovery). So electricity and magnetism are linked.
• A magnetic field is shown using field lines. Outside a magnet they go from N to S, they form closed loops, they never cross, and they are close together where the field is strong.
• The field around a straight wire is a set of circles, one inside the other, around the wire. To find their direction use the right-hand thumb rule (thumb = current, curled fingers = field).
• A circular loop makes a field through its centre. A solenoid acts like a bar magnet and has a uniform field inside. Putting a soft-iron core inside makes an electromagnet, which you can switch on/off and make stronger.
• A current in a magnetic field feels a force. This force is biggest when the current is at right angles to the field. To find its direction use Fleming’s left-hand rule (First finger = Field, Middle = Current, Thumb = Thrust).
• In an electric motor, the coil’s two arms carry current in opposite directions, so one is pushed up and the other down — and the coil spins. A split-ring commutator reverses the current every half turn, so the coil keeps turning the same way instead of rocking back and forth.
• Electromagnetic induction: moving a magnet near a coil (or a coil near a magnet) induces a current in it. The current flows only while the field through the coil is changing — a still magnet gives nothing. The reason is that the field can only shove a charge that is being carried across it, and that shove along the wire is the current.
• The induced current is bigger if you move faster, use more turns, or use a stronger magnet — all of which mean a bigger, faster change in the field.
• The induced current always opposes the motion that made it. If it did not, you would get energy from nothing, breaking the conservation of energy. This is why a dynamo makes a bicycle harder to pedal — and why making electricity always takes effort.
• Use Fleming’s right-hand rule for the induced current (First finger = Field, Thumb = Motion, Middle = Current). Remember: LEFT hand = motor (a current gives you a force); RIGHT hand = generator (a motion gives you a current).
• An AC generator spins a coil in a magnetic field. Each arm sweeps up through the field for half a turn and down for the other half, so the current reverses every half turn by itself. Slip rings (two complete rings) pass that reversal out to the circuit. A split ring instead would cancel it, giving a DC generator.
• DC always flows one way and stays steady (cells, batteries). AC keeps reversing. India’s mains AC is 50 Hz — 50 cycles a second — and since the current reverses twice per cycle, it changes direction 100 times a second.
• AC is used for long-distance transmission because only AC can be stepped up to a very high voltage by a transformer. High voltage means a small current, and since heat lost is H = I²Rt, a small current wastes far less energy as heat in the wires.
• Home circuits: the live wire (red), neutral wire (black), and earth wire (green). Appliances are joined in parallel at 220 V. The fuse melts when the current is too high. The earth wire keeps you safe from shocks. Overloading and short-circuiting both cause dangerous high currents.

What’s Next

You have now seen two effects of electric current: the heating effect (Chapter 11) and the magnetic effect (this chapter). With these two done, the syllabus now moves away from the lab and out into the living world around you.

In Chapter 13: Our Environment, you will learn how energy and matter move through nature. You will study food chains and food webs, the levels of feeding (trophic levels) and the 10% rule, and the jobs of producers, consumers, and decomposers. You will also see how things people do — like throwing away waste and damaging the ozone layer — upset the careful balance of nature.