Magnetic Effects of Electric Current: Class 10 Science · Chapter 12

In the last chapter you saw an electric current warm up a wire. A current does something else as well, something you cannot feel with your hand: it fills the space around the wire with magnetism. A scrapyard crane lifts an old car with a block of iron that is a magnet only while current flows through it. The bell at your school gate and the motor in a ceiling fan rely on the same link between electricity and magnetism.

So a wire carrying a current acts like a magnet. In this chapter you will map magnetic fields and find their shape around wires and coils. You will see how a magnet pushes back on a current. Finally, you will look at the wiring that brings electricity safely into a home.

12.1 Magnetic Field and Field Lines

From earlier classes you know the basic rule of magnets: like poles repel and unlike poles attract. A compass needle (दिक्सूचक सुई) is simply a tiny, light magnet balanced on a pin, so it obeys the same rule. Left to itself, it settles roughly along the north–south line. The end that turns to the north is its north-seeking end, or north pole, and the other end is its south pole.

Bring a bar magnet near a compass and the needle swings round. The magnet's north pole pushes the north end of the needle away and pulls its south end closer. Its south pole does the opposite. Together these pushes and pulls turn the needle until it settles along the magnet's influence at that spot.

The pattern shows that a magnet acts on the space around it, not only on the things it touches. That space has a name.

The curves that the filings trace are called magnetic field lines. They are a picture of the field. Their direction at any point shows which way the field points there, and their spacing shows how strong it is.

Follow any one field line all the way round and you come back to where you started: field lines are closed loops. Outside the magnet a loop runs from the north pole round to the south pole, and inside the magnet it completes the journey from the south pole back to the north pole. The arrows follow a simple agreement: at every point, a line points the way the north pole of a compass needle would point if it were placed there.

You will never see two field lines cross. At a crossing, a compass would have to line up with both lines at once, yet a single needle can point only one way. Notice also how the lines bunch up at the poles: the more tightly the lines are packed, the stronger the field in that region.

In short, magnetic field lines:

• come out of the north pole and go into the south pole outside a magnet, and run from south to north inside it;
• form closed curves;
• are closer together where the field is stronger;
• never cross one another;
• show the field direction at any point by the direction of the line there, which is where a compass needle would point.

12.2 Magnetic Field due to a Current-Carrying Conductor

Activity 12.1 showed that a current creates a magnetic field. Two questions follow: what shape is that field, and which way does it point? The answer depends on how the wire is bent, so we look at three shapes in turn: a straight wire, a single circular loop and a long coil.

12.2.1 Magnetic Field due to a Current through a Straight Conductor

Start with the simplest shape of all, a long straight wire. First we find out how the direction of its field depends on the current.

Next we map the shape of the field and see how its strength changes.

The activity gives three results worth remembering. The field lines of a straight wire are concentric circles, centred on the wire, in planes at right angles to it. At a given point the field gets stronger when the current increases. And for a fixed current the field gets weaker the further you go from the wire. That is why the circles drawn in Fig. 12.7 spread further apart as they get larger.

So doubling the current doubles the field at a point, and moving twice as far away halves it. (In the compass readings the needle also feels the Earth's field, so the angle does not simply double.)

12.2.2 Right-Hand Thumb Rule

You do not need a compass every time to find which way the field goes round a wire. Your right hand is enough.

Test the rule on Activity 12.5, where the current came up through the card towards you. Point your right thumb at your face: your fingers curl anticlockwise, just like the arrows on the rings in Fig. 12.7.

12.2.3 Magnetic Field due to a Current through a Circular Loop

Now bend the straight wire into a circle and pass a current round it. What does its field look like? Zoom in close to the wire and any short piece of the loop looks straight, so near the wire the field lines are small rings around it, just as in Fig. 12.7.

Imagine the loop cut in half through its centre, as in Fig. 12.9. The current comes out of the page at the top (A) and goes in at the bottom (B). Use the thumb rule at A and then at B: both give a field pointing the same way through the middle of the loop. Every part of the loop adds to the field inside in this way. So, although the lines curl tightly round the wire itself, near the centre the field is strong and its lines are almost straight and parallel.

Now wind the wire round several times instead of once. With $n$ turns, the same current goes round the same way $n$ times, and each turn adds its own share to the field at the centre. The field becomes $n$ times that of a single turn, so you can strengthen it by adding turns or by increasing the current.

12.2.4 Magnetic Field due to a Current in a Solenoid

Wind many turns of insulated wire side by side, like thread on a reel, and you get a solenoid (परिनालिका). The fields of all the turns add together, and the result is a pattern you have already met.

Lay this picture beside the bar magnet of Fig. 12.4 and, outside the coil, you will find it hard to tell them apart: one end of the coil acts like a north pole and the far end like a south pole. To find which end is which, use the clock rule on an end. Or curl the fingers of your right hand the way the current goes round the turns: your thumb then points to the north end.

Inside a long solenoid the field lines are straight, parallel and evenly spaced. This means the field has the same strength and the same direction at every point inside: it is a uniform magnetic field.

The strong field inside a solenoid can turn a bar of iron or steel placed in it into a magnet. A bar of soft iron inside a solenoid turns it into an electromagnet (विद्युत चुंबक).

12.3 Force on a Current-Carrying Conductor in a Magnetic Field

Every push has a partner. Newton's third law says that if one body pushes another, the second pushes back equally hard in the opposite direction. We have seen a current push a compass needle, so the magnet in the needle must push back on the current. André-Marie Ampère (1775–1836), the French scientist after whom the unit of current is named, made this argument. A wire that is free to move should therefore move.

So the force on the rod depends on two directions: the current's and the field's. Reverse either one and the force reverses; reverse both and it stays as it was. Experiments also show that the push is largest when the current is at right angles to the field. Turn the rod so that it lies along the field lines and the force disappears altogether.

With the current and the field crossing at right angles, the push comes out at right angles to each of them. Picture the corner of a box, where three edges meet, each square to the other two. A rule using your left hand keeps track of which edge is which.

12.4 Domestic Electric Circuits

The electricity in Indian homes is alternating current (AC): it surges back and forth through 50 complete cycles every second (50 Hz), and the supply voltage is 220 V. It arrives from the street along cables carried on poles or laid under the ground. Two wires do the main work. The live wire is at a high potential; in the older colour code it has red insulation. The neutral wire, with black insulation, stays close to the potential of the earth. The 220 V supply is the potential difference between these two.11NCERT also calls the live and neutral wires positive and negative. With AC, strictly, the live wire swings above and below the neutral, through 50 cycles a second.

At the meter board, the supply passes first through a fuse fitted by the electricity board. Next come the electricity meter, which records the energy you use, and the main switch, which leads into a fuse box (distribution box). From there it is split into separate circuits, each with its own fuse. A 5 A circuit feeds LED lights, tube lights, fans and phone chargers. A 15 A circuit feeds appliances that draw large currents, such as geysers, room heaters, washing machines and air conditioners.

A third wire, the earth wire, has green insulation. It is joined to a thick metal plate buried deep in damp ground near the house.

In each circuit the appliances are connected in parallel, each one bridging the live and neutral wires (Fig. 12.15). This gives every appliance the same 220 V, lets each one be switched on or off by itself, and keeps the others working if one fails. Switches and fuses are always put in the live wire, so that when they are off, the appliance is cut off from the high potential.

Why is the earth wire needed? Many appliances, such as washing machines, room heaters, steel electric kettles and refrigerators, have metal bodies. If the insulation inside wears away, the live wire may touch the body, which then sits at 220 V. Anyone who touches it gives the current a path through their body to the ground and gets a severe shock. If the body is joined to the earth wire, the current takes this low-resistance path to the ground instead. The body stays at the potential of the earth, so touching it is safe. The large current that flows usually melts the fuse and cuts off the supply.

The second safety device is the fuse, which you met in the previous chapter. It is a short piece of wire with a low melting point, connected in series in the live wire. If the current becomes too large, Joule heating melts the fuse wire and breaks the circuit before the wiring or the appliances are damaged or catch fire.

Too much current can flow for two reasons. Overloading happens when a circuit is made to carry more current than it is built for. Too many appliances may be running on one circuit or from one socket through a multi-plug adapter, or the supply voltage may rise suddenly. A short circuit is the most extreme case: the live and neutral wires touch directly, for example where a cord's insulation has cracked or something has come loose inside an appliance. The resistance of the path drops almost to zero, and the current shoots up enormously.

To check whether a circuit is overloaded, find the current each appliance draws from its power rating, using $P=VI$ from the previous chapter.

Because the appliances are in parallel, the currents they draw add up in the circuit's live wire and fuse (Fig. 12.17).