Magnetic Effects of

Electric Current

CHAPTER

n the previous Chapter on ‘Electricity’ we learnt about the heating

effects of electric current. What could be the other effects of electric

curr

ent? We know that an electric curr

ent-carrying wire behaves like a

magnet. Let us perform the following Activity to reinforce it.

Activity 12.1Activity 12.1

Activity 12.1

n Take a straight thick copper wire and place it

between the points X and Y in an electric circuit,

as shown in Fig. 12.1. The wire XY is kept

perpendicular to the plane of paper.

n Horizontally place a small compass near to this

copper wire. See the position of its needle.

n Pass the current through the circuit by

inserting the key into the plug.

n Observe the change in the position of the

compass needle.

Figure 12.1Figure 12.1

Figure 12.1

Compass needle is deflected on passing an electric

current through a metallic conductor

We see that the needle is deflected. What does it mean? It means that

the electric current through the copper wire has produced a magnetic

effect. Thus we can say that electricity and magnetism are linked to each

other. Then, what about the reverse possibility of an electric effect of

moving magnets? In this Chapter we will study magnetic fields and such

electromagnetic effects. We shall also study about electromagnets which

involve the magnetic effect of electric current.

Hans Christian Oersted (1777–1851)

Hans Christian Oersted, one of the leading scientists of the 19

century, played a crucial role in understanding electromagnetism. In

1820 he accidentally discovered that a compass needle got deflected

when an electric current passed through a metallic wire placed nearby.

Through this observation Oersted showed that electricity and

magnetism were related phenomena. His research later created

technologies such as the radio, television and fiber optics. The unit of

magnetic field strength is named the oersted in his honor.

Resistor

Long straight

conductor

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12.1 MAGNETIC FIELD AND FIELD LINES12.1 MAGNETIC FIELD AND FIELD LINES

12.1 MAGNETIC FIELD AND FIELD LINES

We are familiar with the fact that a compass needle gets deflected when

brought near a bar magnet. A compass needle is, in fact, a small bar

magnet. The ends of the compass needle point approximately towards

north and south directions. The end pointing towards north is called north

seeking or north pole. The other end that points towards south is called

south seeking or south pole. Through various activities we have observed

that like poles repel, while unlike poles of magnets attract each other.

QUESTION

1. Why does a compass needle get deflected when brought near

a bar magnet?

Activity 12.2Activity 12.2

Activity 12.2

n Fix a sheet of white paper on a drawing

board using some adhesive material.

n Place a bar magnet in the centre of it.

n Sprinkle some iron filings uniformly

around the bar magnet (Fig. 12.2). A

salt-sprinkler may be used for this

purpose.

n Now tap the board gently.

n What do you observe?

Figure 12.2Figure 12.2

Figure 12.2

Iron filings near the bar magnet align

themselves along the field lines.

The iron filings arrange themselves in a pattern as shown

Fig. 12.2. Why do the iron filings arrange in such a pattern? What does

this pattern demonstrate? The magnet exerts its influence in the region

surrounding it. Therefore the iron filings experience a force. The force

thus exerted makes iron filings to arrange in a pattern. The region

surrounding a magnet, in which the force of the magnet can be detected,

is said to have a magnetic field. The lines along which the iron filings

align themselves represent magnetic field lines.

Are there other ways of obtaining magnetic field lines around a bar

magnet? Yes, you can yourself draw the field lines of a bar magnet.

Activity 12.3Activity 12.3

Activity 12.3

n Take a small compass and a bar magnet.

n Place the magnet on a sheet of white paper fixed on a drawing

board, using some adhesive material.

n Mark the boundary of the magnet.

n Place the compass near the north pole of the magnet. How does

it behave? The south pole of the needle points towards the north

pole of the magnet. The north pole of the compass is directed

away from the north pole of the magnet.

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Magnetic field is a quantity that has both direction and magnitude.

The direction of the magnetic field is taken to be the direction in which a

north pole of the compass needle moves inside it. Therefore it is taken

by convention that the field lines emerge from north pole and merge at

the south pole (note the arrows marked on the field lines in Fig. 12.4).

Inside the magnet, the direction of field lines is from its south pole to its

north pole. Thus the magnetic field lines are closed curves.

The relative strength of the magnetic field is shown by the degree of

closeness of the field lines. The field is stronger, that is, the force acting

on the pole of another magnet placed is greater where the field lines are

crowded (see Fig. 12.4).

No two field-lines are found to cross each other. If they did, it would

mean that at the point of intersection, the compass needle would point

towards two directions, which is not possible.

12.212.2

12.2

MAMA

GNETIC FIELD DUE TO A CURRENTGNETIC FIELD DUE TO A CURRENT

GNETIC FIELD DUE TO A CURRENT

ARRYING

ARRYINGARRYING

ARRYING

CONDUCTORCONDUCTOR

CONDUCTOR

In Activity 12.1, we have seen that an electric current through a

metallic conductor produces a magnetic field around it. In order to

find the direction of the field produced let us repeat the activity in the

following way –

Figure 12.3Figure 12.3

Figure 12.3

Drawing a magnetic field line with the help of a

compass needle

n Mark the position of two ends of the needle.

n Now move the needle to a new position

such that its south pole occupies the

position previously occupied by its north

pole.

n In this way, proceed step by step till you

reach the south pole of the magnet as

shown in Fig. 12.3.

n Join the points marked on the paper by a

smooth curve. This curve represents

a field line.

n Repeat the above procedure and draw as

many lines as you can. You will get a

pattern shown in Fig. 12.4. These lines

represent the magnetic field around the

magnet. These are known as magnetic

field lines.

n Observe the deflection in the compass

needle as you move it along a field line.

The deflection increases as the needle is

moved towards the poles.

Figure 12.4Figure 12.4

Figure 12.4

Field lines around a bar magnet

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12.2.1 Magnetic Field due to a Current through a Straight

Conductor

What determines the pattern of the magnetic field generated by a current

through a conductor? Does the pattern depend on the shape of the

conductor? We shall investigate this with an activity.

We shall first consider the pattern of the magnetic field around a

straight conductor carrying current.

Activity 12.4Activity 12.4

Activity 12.4

n Take a long straight copper wire, two or three cells of 1.5 V each, and a plug key. Connect

all of them in series as shown in Fig. 12.5 (a).

n Place the straight wire parallel to and over a compass needle.

n Plug the key in the circuit.

n Observe the direction of deflection of the north pole of the needle. If the current flows from

north to south, as shown in Fig. 12.5 (a), the north pole of the compass needle would move

towards the east.

n Replace the cell connections in the circuit as shown in Fig. 12.5 (b). This would result in

the change of the direction of current through the copper wire, that is, from south to

north.

n Observe the change in the direction of deflection of the needle. You will see that now the

needle moves in opposite direction, that is, towards the west [Fig. 12.5 (b)]. It means that

the direction of magnetic field produced by the electric current is also reversed.

Figure 12.5Figure 12.5

Figure 12.5 A simple electric circuit in which a straight copper wire is placed parallel to and over a compass

needle. The deflection in the needle becomes opposite when the direction of the current is reversed.

(a) (b)

Activity 12.5Activity 12.5

Activity 12.5

n Take a battery (12 V), a variable resistance (or a rheostat), an

ammeter (0–5 A), a plug key, connecting wires and a long straight

thick copper wire.

n Insert the thick wire through the centre, normal to the plane of a

rectangular cardboard. Take care that the cardboard is fixed and

does not slide up or down.

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What happens to the deflection of the compass needle placed at a

given point if the current in the copper wire is changed? To see this, vary

the current in the wire. We find that the deflection in the needle also

changes. In fact, if the current is increased, the deflection also increases.

It indicates that the magnitude of the magnetic field produced at a given

point increases as the current through the wire increases.

What happens to the deflection of the needle if the compass is moved

away from the copper wire but the current through the wire remains the

same? To see this, now place the compass at a farther point from the

conducting wire (say at point Q). What change do you observe? We see

that the deflection in the needle decreases. Thus the magnetic field

produced by a given current in the conductor decreases as the distance

from it increases. From Fig. 12.6, it can be noticed that the concentric

circles representing the magnetic field around a current-carrying straight

wire become larger and larger as we move away from it.

12.2.2 Right-Hand Thumb Rule

A convenient way of finding the direction of magnetic field associated

with a current-carrying conductor is given in Fig. 12.7.

n Connect the copper wire vertically between the

points X and Y, as shown in Fig. 12.6 (a), in

series with the battery, a plug and key.

n Sprinkle some iron filings uniformly on the

cardboard. (You may use a salt sprinkler for this

purpose.)

n Keep the variable of the rheostat at a fixed

position and note the current through the

ammeter.

n Close the key so that a current flows through

the wire. Ensure that the copper wire placed

between the points X and Y remains vertically

straight.

n Gently tap the cardboard a few times. Observe

the pattern of the iron filings. You would find

that the iron filings align themselves showing

a pattern of concentric circles around the

copper wire (Fig. 12.6).

n What do these concentric circles represent?

They represent the magnetic field lines.

n How can the direction of the magnetic field be

found? Place a compass at a point (say P) over

a circle. Observe the direction of the needle. The

direction of the north pole of the compass

needle would give the direction of the field lines

produced by the electric current through the

straight wire at point P. Show the direction by

an arrow.

n Does the direction of magnetic field lines get

reversed if the direction of current through the

straight copper wire is reversed? Check it.

Figure 12.6Figure 12.6

Figure 12.6

(a) A pattern of concentric circles indicating

the field lines of a magnetic field around a

straight conducting wire. The arrows in the

circles show the direction of the field lines.

(b) A close up of the pattern obtained.

(a)

(b)

Variable

resistance

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Imagine that you are holding a current-carrying straight

conductor in your right hand such that the thumb points towards

the direction of current. Then your fingers will wrap around the

conductor in the direction of the field lines of the magnetic field, as

shown in Fig. 12.7. This is known as the right-hand thumb rule*.

Example 12.1

A current through a horizontal power line flows in east to west

direction. What is the direction of magnetic field at a point directly

below it and at a point directly above it?

Solution

The current is in the east-west direction. Applying the right-hand

thumb rule, we get that the magnetic field (at any point below or

above the wire) turns clockwise in a plane perpendicular to the wire,

when viewed from the east end, and anti-clockwise, when viewed

from the west end.

Figure 12.7Figure 12.7

Figure 12.7

Right-hand thumb rule

* This rule is also called Maxwell’s corkscrew rule. If we consider ourselves driving a

corkscrew in the direction of the current, then the direction of the rotation of

corkscrew is the direction of the magnetic field.

12.2.3 Magnetic Field due to a Current through a

Circular Loop

We have so far observed the pattern of the magnetic field lines

produced around a current-carrying straight wire. Suppose

this straight wire is bent in the form of a circular loop and a

current is passed through it. How would the magnetic field

lines look like? We know that the magnetic field produced

by a current-carrying straight wire depends inversely on the

distance from it. Similarly at every point of a current-carrying

circular loop, the concentric circles representing the magnetic

field around it would become larger and larger as we move

away from the wire (Fig. 12.8). By the time we reach at the

centre of the circular loop, the arcs of these big circles would

appear as straight lines. Every point on the wire carrying

current would give rise to the magnetic field appearing as

straight lines at the center of the loop. By applying the right

hand rule, it is easy to check that every section of the wire

contributes to the magnetic field lines in the same direction

within the loop.

Figure 12.8Figure 12.8

Figure 12.8

Magnetic field lines of the field

produced by a current-carrying

circular loop

QUESTIONS

1. Draw magnetic field lines around a bar magnet.

2. List the properties of magnetic field lines.

3. Why don’t two magnetic field lines intersect each other?

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We know that the magnetic field produced by a current-carrying

wire at a given point depends directly on the current passing through it.

Therefore, if there is a circular coil having n turns, the field produced is

n times as large as that produced by a single turn. This is because the

current in each circular turn has the same direction, and the field due to

each turn then just adds up.

Activity 12.6Activity 12.6

Activity 12.6

n Take a rectangular cardboard having two holes.

Insert a circular coil having large number of turns

through them, normal to the plane of the cardboard.

n Connect the ends of the coil in series with a battery,

a key and a rheostat, as shown in Fig. 12.9.

n Sprinkle iron filings uniformly on the cardboard.

n Plug the key.

n Tap the cardboard gently a few times. Note the

pattern of the iron filings that emerges on the

cardboard.

Figure 12.9Figure 12.9

Figure 12.9

Magnetic field produced by a current-

carrying circular coil.

12.2.4 Magnetic Field due to a Current in a Solenoid

A coil of many circular turns of insulated copper wire wrapped

closely in the shape of a cylinder is called a solenoid. The pattern

of the magnetic field lines around a current-carrying solenoid is

shown in Fig. 12.10. Compare the pattern of the field with the

magnetic field around a bar magnet (Fig. 12.4). Do they look

similar? Yes, they are similar. In fact, one end of the solenoid

behaves as a magnetic north pole, while the other behaves as the

south pole. The field lines inside the solenoid are in the form of

parallel straight lines. This indicates that the magnetic field is

the same at all points inside the solenoid. That is, the field is

uniform inside the solenoid.

A strong magnetic field produced inside a solenoid can be

used to magnetise a piece of magnetic material, like soft iron,

when placed inside the coil (Fig. 12.11). The magnet so formed is

called an electromagnet.

Figure 12.10Figure 12.10

Figure 12.10

Field lines of the magnetic field

through and around a current

carrying solenoid.

Figure 12.11Figure 12.11

Figure 12.11

A current-carrying solenoid coil

is used to magnetise steel rod

inside it – an electromagnet.

QUESTIONS

1. Consider a circular loop of wire lying in

the plane of the table. Let the current

pass through the loop clockwise. Apply

the right-hand rule to find out the

direction of the magnetic field inside

and outside the loop.

2. The magnetic field in a given region is

uniform. Draw a diagram to represent it.

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12.312.3

12.3

FORCE ON A CURRENTFORCE ON A CURRENT

FORCE ON A CURRENT

ARRYING CONDUCTORARRYING CONDUCTOR

ARRYING CONDUCTOR

IN A MAGNETIC FIELDIN A MAGNETIC FIELD

IN A MAGNETIC FIELD

We have learnt that an electric curr

ent flowing through a conductor

produces a magnetic field. The field so produced exerts a force on a

magnet placed in the vicinity of the conductor. French scientist Andre

Marie Ampere (1775–1836) suggested that the magnet must also exert

an equal and opposite force on the current-carrying conductor. The force

due to a magnetic field acting on a current-carrying conductor can be

demonstrated through the following activity.

3. Choose the correct option.

The magnetic field inside a long straight solenoid-carrying current

(a) is zero.

(b) decreases as we move towards its end.

(d) is the same at all points.

Activity 12.7Activity 12.7

Activity 12.7

n Take a small aluminium rod AB (of about 5 cm). Using

two connecting wires suspend it horizontally from a

stand, as shown in Fig. 12.12.

n Place a strong horse-shoe magnet in such a way that

the rod lies between the two poles with the magnetic

field directed upwards. For this put the north pole of

the magnet vertically below and south pole vertically

above the aluminium rod (Fig. 12.12).

n Connect the aluminium rod in series with a battery,

a key and a rheostat.

n Now pass a current through the aluminium rod from

end B to end A.

n What do you observe? It is observed that the rod is

displaced towards the left. You will notice that the rod

gets displaced.

n Reverse the direction of current flowing through the

rod and observe the direction of its displacement. It is

now towards the right.

Why does the rod get displaced?

Figure 12.12Figure 12.12

Figure 12.12

A current-carrying rod, AB, experiences

a force perpendicular to its length and

the magnetic field. Support for the

magnet is not shown here, for simplicity.

The displacement of the rod in the above activity suggests that a

force is exerted on the current-carrying aluminium rod when it is placed

in a magnetic field. It also suggests that the direction of force is also

reversed when the direction of current through the conductor is reversed.

Now change the direction of field to vertically downwards by

interchanging the two poles of the magnet. It is once again observed that

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the direction of force acting on the current-carrying rod gets reversed. It

shows that the direction of the force on the conductor depends upon the

direction of current and the direction of the magnetic field. Experiments

have shown that the displacement of the rod is largest (or the magnitude

of the force is the highest) when the direction of current is at right angles

to the direction of the magnetic field. In such a condition we can use a

simple rule to find the direction of the force on the conductor.

In Activity 12.7, we considered the

direction of the current and that of the magnetic

field perpendicular to each other and found

that the force is perpendicular to both of them.

The three directions can be illustrated through

a simple rule, called Fleming’s left-hand rule.

According to this rule, stretch the thumb,

forefinger and middle finger of your left hand

such that they are mutually perpendicular

(Fig. 12.13). If the first finger points in the

direction of magnetic field and the second

finger in the direction of current, then the

thumb will point in the direction of motion or

the force acting on the conductor.

Devices that use current-carrying conductors and magnetic fields

include electric motor, electric generator, loudspeakers, microphones

and measuring instruments.

Example 12.2

An electron enters a magnetic field at right angles to it, as shown

in Fig. 12.14. The direction of force acting on the electron will be

(a) to the right.

(b) to the left.

(d) into the page.

Solution

Answer is option (d). The direction of force is perpendicular to the

direction of magnetic field and current as given by Fleming’s left hand

rule. Recall that the direction of current is taken opposite to the direction

of motion of electrons. The force is therefore directed into the page.

Figure 12.13Figure 12.13

Figure 12.13

Fleming’s left-hand rule

Figure 12.14Figure 12.14

Figure 12.14

QUESTIONS

1. Which of the following property of a proton can change while it moves

freely in a magnetic field? (There may be more than one correct answer.)

(a) mass (b) speed

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12.4 DOMESTIC ELECTRIC CIRCUITS12.4 DOMESTIC ELECTRIC CIRCUITS

12.4 DOMESTIC ELECTRIC CIRCUITS

In our homes, we receive supply of electric power through a main supply

(also called mains), either supported through overhead electric poles or

by underground cables. One of the wires in this supply, usually with

red insulation cover, is called live wire (or positive). Another wire, with

black insulation, is called neutral wire (or negative). In our country, the

potential difference between the two is 220 V.

At the meter-board in the house, these wires pass into an electricity

meter through a main fuse. Through the main switch they are connected

to the line wires in the house. These wires supply electricity to separate

circuits within the house. Often, two separate circuits are used, one of

15 A current rating for appliances with higher power ratings such as

geysers, air coolers, etc. The other circuit is of 5 A current rating for

bulbs, fans, etc. The earth wire, which has insulation of green colour, is

usually connected to a metal plate deep in the earth near the house.

This is used as a safety measure, especially for those appliances that

have a metallic body, for example, electric press, toaster, table fan,

refrigerator, etc. The metallic body is connected to the earth wire, which

provides a low-resistance conducting path for the current. Thus, it

ensures that any leakage of current to the metallic body of the appliance

keeps its potential to that of the earth, and the user may not get a severe

electric shock.

2. In Activity 12.7, how do we think the displacement of rod AB will be

affected if (i) current in rod AB is increased; (ii) a stronger horse-shoe

magnet is used; and (iii) length of the rod AB is increased?

3. A positively-charged particle (alpha-particle) projected towards west is

deflected towards north by a magnetic field. The direction of magnetic

field is

(a) towards south (b) towards east

Magnetism in medicine

An electric current always produces a magnetic field. Even weak ion currents that

travel along the nerve cells in our body produce magnetic fields. When we touch

something, our nerves carry an electric impulse to the muscles we need to use. This

impulse produces a temporary magnetic field. These fields are very weak and are

about one-billionth of the earth’s magnetic field. Two main organs in the human

body where the magnetic field produced is significant, are the heart and the brain. The

magnetic field inside the body forms the basis of obtaining the images of different body

parts. This is done using a technique called Magnetic Resonance Imaging (MRI). Analysis

of these images helps in medical diagnosis. Magnetism has, thus, got important uses

in medicine.

More to Know!

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Figure 12.15Figure 12.15

Figure 12.15 A schematic diagram of one of the common domestic circuits

QUESTIONS

Figure 12.15 gives a schematic diagram of one of the common

domestic circuits. In each separate circuit, different appliances can be

connected across the live and neutral wires. Each appliance has a

separate switch to ‘ON’/‘OFF’ the flow of current through it. In order

that each appliance has equal potential difference, they are connected

parallel to each other.

Electric fuse is an important component of all domestic circuits. We

have already studied the principle and working of a fuse in the previous

chapter (see Section 11.7). A fuse in a circuit prevents damage to the

appliances and the circuit due to overloading. Overloading can occur

when the live wire and the neutral wire come into direct contact. (This

occurs when the insulation of wires is damaged or there is a fault in the

appliance.) In such a situation, the current in the circuit abruptly

increases. This is called short-circuiting. The use of an electric fuse

prevents the electric circuit and the appliance from a possible damage

by stopping the flow of unduly high electric current. The Joule heating

that takes place in the fuse melts it to break the electric circuit.

Overloading can also occur due to an accidental hike in the supply

voltage. Sometimes overloading is caused by connecting too many

appliances to a single socket.

1. Name two safety measures commonly used in electric circuits and

appliances.

2. An electric oven of 2 kW power rating is operated in a domestic electric

circuit (220 V) that has a current rating of 5 A. What result do you

expect? Explain.

3. What precaution should be taken to avoid the overloading of domestic

electric circuits?

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What you have learnt

n A compass needle is a small magnet. Its one end, which points towards north, is

called a north pole, and the other end, which points towards south, is called a

south pole.

n A magnetic field exists in the region surrounding a magnet, in which the force of

the magnet can be detected.

n Field lines are used to represent a magnetic field. A field line is the path along

which a hypothetical free north pole would tend to move. The direction of the

magnetic field at a point is given by the direction that a north pole placed at that

point would take. Field lines are shown closer together where the magnetic field is

greater.

n A metallic wire carrying an electric current has associated with it a magnetic field.

The field lines about the wire consist of a series of concentric circles whose direction

is given by the right-hand rule.

n The pattern of the magnetic field around a conductor due to an electric current

flowing through it depends on the shape of the conductor. The magnetic field of a

solenoid carrying a current is similar to that of a bar magnet.

n An electromagnet consists of a core of soft iron wrapped around with a coil of

insulated copper wire.

n A current-carrying conductor when placed in a magnetic field experiences a force.

If the direction of the field and that of the current are mutually perpendicular to

each other, then the force acting on the conductor will be perpendicular to both

and will be given by Fleming’s left-hand rule.

n In our houses we receive AC electric power of 220 V with a frequency of 50 Hz. One

of the wires in this supply is with red insulation, called live wire. The other one is of

black insulation, which is a neutral wire. The potential difference between the two

is 220 V. The third is the earth wire that has green insulation and this is connected

to a metallic body deep inside earth. It is used as a safety measure to ensure that

any leakage of current to a metallic body does not give any severe shock to a user.

n Fuse is the most important safety device, used for protecting the circuits due to

short-circuiting or overloading of the circuits.

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EXERCISES

1. Which of the following correctly describes the magnetic field near a long

straight wire?

(a) The field consists of straight lines perpendicular to the wire.

(b) The field consists of straight lines parallel to the wire.

(d) The field consists of concentric circles centred on the wire.

2. At the time of short circuit, the current in the circuit

(a) reduces substantially.

(b) does not change.

(d) vary continuously.

3. State whether the following statements are true or false.

(a) The field at the centre of a long circular coil carrying current will be

parallel straight lines.

(b) A wire with a green insulation is usually the live wire of an electric supply.

4. List two methods of producing magnetic fields.

5. When is the force experienced by a current–carrying conductor placed in a magnetic

field largest?

6. Imagine that you are sitting in a chamber with your back to one wall. An electron

beam, moving horizontally from back wall towards the front wall, is deflected by a

strong magnetic field to your right side. What is the direction of magnetic field?

7. State the rule to determine the direction of a (i) magnetic field produced around a

straight conductor-carrying current, (ii) force experienced by a current-carrying

straight conductor placed in a magnetic field which is perpendicular to it, and

(iii) current induced in a coil due to its rotation in a magnetic field.

8. When does an electric short circuit occur?

9. What is the function of an earth wire? Why is it necessary to earth metallic

appliances?

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