Magnetic effect of a current

Oersted’s experiment on the magnetic effect of

electric current; magnetic field (B) and field lines due
to current in a straight wire (qualitative only), right
hand thumb rule – magnetic field due to a current in
a loop; Electromagnets: their uses; comparisons with
a permanent magnet; conductor carrying current in a
magnetic field experiences a force, Fleming’s Left
Hand Rule and its understanding, Simple introduction
to electromagnetic induction; a magnet moved along
the axis of a solenoid induces current, Fleming’s
Right Hand Rule and its application in understanding
the direction of current in a coil and Lenz’s law,
Comparison of AC and DC.
 Oersted’s experiment on the magnetic effect
of electric current;
Hans Oersted, in 1820, in his experiments observed that when an electric current is
passed through a conducting wire, a magnetic field is produced around it. The
presence of magnetic field at any point around the current carrying wire can be
detected with the help of a campass needle.
Experiment : In Fig. 10.1, AB is a wire lying in the
north-south direction and connected to a battery
through a rheostat and a taping key. A compass
needle is placed just below the wire
.
Observations : (1) When the key is open i.e ., no
current passes through the wire, the needle points in
the N-S direction (i.e ., along the earth's magnetic
field) with the north pole of needle pointing towards
the north direction. In this
position, the needle is parallel to the wire as shown in
Fig. 10.1 (a).

When the key is pressed, a current passes in the

wire in the direction from A to B (i.e., from south to
north) and the north pole (N) of the needle deflects
towards the west [Fig. 10.1 (b)]
. If current in the wire is increased, the deflection of
the needle also increases.
When the direction of current in the wire is reversed
by reversing the connections at the terminals of the
battery, the north pole (N) of the needle deflects
towards the east [Fig. 10.1(c)].

If the compass needle is placed just above the wire,

the north pole (N) deflects towards east when the
direction of current in wire is from A to B [Fig.
10.1(d)], but the needle deflects towards west as
shown in Fig. 10.1 (e) ifthe direction of current in wire
is from B to A.
magnetic field (B) and field lines due to current in
a straight
wire
When a current is passed through a conducting wire, a magnetic field is
produced around it. At a point, the direction of magnetic field will be along the
tangent drawn on the From the magnetic field lines patterns, we note that
the magnetic field lines form the concentric circles around the wire, with their
plane perpendicular to the straight wire and with their centres lying on the
wire

When the direction of current in the wire is reversed, the pattern of iron filings
does not change, but the direction of deflection of the compass needle gets
reversed. The north pole of the compass needle now points in a direction
opposite to the previous direction showing that the direction of magnetic field
has reversed.
(3) On increasing current in the wire, the magnetic field lines become denser
and the iron filings get arranged in circles up to a larger distance from the wire,
showing that the magnetic field strength has increased and it is effective up to a
larger distance.
RULE TO FIND THEDIRECTION OF
MAGNETIC FIELD
Experimentally the direction of magnetic field at a point is determined with the help of
compass needle. But theoretically the direction of magnetic field (or magnetic field lines)
produced due to flow of current in a conductor can be determined by various rules. One
such rule is the right hand thumb rule.

Right hand thumb rule

If we hold the current carrying conductor in our right

hand such that the thumb points in the direction of flow
of current, then the encircle the wire in the direction of
the field lines
Right hand thumb rule
MAGNETIC FIELD DUE TO CURRENT IN A LOOP (OR
CIRCULAR COIL)
The magnetic field lines due to current
in a loop (or circular coil) can be
obtained by the following experiment
Electromagnets: their uses;
1) For lifting and transporting heavy iron scrap, girders, plates, etc. particularly
when it is not convenient to take the help of human labour. Electromagnets are
used to lift as much as 20,000 kg of iron in a single lift. To unload the iron objects
at the desired place, the current in the electromagnet is switched off so that the
electromagnet gets demagnetised and the iron objects get detached.

2) For loading the furnaces with iron.

3) For separating the iron pieces from debris and ores, where iron exists as
impurities (e.g ., for separating iron from the crushed copper ore in copper mines)
.
4) For removing the pieces of iron from wounds.

5) In scientific research, to study the magnetic properties of a substance in a

magnetic field.

6) In several electrical devices such as electric bell, telegraph, electric tram, electric
motor, relay, microphone, loud speaker, etc.
 ADVANTAGES OF AN ELECTRO-MAGNET OVER A
PERMANENT MAGNET

1) An electromagnet can produce a strong magnetic field.

2) The strength of the magnetic field of an electromagnet can easily be

changed by changing the current (or the number of turns) in its solenoid.

(3) The polarity of the electromagnet can be reversed by reversing the

direction of current in its solenoid.
conductor carrying current in a magnetic field
experiences a force

When no current flows in the wire, no force acts on the wire and the wire does not move.

When current I is passed in the wire in direction from A to C (i.e ., in X direction), a force F acts
on the wire in a direction perpendicular to both the direction of current I and the direction of
magnetic field B, as shown in Fig. 10.22 (b). Due to the force F, the conductor begins to move
normal to the plane of paper in outward direction (i.e ., in Z direction).

If the direction of current through the wire is reversed by interchanging the terminals of the
battery, the direction of force (i.e ., the direction of movement of the wire) is also reversed i.e .,
now it moves normal to the .1019 plane of paper in inward direction (or in - Z direction).

On reversing the direction of magnetic field (i.e ., on reversing the polarities N and S of the horse
shoe magnet), the direction of force (i.e ., the direction of movement of wire) is reversed i.e ., now
it movesin - Z direction.
conductor carrying current in a magnetic field
experiences a force
 Fleming’s Left Hand Rule and its
understanding
The direction of force on a current carrying conductor placed in a magnetic field is
obtained by the Fleming's left hand rule.
Fleming's left hand rule : Stretch the forefinger, central finger and the thumb
of your left hand mutually perpendicular to each other as shown in Fig. 10.23. If the
forefinger indicates the direction of magnetic field and the central finger indicates the
direction of current, then the thumb will indicate the direction of motion of conductor
(i.e ., force on conductor).
 ELECTROMAGNETIC INDUCTION

We have read that when an electric current is passed through a conductor, a

magnetic field is produced around the conductor. Faraday thought since a magnetic
field is produced by an electric current, it should be possible to produce an electric
current by the magnetic field. He performed a number of experiments and observed
that whenever there is a change in the number of magnetic field lines linked with a
conductor, an electromotive force (e.m.f.) is developed between the ends of the
conductor which lasts as long as there is a change in the number of magnetic field
lines. This phenomenon is called the electromagnetic induction.
(1) When the magnet is stationary (v = 0), there is no deflection in
galvanometer and its pointer reads zero [Fig. 10.29 (a)].

2) When the magnet with its north pole facing the solenoid is moved towards
it, the galvanometer shows a deflection towards right showing that a current
flows in solenoid in the direction B to A as shown in Fig. 10.29 (b).

3) As the motion of magnet is stopped, the pointer of galvanometer comes to

zero position [Fig. 10.29 (c)]. This shows that the current in solenoid flows as
long as the magnet remains moving.

4) If the magnet is moved away from the solenoid, the pointer of

galvanometer deflects towards left [Fig. 10.29 (d)] showing that the currentin
solenoid flowsagain,but now in direction A to B which is opposite to that
shown in Fig. 10.29 (b).

5) If the magnet is moved away rapidly, the deflection in the galvanometer

increases although the direction of deflection remains same. It shows that
now more current flows.
 6) If the magnet is brought towards the solenoid by keeping its south pole towards
it, the pointer of galvanometer deflects towards left [Fig. 10.29 (e)] showing that
the current in solenoid flows in direction A to B which is opposite to that shown in
Fig. 10.29 (b).

Conclusions :
1) A current flows in the coil only when there is a relative motion between the coil
and magnet.

2) The direction of current is reversed if the direction of motion (or polarity of the
magnet) is reversed.

(3) The current in the coil is increased by

(i) the rapid motion of magnet (or coil),
(ii) the use of a strong magnet, and
(iii) increasing the area of cross section of coil and by increasing the number of
turns in the coil.
Direction of induced e.m.f.
The direction of induced e.m.f. (and hence the direction of induced
current) depends on whether there is an increase or decrease in the
magnetic flux. It can be obtained by any of the following two rules :
(1) Fleming's right hand rule, and (2) Lenz's law.

Fleming's right hand rule

Stretch the thumb, central finger and forefinger of your right hand mutually
perpendicular to each other as shown in Fig. 10.32. If the forefinger
indicates the direction of magnetic field and the thumb indicates the
direction of motion of the conductor, then the central finger will indicate the
direction of induced current.
Fleming's right hand rule
 Lenz's law

According to Lenz's law, the direction of induced e.m.f. (or induced current) is such that it
opposes the cause which produces it.

In Fig. 10.29 (b), when north pole of the magnet is brought towards the end A of solenoid, the
induced current flows in solenoid in direction B to A i.e ., at the end A, the current is
anticlockwise, so the end A of solenoid becomes a north pole to repel the magnet. Thus it
opposes the motion of the north pole of magnet towards the solenoid which is the cause of
producing it.

Similarly, in Fig. 10.29 (d), when north pole of the magnet recedes from this end A of the
solenoid, the direction of induced current in the solenoid is from B to A i.e ., it is clockwise at
the end A, and it becomes the south pole so as to oppose the motion of north pole of the
magnet away from the solenoid which is the cause of producing it.

Lenz's law is based on the law of conservation of energy. It shows that the mechanical energy
is spent in doing work against the opposing force experienced by the moving magnet and it is
transformed into the electrical energy due to which current flows in the solenoid.
 Comparison of AC and DC.

DC
It is the current of constant magnitude.

It flows in one direction in the circuit.

It is obtained from a cell (or battery).

AC
It is the current of magnitude varying perodically with time.

It reverses its direction periodically while flowing in a circuit.

It is obtained from an a.c. generator and mains.