Magnetic field lines of a bar magnet emerge from the north pole and terminate
at the south pole. Inside the magnet, the field lines emerge from the south pole
and terminate at the north pole, as shown in the given figure.
Magnetic Field Lines around a bar magnet
Inside the loop = Pierce inside the table
Outside the loop = Appear to emerge out from the table
For downward direction of current flowing in the circular loop, the direction of
magnetic field lines will be as if they are emerging from the table outside the
loop and merging in the table inside the loop. Similarly, for upward direction
of current flowing in the circular loop, the direction of magnetic field lines
will be as if they are emerging from the table outside the loop and merging in
the table inside the loop, as shown in the given figure.
The magnetic field lines inside a current-carrying long straight solenoid are
uniform.
A solenoid is a long coil of circular loops of insulated copper wire.
Magnetic field lines are produced around the solenoid when a current is allowed
to flow through it. The magnetic field produced by it is similar to the magnetic
field of a bar magnet. The field lines produced in a current-carrying solenoid
is shown in the following figure.
In the above figure, when the north pole of a bar magnet is brought near the end
connected to the negative terminal of the battery, the solenoid repels the bar
magnet. Since like poles repel each other, the end connected to the negative
terminal of the battery behaves as the north pole of the solenoid and the other
end behaves as a south pole. Hence, one end of the solenoid behaves as a north
pole and the other end behaves as a south pole.
An electric motor converts electrical energy into mechanical energy.
Principle: It works on the principle of the magnetic effect of current. A
current-carrying coil rotates in a magnetic field. The following figure shows a
simple electric motor.
When a current is allowed to flow through the coil MNST by closing the switch,
the coil starts rotating anti-clockwise. This happens because a downward force
acts on length MN and at the same time, an upward force acts on length ST. As a
result, the coil rotates anti-clockwise.
Current in the length MN flows from M to N and the magnetic field acts from left
to right, normal to length MN. Therefore, according to Fleming's left hand rule,
a downward force acts on the length MN. Similarly, current in the length ST
flows from S to T and the magnetic field acts from left to right, normal to the
flow of current. Therefore, an upward force acts on the length ST. These two
forces cause the coil to rotate anti-clockwise.
After half a rotation, the position of MN and ST interchange. The half-ring D
comes in contact with brush A and half-ring C comes in contact with brush B.
Hence, the direction of current in the coil MNST gets reversed.
The current flows through the coil in the direction TSNM. The reversal of
current through the coil MNST repeats after each half rotation. As a result, the
coil rotates unidirectional. The split rings help to reverse the direction of
current in the circuit. These are called the commutator.
Principle: An electric generator works on the principle of electromagnetic
induction phenomenon. According to it, whenever a coil is rotated between the
poles of a magnet, an induced current is set up in the coil, whose direction is
given by Fleming�s right hand rule.
Diagram of Electric Generator
Working: Let in the beginning, as shown in Fig. brushes B1 and B2are kept
pressed separately on rings R1 and R2 respectively. Let the axle attached to the
rings is rotated such that arm AB of the coil moves up and arm CD moves down in
the magnetic field. Due to rotation of arms AB and CD induced currents are set
up in them. As per Fleming�s right hand rule induced currents in these arms are
along the directions AB and CD. Thus an induced current flows along ABCD and
current in the external circuit flows from B2 to B1.
After half a rotation, arm AB starts moving down and the arm CD upward.
Therefore, directions of induced currents in these arms change. Thus net induced
current now becomes in the direction DCBA. In the external circuit now current
flows from B1 to B2. Thus after every half rotation current changes its
direction and an alternating current is obtained from the generator.
Function of Brushes: Brushes are kept pressed on the two slip
tings separately. Outer ends of the brushes are connected to the galvanometer
(or the external load). Thus brushes help in transferring current from the coil
ABCD to the external circuit.
1. Choose the incorrect statement from the following regarding magnetic lines of field
(a) The direction of magnetic field at a point is taken to be the direction in which the north pole of a magnetic compass needle points
(b) Magnetic field lines are closed curves
(c) If magnetic field lines are parallel and equidistant, they represent zero field strength
(d) Relative strength of magnetic field is shown by the degree of closeness of the field lines
Ans. (c) If magnetic field lines are parallel and equidistant, they represent zero field strength Explanation: Magnetic field lines appear parallel when they are far from the magnet. But this does not mean that field strength is zero. No field line would be present where field strength becomes zero.
2. If the key in the arrangement (Figure 13.1) is taken out (the circuit is made open) and magnetic field lines are drawn over the horizontal plane ABCD, the lines are
(a) concentric circles
(b) elliptical in shape
(c) straight lines parallel to each other
(d) concentric circles near the point O but of elliptical shapes as we go away from it
Ans. (a) Concentric circles
Explanation: Magnetic field lines around a straight current carrying conductor are in the form of concentric circles.
3. A circular loop placed in a plane perpendicular to the plane of paper carries a current when the key is ON. The current as seen from points A and B (in the plane of paper and on the axis of the coil) is anti-clockwise and clockwise respectively.
The magnetic field lines point from B to A. The N-pole of the resultant magnet is on the face close to
(a) A
(b) B
(c) A if the current is small, and B if the current is large
(d) B if the current is small and A if the current is large
Ans. (a) A
Explanation: Magnetic field lines are in the direction from south pole to north pole. Point
A is showing north pole because field lines are pointing from B to A.
4. For a current in a long straight solenoid N- and S-poles are created at the two ends. Among the following statements, the incorrect statement is
(a) The field lines inside the solenoid are in the form of straight lines which indicates that the magnetic field is the same at all points inside the solenoid
(b) The strong magnetic field produced inside the solenoid can be used to magnetise a piece of magnetic material like soft iron, when placed inside the coil
(c) The pattern of the magnetic field associated with the solenoid is different from the pattern of the magnetic field around a bar magnet
(d) The N- and S-poles exchange position when the direction of current through the solenoid is reversed
Ans. (c) The pattern of the magnetic field associated with the solenoid is different from the pattern of the magnetic field around a bar magnet
Explanation: A solenoid behaves like a bar magnet. Hence, the pattern of the magnetic field associated with the solenoid is same as the pattern of the magnetic field around a bar magnet.
5. A uniform magnetic field exists in the plane of paper pointing from left to right as shown in Figure 13.3. In the field an electron and a proton move as shown. The electron and the proton experience
(a) forces both pointing into the plane of paper
(b) forces both pointing out of the plane of paper
(c) forces pointing into the plane of paper and out of the plane of paper, respectively
(d) force pointing opposite and along the direction of the uniform magnetic field respectively
Ans. (a) forces both pointing into the plane of paper
Explanation: Direction of current is opposite to the direction of movement of electron. So, current will move upwards. If index finger is showing the direction of magnetic field, ring finger is showing the direction of current, then direction of thumb is into the paper.
6. Commercial electric motors do not use
(a) an electromagnet to rotate the armature
(b) effectively large number of turns of conducting wire in the current carrying coil
(c) a permanent magnet to rotate the armature
(d) a soft iron core on which the coil is wound
Ans. (c) A permanent magnet to rotate the armature
Explanation: Electromagnet is used instead of permanent magnet in commercial electric motor.
7. In the arrangement shown in Figure 13.4 there are two coils wound on a nonconducting cylindrical rod. Initially the key is not inserted. Then the key is inserted and later removed. Then
(a) the deflection in the galvanometer remains zero throughout
(b) there is a momentary deflection in the galvanometer but it dies out shortly and there is no effect when the key is removed
(c) there are momentary galvanometer deflections that die out shortly; the deflections are in the same direction
(d) there are momentary galvanometer deflections that die out shortly; the deflections are in opposite directions
Ans. (d) there are momentary galvanometer deflections that die out shortly; the deflections are in opposite directions.
Explanation: When key is plugged, galvanometer shows momentary deflection in one direction. When the key is removed, galvanometer shows momentary deflection in opposite direction.
8. Choose the incorrect statement
(a) Fleming’s right-hand rule is a simple rule to know the direction of induced current
(b) The right-hand thumb rule is used to find the direction of magnetic fields due to current carrying conductors
(c) The difference between the direct and alternating currents is that the direct current always flows in one direction, whereas the alternating current reverses its direction periodically
(d) In India, the AC changes direction after every 1/50 second
Ans. (d) In India, the AC changes direction after every 1/50 second
Explanation: In India, the AC changes direction after every 1/100 second.
9. A constant current flows in a horizontal wire in the plane of the paper from east to west as shown in Figure 13.5. The direction of magnetic field at a point will be North to South
(a) directly above the wire
(b) directly below the wire
(c) at a point located in the plane of the paper, on the north side of the wire
(d) at a point located in the plane of the paper, on the south side of the wire
Ans. (b) directly below the wire
Explanation: Line WE shows a straight conductor through which current is moving from E to W. When seen from east, the magnetic field lines appear in clockwise direction, i.e. S to N above the wire and N to S below the wire. This is in accordance with Right Hand Thumb rule.
10. The strength of magnetic field inside a long current carrying straight solenoid is
(a) more at the ends than at the centre
(b) minimum in the middle
(c) same at all points
(d) found to increase from one end to the other
Ans. (c) Same at all points
Explanation: Magnetic field lines are straight and parallel inside the solenoid. This indicates a strong magnetic field. Hence, inside the solenoid, the magnetic field is same throughout.
11. To convert an AC generator into DC generator
(a) split-ring type commutator must be used
(b) slip rings and brushes must be used
(c) a stronger magnetic field has to be used
(d) a rectangular wire loop has to be used
Ans. (a) split-ring type commutator must be used
Explanation: Split ring commutator reverses the direction of current after each half turn of armature. This maintains a DC current.
12. The most important safety method used for protecting home appliances from short circuiting or overloading is
(a) earthing
(b) use of fuse
(c) use of stabilizers
(d) use of electric meter
Ans. (b) Use of fuse
A magnetic compass needle is placed in the plane of paper near point A as
shown in Figure 13.6. In which plane should a straight current carrying
conductor be placed so that it passes through A and there is no change in the
deflection of the compass? Under what condition is the deflection maximum and
why?
Ans. In the plane of the paper itself. The axis of the compass is vertical and
the field due to the conductor is also vertical. It could result in a dip of
compass needle which is not possible in this case (dips result only if axis of
compass is horizontal). The deflection is maximum when the conductor through A
is perpendicular to the plane of paper and the field due to
it is maximum in the plane of the paper.
Under what conditions permanent electromagnet is obtained if a current carrying solenoid is used? Support your answer with the help of a labelled circuit diagram.
Ans. Following conditions are necessary for making electromagnet from a current carrying
solenoid.
(a) Circuit should be closed.
(b) A core of soft iron should be used.
According to Right Hand Thumb Rule, if all fingers of right hand we wrapped in a fist and thumb shows the direction of electric current, then direction of wrapped fingers shows the direction of magnetic field. In case of r1, the direction of magnetic field is towards the paper. In case of r2, the direction of magnetic field is out of the paper. Since r1 > r2, so strength of magnetic field at P is less than that at Q.
Right hand thumb rule states that if a current carrying straight conductor is supposedly held in the right hand with the thumb pointing towards the direction of current, then the fingers will wrap around the conductor in the direction of the field lines of the magnetic field.
In a current carrying circular loop also the Right-Hand Thumb Rule is being obeyed. This means magnetic field lines are around the conducting wire. But circular shape of the conductor means that field lines at different points of the loop appear to be making ring around the periphery of the loop. It can be visualized like many small rings looping around the periphery of a big ring. Effect of Number of turns in a coil: If number of turns in coil is increased, then magnetic field has add on effect. Due to this, strength of magnetic field increases with increased number of turns in a coil.
Take a small rod (AB) of aluminium and suspend it with a stand; using two connecting wires.
• Take a horse-shoe magnet and keep it in a way that its magnetic field is in upward direction. For this, the north pole should be at the bottom and south pole should be at the top. The aluminium rod should be between the two poles of this magnet.
• Now connect the aluminium rod to a battery and plug key so that current flows through it
from B to A.
• You will notice that the aluminum rod moves towards left.
• Now, change the direction of current from A to B.
• In this case, the aluminium rod moves towards right.
In this experiment, direction of current and direction of magnetic field are mutually perpendicular. Direction of movement of aluminium rod is perpendicular to direction of current and direction of magnetic field.
Fleming’s Left-Hand Rule: If the thumb, index finger and ring finger of left hand are kept in mutually perpendicular direction then index finger shows the direction of magnetic field, ring finger shows the direction of electric current and thumb shows the direction of deflection in current carrying conductor. By using this rule, we can easily find the direction of deflection.
ABCD is a coil which is placed between the two poles of a permanent magnet. The coil is placed in such a way that the direction of current is perpendicular to the direction of magnetic field.
• Arms of the coil are attached to a split ring; which has two halves P and Q. Arms AB is attached to the half P, and arm CD is attached to the half Q.
• The split rings are touching two static brushes X and Y from which they get electricity supply. The split rings are insulated from inside. An axle is passing through the split rings.
• A battery supplies electric current to the coil. The flow of current is from A to B and C to D.
Working of Electric Motor:
Obeying Fleming’s Left Hand rule, when current passes from A to B; the arm AB of coil moves down. On the contrary, when current passes from C to D; the arm CD moves up.
When the coil makes half a turn, P of split ring touches brush Y and its opposite happens with Q of split ring. As a result, the direction of current gets reversed. This means current now flows D to C and from B to A. This pushes CD down and AB up. As a result, the coil turns continuously.
Electromagnetic Induction: This was first demonstrated by Michael faraday in 1831. When a conductor and magnetic field are in motion relative to each other, electric currents is induced in the conductor. This phenomenon is called electromagnetic induction.
• Take two coils with large number of turns. Let us assume that number of turns in one coil
is 100 and in another coil it is 50.
• Insert a cylinder of a non-conducting material through these coils.
• Connect the coil with 50 turns to a galvanometer.
• Connect the coil with 100 turns to a battery and plug key.
Observation:
• When key is switched on, deflection is seen in galvanometer.
• When key is switched off, deflection is seen in galvanometer; but in opposite direction.
• This shows that whenever there is a change in strength of magnetic field, current is induced in the coil.
Important of Fuse:
• Fuse is an important safety device in household wiring and in electrical appliance.
• Fuse prevents any damage to the circuit and appliance which may happen due to overload.
• Fuse prevents accidental fire which may happen because of short circuit.
Fuse of a particular rating is used with a particular appliance. A fuse with lower rating would result in lot of inconvenience because of frequent need of changing the fuse. A fuse with higher rating would not serve the purpose because it will not melt even if the current exceeds the limit of the device. Hence, a burnt out fuse should be replaced by another fuse of identical rating.
ABCD is rectangular coil placed between the two poles of a permanent magnet.
• Arms AB of coil is attached to a ring R1 and arm CD is attached to the ring R2.
• R1 and R2 are insulated from inside and are attached to an axle.
• A brush B1 touches R1 and another brush B2 touches R2.
• A galvanometer is connected to B1 and B2 to show the flow and direction of current in the circuit.
Working of AC Generator:
• AC generator works in Fleming’s Right Hand Rule.
• When coil is rotated within the magnetic field, electric current is induced in it.
• Let us assume that the coil is rotating clocking in this figure. This means, AB is moving up and CD is moving down.
• When AB is moving up, current flows in it from A to B.
• When CD is moving down, current flows in it from C to D.
• After half a turn; relative positions of AB and CD change. Now, CD is moving up hence current is moving from D to C. Similarly, AB is moving down hence current is moving from B to A.
• So, after every half turn the direction of current changes in this generator. This means alternating current is being generated.
Conversion of AC generator to DC generator: For this, a split ring commutator is used in place of two separate rings. A split ring commutator maintains the direction of current in one direction and it becomes a DC generator
