Class X

Magnetic effects of electric current

Magnetic effects of current – A current carrying conductor always creates a magnetic field around it.
This is known as magnetic effects of current.
Magnetic effects of current was demonstrated by Hans Christian Oersted by his experiment.

Oersted Experiment

 In 1820, Hans Christian Oersted performed an important experiment which showed that
there is a relation between electricity and magnetism.
 When the current is passed through the conductor, a magnetic needle placed close to the
conductor deflects indicating the presence of magnetic field around it.
 When the current was switched off, magnetic needle of magnetic compass comes back to
original N-S position
 When direction of current was reversed by reversing the terminals of the battery, it was
observed that magnetic needle also showed deflection in opposite direction.
 The direction of deflection of the needle is given by SNOW RULE

SNOW RULE – According to this rule, if current flows from South to North, North of magnetic
needle deflects towards the west.

What is magnetic field?

 It is the space around the magnet or current carrying conductor in which attraction or repulsion
on another magnetic pole can be felt.
 Magnetic field is a vector quantity having magnitude as well as direction.
 The direction of the magnetic field is taken to be the direction in which a north pole of the
compass needle moves inside it.

Magnetic field lines -

They are the path straight or curved in the magnetic field. Tangent drawn to any point on magnetic field
lines gives the direction of magnetic field.
Magnetic field lines are the path along which north of magnetic compass will move.

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Bar magnet
A bar magnet is a long rectangular bar of uniform cross section which attracts pieces of Iron, Nickel
Cobalt towards it.

Magnetic compass

 A compass needle is a small bar magnet whose ends always point towards north south direction.
 The end pointing towards north is called North Pole and the end pointing towards south is called
South Pole.

Magnetic field lines due to a Bar magnet

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Magnetic field pattern due to a Bar magnet

 The field lines emerge from the North pole and end at the south pole outside magnet.
 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.
 Magnetic field lines are closer to each other near the poles showing greater strength of
magnetic field near poles.

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Properties of magnetic field lines
 Magnetic field lines are path straight or curved such that tangent drawn at any point on
magnetic field lines gives the direction of magnetic field.
 Magnetic field lines forms closed curves outside the magnet.
 The direction of field lines is from North to South outside the magnet and from South to North
inside the magnet.
 No two magnetic field lines can intersect each other. This is because if they intersect, there will
two different directions of magnetic field given by two tangents at the point of intersection
which is not possible.

 The degree closeness of magnetic field lines decides the relative strength of magnetic field. If
field lines are closer to each other, it denotes that magnetic field will be stronger in that region.
 Parallel magnetic field lines denotes uniform magnetic field.

Why does a compass gets deflected when brought closer to a bar magnet?
Compass needle is a small magnet itself and it deflects due to interaction between the two magnets.

Magnetic field due to current carrying straight conductor

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The shape of field lines is in form of concentric circles (around a common) center
 The magnitude of the magnetic field produced at a given point increases as the current through
the wire increases.
 The direction of magnetic field produced by the electric current depends upon the direction of
flow of current.
 If we reverse the direction of current then the direction of magnetic field produced by the
electric current gets changed.
 The concentric circles representing the magnetic field around a current-carrying straight wire
become larger and larger as we move away from it.
 Strength of magnetic field produced – increases on increasing the current and decreases as we
move away from the current carrying wire.
 The direction of magnetic field is given by Right Hand Thumb Rule.

Factors affecting strength of magnetic field around a current carrying straight

conductor
 Strength of magnetic field is directly proportional to the current passing through the conductor
and inversely proportional to the distance from the conductor.

Maxwell’s Right Hand Thumb Rule

 If we hold a straight current conductor in our right hand such that the thumb represents the
direction of current then the fingers encircling represent the direction of magnetic field lines.

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Current – upwards, Magnetic field lines – anticlockwise direction
Current – downwards, Magnetic field lines – clockwise direction

Magnetic field due to current carrying circular loop:

We consider a circular loop of radius R carrying current I in the direction shown below

 Magnetic field produced at the center of the loop increases with increase in current
 Magnetic field at the center of the loop decreases if the radius of the circular loop is increased
 The direction of magnetic field lines is given by Right Hand Thumb rule
 This pattern is obtained when current is passed through the circular loop and iron fillings
arrange themselves in the pattern of concentric circles.
 When the current is passed through circular loop or coil, the lines of force are circular near the
wire but straight and parallel near the centre of loop or coil.

Factors affecting magnetic field due to current carrying circular loop or coil.
Magnetic field due to current carrying circular loop at its centre is–
 Directly proportional to the current passing through it.
 Inversely proportional to the radius of loop.

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Solenoid

Magnetic field due to a current carrying solenoid

The field lines inside the solenoid are in the form of straight parallel lines showing uniform magnetic
field.

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Electromagnet
 An electromagnet consists of a long coil of insulated copper wire wrapped around a soft iron core.
 Iron core inserted inside the current carrying solenoid becomes magnetized and is called
electromagnet.
 Soft iron core is the best suited material to make electromagnet as it looses its magnetism
immediately as the current is switched off.
 On the other hand if we use steel, it will not lose its magnetism immediately when current is
switched off and turns into a permanent magnet.

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Factors affecting the strength of magnetic field of an electromagnet
The strength of magnetic field of an electromagnet is –
1. Directly proportional to the number of turns.
2. Directly proportional to the current flowing through it.
3. Inversely proportional to the length of air gaps between the poles.

Uses of electromagnet
 They are used in electrical devices such as electric bell, electric fan, motor, and generator.
 They are used for lifting and transporting large mass of iron.
 They are used in medical practices for removing pieces of iron from wound and used in MRI.

Force on a current carrying conductor in a magnetic field

A current carrying conductor placed in a magnetic field experiences a force due to the interaction
between ---
 Magnetic field due to current carrying conductor and
 External magnetic field in which conductor is placed.

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Fleming’s left hand rule

If we stretch our thumb, index finger and the central finger of our left hand such that they are mutually
perpendicular to each other, if the index finger represents the direction of magnetic field and the central
finger represents the direction of current then the thumb represents the direction of Force experienced
by the conductor.

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Domestic circuits

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