Magnetic effect of_current_1

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MAGNETIC EFFECT OF ELECTRIC CURRENT1. Magnetic Effect of Current – Oersted’s Experiment

2. Ampere’s Swimming Rule, Maxwell’s Cork Screw Rule and

Right Hand Thumb Rule

3. Magnetic Field shown by Iron Filings – Activity

4. Magnetic Field Lines around a Bar Magnet

5. Properties of Magnetic Field Lines

6. Magnetic Field due to a Straight Current carrying Conductor

7. Magnetic Field due to a Circular Loop of a Coil

8. Magnetic Field due to a Current in a Solenoid & Electromagnet

9. Force on a Current carrying Conductor in a Magnetic Field

10.Fleming’s Left Hand Rule

11.Faraday’s Experiments and Laws of Electromagnetic Induction

12.Fleming’s Right Hand Rule

13.A.C. Generator

Created by : shivam kr. Dhawal (sam)

Magnetic Effect of Current:

An electric current (i.e. flow of electric charge) produces magnetic effect in the space around the conductor called strength of Magnetic field or simply Magnetic field.

Oersted’s Experiment:(Hans Christian Oersted (1777-1851)

When current was allowed to flow through a wire placed parallel to the axis of a magnetic needle kept directly below the wire, the needle was found to deflect from its normal position.

NWhen current was reversed through the wire, the needle was found to deflect in the opposite direction to the earlier case.

Rules to determine the direction of magnetic field:

Imagining a man who swims in the direction of current from south to north facing a magnetic needle kept under him such that current enters his feet then the North pole of the needle will deflect towards his left hand, i.e. towards West.

Ampere’s Swimming Rule or SNOW Rule:

If the forward motion of an imaginary right handed screw is in the direction of the current through a linear conductor, then the direction of rotation of the screw gives the direction of the magnetic lines of force around the conductor.

Maxwell’s Cork Screw Rule or Right Hand Screw Rule:

If a current carrying conductor is imagined to be held in the right hand such that the thumb points in the direction of the current, then the tips of the fingers encircling the conductor will give the direction of the magnetic lines of force.

Right Hand Thumb Rule or Curl Rule:

Magnetic Field - Activity

Iron filings sprinkled

Iron filings alligned along with magnetic field lines

Courtesy: Pattern of iron filings from Internet

Magnetic Field Lines around a Magnetic Dipole or Bar Magnet

Properties of Magnetic Field Lines:

1. Magnetic field lines, by convention, emerge from North pole and enter at the South pole.

2. Inside the magnet, the field line is from South to North pole.

3. Thus, the magnetic field lines are closed curves.

4. No two magnetic field lines ever cross each other. If they did, it would mean that at the point of intersection, two magnetic fields would exist and the compass needle would point to two directions, which is not possible.

5. The relative strength of the magnetic field is shown by the degree of closeness of the field lines. The crowded lines indicate stronger magnetic field and the sparse lines indicate weaker field.

6. Magnetic field is a vector quantity having both magnitude and direction.

MAGNETIC FIELD DUE TO A STRAIGHT CURRENT CARRYING CONDUCTOR

Magnetic Field Lines

Magnetic Field Lines due to a straight current indicated by iron filings

Courtesy: Internet

Different views of direction of current and magnetic field due to circular loop of a coil

Magnetic Field due to a Current in a Solenoid

When we look at any end of the coil carrying current, if the current is in anti-clockwise direction then that end of coil behaves like North Pole and if the current is in clockwise direction then that end of the coil behaves like South Pole.

Electromagnet

Force on a Current Carrying Conductor in a Magnetic Field

Fleming’s Left Hand Rule:

Force (F)

Magnetic Field (B)

ElectricCurrent (I)

If the central finger, fore finger and thumb of left hand are stretched mutually perpendicular to each other and the central finger points to current, fore finger points to magnetic field, then thumb points in the direction of motion (force) on the current carrying conductor.

Remember the phrase ‘e m f’ to represent electric current, magnetic field and force in anticlockwise direction of the fingers of left hand.

Faraday’s Experiment - 1:

NS N S

Magnetic flux linked with the coil changes relative to the positions of the coil and the magnet due to the magnetic lines of force cutting at different angles at the same cross sectional area of the coil.

Observe:

i) the relative motion between the coil and the magnet

ii) the induced polarities of magnetism in the coil

iii) the direction of current through the galvanometer and hence the

deflection in the galvanometer

iv) that the induced current (e.m.f) is available only as long as there is

relative motion between the coil and the magnet

Note: i) coil can be moved by fixing the magnet

ii) both the coil and magnet can be moved (towards each other or

away from each other) i.e. there must be a relative velocity between

iii) magnetic flux linked with the coil changes relative to the positions

of the coil and the magnet

iv) current and hence the deflection is large if the relative velocity

between the coil and the magnet and hence the rate of change of

flux across the coil is more

N S NS

Faraday’s Experiment - 2:

When the primary circuit is closed current grows from zero to maximum value.

During this period, changing current induces changing magnetic flux across the primary coil.

This changing magnetic flux is linked across the secondary coil and induces e.m.f (current) in the secondary coil.

Induced e.m.f (current) and hence deflection in galvanometer lasts only as long as the current in the primary coil and hence the magnetic flux in the secondary coil change.

When the primary circuit is open current decreases from maximum value to zero.

During this period changing current induces changing magnetic flux across the primary coil.

This changing magnetic flux is linked across the secondary coil and induces current (e.m.f) in the secondary coil.

However, note that the direction of current in the secondary coil is reversed and hence the deflection in the galvanometer is opposite to the previous case.

Faraday’s Laws of Electromagnetic Induction:

I Law:

Whenever there is a change in the magnetic flux linked with a circuit, an emf and hence a current is induced in the circuit. However, it lasts only so long as the magnetic flux is changing.

II Law:The magnitude of the induced emf is directly proportional to the rate of change of magnetic flux linked with a circuit.

E α dΦ / dt

E = k dΦ / dt(where k is a constant and units are chosen such that k = 1)

E = dΦ / dt E = (Φ2 – Φ1) / t

ElectricCurrent (I)

Force (F)

Magnetic Field (B)

Fleming’s Right Hand Rule:

If the central finger, fore finger and thumb of right hand are stretched mutually perpendicular to each other and the fore finger points to magnetic field, thumb points in the direction of motion (force), then central finger points to the direction of induced current in the conductor.

A.C. Generator

A.C. Generator or A.C. Dynamo or Alternator is a device which converts mechanical energy into alternating current (electrical energy).

A.C. Generator is based on the principle of Electromagnetic Induction.

Principle:

(i) Field Magnet with poles N and S(ii) Armature (Coil) PQRS(iii)Slip Rings (R1 and R2)(iv)Brushes (B1 and B2)(v) Load

Construction:

Working:

Let the armature be rotated in such a way that the arm PQ goes down and RS comes up from the plane of the diagram. Induced emf and hence current is set up in the coil. By Fleming’s Right Hand Rule, the direction of the current is PQRSR2B2B1R1P.

After half the rotation of the coil, the arm PQ comes up and RS goes down into the plane of the diagram. By Fleming’s Right Hand Rule, the direction of the current is PR1B1B2R2SRQP.

If one way of current is taken +ve, then the reverse current is taken –ve.

Therefore the current is said to be alternating and the corresponding wave is sinusoidal.

Theory:

Φ = N B A cos θ

At time t, with angular velocity ω,

θ = ωt (at t = 0, loop is assumed to be perpendicular to the magnetic field and θ = 0°)

Φ = N B A cos ωt

Differentiating w.r.t. t,

dΦ / dt = - NBAω sin ωt

E = - dΦ / dt

E = NBAω sin ωt

E = E0 sin ωt (where E0 = NBAω)

0π 2π 3π 4π

T/4 T/2 3T/4 T 5T/4 3T/2 7T/4 2Tt

π/2 3π/2 5π/2 7π/2 θ = ωt

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