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MAGNETIC EFFECT OF ELECTRIC CURRENT
1. 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
ADARSH SHARMA K.V OEF CANTT KANPUR
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)
N
E
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.
I
K
I
When current was reversed through the wire,
the needle was found to deflect in the
opposite direction to the earlier case.
N
E
K
Rules to determine the direction of magnetic field:
Ampere’s Swimming Rule or SNOW Rule:
S
I
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.
Maxwell’s Cork Screw Rule or Right Hand Screw Rule:
I
I
B
B
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.
Right Hand Thumb Rule or Curl Rule:
I
B
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.
Magnetic Field
- Activity
Iron filings alligned
alongIron
withfilings
magnetic
sprinkled
field lines
S
N
Courtesy: Pattern
of iron filings from
Internet
Magnetic Field Lines around a Magnetic Dipole or Bar Magnet
S
N
B
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
I
I
B
B
E
E
I
I
Magnetic Field Lines
K
K
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
I
I
I
B
B
I
E
I
I
K
I
B
I
I
Eye
Magnetic Field due to a Current in a Solenoid
B
I
I
E
K
TIP:
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
I
I
E
K
Force on a Current Carrying Conductor in a Magnetic Field
N
I
S
E
K
I
Fleming’s Left Hand Rule:
Force
(F)
Magnetic
Field
(B)
Electric
Current
(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.
TIP:
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:
S
N
N
N
S
G
N
G
S
S
S
G
S
N
N
G
S
N
N
S
G
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
them
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
Faraday’s Experiment - 2:
N
S
S
P
E
During this period, changing
current induces changing
magnetic flux across the primary
coil.
S
G
K
N
S
P
E
N
N
S
S
K
G
When the primary circuit is closed
current grows from zero to
maximum value.
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 /
E = dΦ / dt
E = (Φ2 – Φ1) / t
dt
(where k is a constant and units are chosen such that k = 1)
Fleming’s Right Hand Rule:
Magnetic
Field
(B)
Force
(F)
Electric
Current
(I)
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
R1
R1
B1
R2
B1
R2
B2
Load
B2
Load
A.C. Generator or A.C. Dynamo or Alternator is a device which converts
mechanical energy into alternating current (electrical energy).
Principle:
A.C. Generator is based on the principle of Electromagnetic Induction.
Construction:
(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
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
ω
At time t, with angular velocity ω,
R
θ = ωt
(at t = 0, loop is assumed to
be perpendicular to the magnetic field
and θ = 0°)
Φ = N B A cos ωt
Q
θ
B
n
Differentiating w.r.t. t,
dΦ / dt = - NBAω sin ωt
S
E = - dΦ / dt
E = NBAω sin ωt
E = E0 sin ωt
(where E0 = NBAω)
P
E0
0
π/2
π
T/4
T/2
3π/2 2π 5π/2 3π
3T/4
T
7π/2 4π θ = ωt
5T/4 3T/2 7T/4 2T
t
More of Magnetic Effect of
Current in Higher Class…