lenses law

2. ELCTROMAGNETIC INDUCTION AND
LENZ”S LAW

3. Current give rise to a magnetic field so
scientists started working for its converse
process i-e
Magnetic field should give rise to
electric current

4. Introduction
Previous cnocept
What do you mean by magnetic
flux?
What do you mean by induced
EMF?
What is the relation ship between
magnetic flux and induced
EMF?
New objective
Induced EMF will give rise to
induced current.We will now
find how to find direction of
induced EMF

5. ELCTROMAGNETIC INDUCTION AND
LENZ”S LAW
Lesson content
Effcts of induction
Lenz’s Law
Electromagnetic induction
Examples
uses

6. Electro-Magnetic Induction
An EMF is induced in the coil, when it is
rotated in an uniform magnetic field due to
change in magnetic flux.

7. Induction Effects
v
v
S N
v
N S
N S
S N
 Bar magnet moves through coil
 Current induced in coil
• Change pole that enters
 Induced current changes sign
 Bar magnet stationary inside coil
 No current induced in coil
 Coil moves past fixed bar magnet
 Current induced in coil

8. Induction Effects
from Currents
• Switch closed (or opened)
 current induced in coil b
• Steady state current in coil a
 no current induced in coil b
a
b
 Conclusion:
A current is induced in a loop when:
• there is a change in magnetic field through it
• loop moves through a magnetic field
 How can we quantify this?

9. EXPERIMENT

10. Faraday's Law
:
 Faraday's Law:
The emf induced in a circuit is determined by the time rate of
change of the magnetic flux through that circuit.
The minus sign indicates direction of induced current (given by Lenz's
Law).

11. Heinrich F.E. Lenz
 Russian physicist
 (1804-1865)
 1834 Lenz’s Law
 There is an induced current
in a closed conducting loop
if and only if the magnetic
flux through the loop is
changing. The direction of
the induced current is such
that the induced magnetic
field always opposes the
change in the flux.
LENZ”S LAW . ABOUT THE SCIENTIST

12. Lenz's Law
• Lenz's Law:
The induced current will appear in such a direction that it opposes the
change in flux that produced it.
 Conservation of energy considerations:
Claim: Direction of induced current must be so as to oppose the
change; otherwise conservation of energy would be violated.
 Why???
 If current reinforced the change, then the change
would get bigger and that would in turn induce a larger
current which would increase the change, etc..
v
B
S N
v
B
N S

13. Right Hand Rule
 If you wrap your
fingers around the
coil in the
direction of the
current, your
thumb points
north.
DIRECTION OF FIELD IS GIVEN BY
RIGHT HAND RULE

14. There is an induced current in a closed conducting loop if and only if
the magnetic flux through the loop is changing. The direction of the
induced current is such that the induced magnetic field always opposes
the change in the flux.

15. If the field of the bar magnet is already in
the loop and the magnet is removed, the induced
current is in the direction that tries to keep
the field constant.
,

16. 2.DIRECTION OF INDUCED
CURRENT
In both cases, magnet
moves against a force.
Work is done during
the motion & it is
transferred as electrical
energy.
Induced I always flows to oppose the
movement which started it.
b Lenz's law

17. Current induced along a coil
A bar magnet passes through a coil:
(a) Indicate the direction of the induced I in
each case. Explain briefly.
(i) (ii) (iii)

18. Current induced along a coil
(a) Indicate the direction of the induced I. Explain.
(i)
When magnet’s N-pole is moving
into coil,
induced I flows in such a direction as
to produce a N-pole
to oppose the approaching of magnet.
Lenz’s
law
I
S N

19. Current induced along a coil
(a) Indicate the direction of the induced I. Explain.
(ii)
The induced I become zero

I is about to change direction.

20. Current induced along a coil
(a) Indicate the direction of the induced I. Explain.
(iii)
When magnet’s S-pole is leaving the coil,
induced I flows in such a direction as to
produce a N-pole to oppose the leaving of
magnet.
I
N S

21. Lenz’s Law
 Alternating current
through the coil
 What happens to Iron
core?
 What will happen to
the ring?
APPLICATIONS OF LENZ”S LAW

22. LENZ LAW
 The changing magnetic
field in the coil due to the
change in current induces
a current in the metal
ring. Lenz’s Law states
that the magnetic field in
the ring opposes the
magnetic field of the coil
and forces the ring to fly
away.

23. LENZ LAW
 What just happened?
 How often does this
occur?

24. FALLING MAGNET
 the copper tube "sees"
a changing magnetic
field from the falling
magnet.This changing
magnetic field induces
a current in the copper
tube.
 The induced current in
the copper tube creates
its own magnetic field
that opposes the
magnetic field that
created it.

25. APPLICATIONS OF MAGNETIC INDUCTION
 Levitation (Maglev)Trains
 Induced surface (“eddy”) currents produce field in opposite
direction
 Repels magnet
 Levitates train
 Maglev trains today can travel up to 310 mph
Twice the speed of Amtrak’s fastest conventional train!
N
S
rails
“eddy” current

26. BASE ON LENZ LAW
0-70 mph in 3 sec

27. Recapitulation
Effcts of induction
Lenz’s Law
Electromagnetic induction
Examples
uses