Lenz's law states that (1) an induced current will flow in a closed conducting loop when the magnetic flux through the loop changes, and (2) the direction of this induced current will be such that it creates its own magnetic field opposing the original change in flux. (3) Examples are given demonstrating how an induced current in a coil will flow in a direction opposing the motion of a magnet moving in or out of the coil.

1. LENZ’S LAW

2. 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.

3. 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.

4. Right Hand Rule
• If you wrap your
fingers around the
coil in the direction
of the current, your
thumb points
north.

5. 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

6. 2 Direction of induced current
b Lenz's law
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.

7. Example 1
Current induced along a coil
A bar magnet passes through a coil:
(i) (ii) (iii)
(a) Indicate the direction of the induced I in
each case. Explain briefly.

8. Example 1
Current induced along a coil
(a) Indicate the direction of the induced I. Explain.
S N
When magnet’s N-pole is moving
I into coil,
(i) induced I flows in such a direction as
to produce a N-pole
Lenz’
s law to oppose the approaching of magnet.

9. Example 1
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.
∴

10. Example 1
Current induced along a coil
(a) Indicate the direction of the induced I. Explain.
N S
I
(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.

11. Lenz’s Law
• Alternating current
through the coil
• What happens to Iron
core?
• What will happen to
the ring?

12. Lenz’s 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.

13. Lenz’s Law
• What just happened?
• How often does this
occur?

14. Applications of Magnetic Induction
• Magnetic Levitation (Maglev) Trains
• Induced surface (“eddy”) currents produce field in
opposite direction
 Repels magnet
 Levitates train
S
N
rails
“eddy” current
• Maglev trains today can travel up to 310 mph
 Twice the speed of Amtrak’s fastest conventional
train!

15. liner induction
0-70 mph in 3 sec

16. liner induction

17. 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.