Download Lenz`s Law

Chapter 23
Magnetic Flux and Faraday’s Law
of Induction (Lecture II)
Dr. Jie Zou
PHY 1161
1
Outline

Lenz’s law


Motional EMF





Several examples
A qualitative discussion
A quantitative discussion
Mechanical work/electrical energy
An example
Real world applications

Electric generators and motors
Dr. Jie Zou
PHY 1161
2
Lenz’s Law


Lenz’s Law: An induced current always flows
in a direction that opposes the change that
caused it.
Example 1: consider a bar magnet that is
moved toward or away from a conducting ring.
Dr. Jie Zou
PHY 1161
3
More Examples of Lenz’s Law


Example 2 (Conceptual
checkpoint 23-2): As the
magnets drop toward the
rings, does the magnet on the
left have an acceleration that
is (a) more than, (b) less
than, or (c) the same as that
of the magnet on the right?
Example 3: Suppose that a
magnetic field is decreased
with time. What is the
direction of the induced
current in the ring?
Dr. Jie Zou
PHY 1161
4
More Examples of Lenz’s Law
(Cont.)

Dr. Jie Zou
Example 4 (conceptual
checkpoint 23-3):
Consider a system in
which a metal ring is
falling out of a region
with a magnetic field
and into a field-free
region. According to
Lenz’s law, is the
induced current in the
ring (a) clockwise or (b)
counterclockwise?
PHY 1161
5
Motional EMF: Qualitative



Motional emf is created in
this system as the rod falls.
The result is an induced
current, which causes the
light to shine.
What is the direction of the
induced current when the
rod is released from rest and
allowed to fall?
Connections between
mechanical work and
electrical energy.
Dr. Jie Zou
PHY 1161
6
Motional EMF: Quantitative


Example: Consider a rod that
slides horizontally without
friction. A constant magnetic
field points out of the page, and
the rod is pushed by an external
agent so that it moves to the
right with a constant speed v.
Find the following:



The increase in magnetic flux, .
The magnitude of the induced emf,
||.
The electric filed caused by the
motion of the rod, E.
Dr. Jie Zou
PHY 1161
7
Mechanical Work/Electrical
Energy

Find the following:




Dr. Jie Zou
The magnetic force acting
on the rod, F.
The mechanical power
delivered by the external
force Pmechanical.
The electrical power
converted to light and heat
in the light bulb Pelectrical.
With the aid of a magnetic
field, we can convert
mechanical power directly
to electrical power.
PHY 1161
8
Example 23-3 Light Power



The light bulb in the circuit
shown has a resistance of 12
 and consumes 5.0 W of
power; the rod is 1.25 m long
and moves to the left with a
constant speed of 3.1 m/s.
(a) What is the strength of the
magnetic field?
(b) What external force is
required to maintain the rod’s
constant speed?
Dr. Jie Zou
PHY 1161
9
Electric Generators and Motors
(a) An Electric Generator
Dr. Jie Zou
(b) An Electric Motor
PHY 1161
10
Homework #9

Chapter 23, P. 830-831, Problems:
#29, 32, 38 (Physics, Walker, 4th
edition).
Dr. Jie Zou
PHY 1161
11