Electromagnetic induction. Lesson for MaCS and Academic classes.

1. Lesson 4Lesson 4
Electromagnetic Induction
Nelson Reference: 588-594
(Not a complete reference)
McGraw-Hill Reference Pages:
718-719, 723-725

2.  In 1820 Ampere showed that a constant currentIn 1820 Ampere showed that a constant current
produced a steady magnetic field. In 1831, afterproduced a steady magnetic field. In 1831, after
reading Ampere’s paper on electromagnetism,reading Ampere’s paper on electromagnetism,
Michael Faraday believed that a steady magneticMichael Faraday believed that a steady magnetic
field could produce a constant current. Faradayfield could produce a constant current. Faraday
used the experiment below (calledused the experiment below (called Faraday’s Ring))
toto trytry to show this.to show this.
A
IR O N R IN G
S w it c h

3. To detect small currents, Faraday used aTo detect small currents, Faraday used a
galvanometer (not just an ammeter)(not just an ammeter)
What Faraday Found:
 When the switch was closed, the galvanometerWhen the switch was closed, the galvanometer
briefly deflected one way and then returned tobriefly deflected one way and then returned to
zero.zero.
 When the switch was opened, the galvanometerWhen the switch was opened, the galvanometer
briefly deflected the other way and then returnedbriefly deflected the other way and then returned
to zero.to zero.
 The conclusion was that a changing magneticThe conclusion was that a changing magnetic
field (not constant) could produce a current. Thefield (not constant) could produce a current. The
changing current direction leads to the creationchanging current direction leads to the creation
of alternating or AC current that we use today.of alternating or AC current that we use today.

4. The Generator Effect
 Moving a magnetMoving a magnet
towards a copper ringtowards a copper ring
causes a current to becauses a current to be
generated in the ring.generated in the ring.
If the movement of theIf the movement of the
magnet stops, so doesmagnet stops, so does
the current. Thethe current. The
direction of the currentdirection of the current
can be found fromcan be found from
RHR #4 or Lenz Law.RHR #4 or Lenz Law.
N
S
F r o n t
B a c k
C o p p e r
R in g
DOM
I
I

5. RHR #4 – A Palm Rule (Refer to previous(Refer to previous
diagram)diagram)
(Not taught to Academic Class)
 Fingers point in the direction of the magneticFingers point in the direction of the magnetic
field as it crosses the conductor.field as it crosses the conductor.
 Thumb points in the direction ofThumb points in the direction of relative
motion of the coil. (If the magnet moves upof the coil. (If the magnet moves up
this means thethis means the relative motion of the coil isof the coil is
down – so thumb points down)down – so thumb points down)
 Out of the palm is the direction of the inducedOut of the palm is the direction of the induced
current.current.

6. Lenz Law
{All students should be able to apply this law}
A few years after Faraday’s discovery ofA few years after Faraday’s discovery of
electromagnetic induction, Germanelectromagnetic induction, German
physicist Heinrich Lenz applied the law ofphysicist Heinrich Lenz applied the law of
conservation of energy to the same typeconservation of energy to the same type
of problem. What he found was that theof problem. What he found was that the
induced current produced a magnetic fieldinduced current produced a magnetic field
that always opposed the motion of the coilthat always opposed the motion of the coil
or the magnet. This is shown in theor the magnet. This is shown in the
following diagram.following diagram.

7.  In the diagram below, the bar magnet is beingIn the diagram below, the bar magnet is being
brought towards the solenoid (coils arebrought towards the solenoid (coils are
connected so a current can flow). The fieldconnected so a current can flow). The field
generated is as shown and opposes the motiongenerated is as shown and opposes the motion
of the bar magnet.of the bar magnet.
 If the direction of the bar magnet is reversed
(moves away from the solenoid), what happens
to the magnetic field and direction of current?
I
N
S
D O M
NS

8. Practice Questions
Nelson TB
 Page 594 #1 (assume solenoid wires arePage 594 #1 (assume solenoid wires are
connected) 2, 3, 4 ( Read page 594)connected) 2, 3, 4 ( Read page 594)
 Page 616 #4, 11, 12, 20, 21Page 616 #4, 11, 12, 20, 21
Workbook
Page 64# 1 - 4Page 64# 1 - 4
Page 65 # 1Page 65 # 1

9. Previously asked questions fromPreviously asked questions from
old textbookold textbook
 McGraw-Hill QuestionsMcGraw-Hill Questions
 Page 728 #1, 2, 4Page 728 #1, 2, 4

10. Previously asked questions fromPreviously asked questions from
old textbookold textbook
 McGraw-Hill QuestionsMcGraw-Hill Questions
 Page 728 #1, 2, 4Page 728 #1, 2, 4