PHYS 142 Lecture 17

1
Positive charged particles headed toward the
earth from space, would tend (at mid-latitudes)
to reach the surface coming down from the
1. North 2. South 3. East 4. West 5. split East
and West
2
A conducting rod is drawn down through the
B-field as shown.
N
Free electrons from the rod will 1)flow through
the hand to ground. 2)tend to bunch up at the far
end. 3)be pushed to upper surface of the rod. 4)
be unaffected.
3
MAGNETIC FLUX ? ? BAcos?
?
Units of ? Tm2 ?weber
4
If a coil is shrinking in a B field pointing
into the page, in what direction is the induced
current? 1) clockwise 2) counter-clockwise
3) no induced current
5

• If the B field pointing out of the page suddenly
  drops to zero, in what direction is the induced
  current?
• 1) clockwise
• 2) counter-clockwise
• 3) no induced current

6

• If a S pole moves towards the loop from behind
  the page, in what direction is the induced
  current?
• 1) clockwise
• 2) counter-clockwise
• 3) no induced current

S
7
x x x x x x x x x x x x x x x x x
x x x x x x x x x x x x x x x x x
x x x x x x x x x x x x x x x x x
x x x x x x x x x
A wire loop is being pulled through a uniform
magnetic field. What is the direction of the
induced current?

• 1) clockwise
• 2) counter-clockwise
• 3) no induced current

8

• A wire loop is being pulled away
• from a current-carrying wire in
• the direction of the green arrow.
• What is the direction of the
• induced current in the loop?
• 1) clockwise
• 2) counter-clockwise
• 3) no induced current

I
What is the induced current if the wire loop
moves in the direction of the red arrow?
9
4. West
2)tend to bunch up at the far end.
Magnetic flux is getting smaller since area is
getting smaller. Induced current will oppose
the change ? try to make the flux bigger Right
hand rule CW current gives B field into of the
page
A) clockwise
Magnetic flux is getting smaller. Induced
current will oppose the change ? try to make the
flux bigger Right hand rule CCW current gives B
field out of the page
2) counter-clockwise
The S pole field points into the page. S pole is
moving towards the loop means the B field is
increasing into the page. The induced current
will oppose the increase, which means it will
have a B field pointing out of the page. Right
Hand Rules gives a CCW current.
2) counter-clockwise
No change in flux B field not changing area
not changing angle not changing
3)no induced current
B field going into the page gets smaller.
Induced current will try to increase the B field.
1) clockwise 3) no induced current
No change in flux since no change in B field.