1. W11D2 Concept Questions Review
Class 18

2. W07D1 Magnetic Dipoles, Force and Torque on a Dipole, Experiment 2
W07D1 Magnetic Dipoles, Torque and Force on a Dipole, Experiment 2: Magnetic Dipole in a Helmholtz Coil
Reading Course Notes: Sections 8.4,8.11.6,
Class 18

3. Concept Question: Magnetic Field Lines
The picture shows the field lines outside a permanent magnet The field lines inside the magnet point: Up
Down
Left to right
Right to left
The field inside is zero
I don’t know
Class 15

4. Concept Q. Answer: Magnetic Field Lines
Answer: 1. They point up inside the magnet
Magnetic field lines are continuous.
E field lines begin and end on charges.
There are no magnetic charges (monopoles) so B field lines never begin or end
Class 15

5. Concept Question: Parallel Wires
Consider two parallel current carrying wires. With the currents running in the opposite direction, the wires are
attracted (opposites attract?)
repelled (opposites repel?)
pushed another direction
not pushed – no net force
I don’t know
Class 18

6. Concept Q. Answer: Parallel Wires
Answer: 1. The wires are repelled
I1 creates a magnetic field into the
page at wire 2. That makes a force
on wire 2 to the right.
I2 creates a magnetic field into the
page at wire 1. That makes a force
on wire 1 to the left.
Class 18

7. Concept Question: Dipole in Uniform Magnetic Field
From rest, the coil in a uniform magnetic field above will:
rotate wise, not move
rotate counterclockwise, not move
move to the right, not rotate
move to the left, not rotate
move in another direction, without rotating
both move and rotate
neither rotate nor move
I don’t know
Class 18

8. Concept Q. Answer: Dipole in Field
Answer: 1. Coil will rotate clockwise (not move)
No net force so no center of mass motion. BUT Magnetic dipoles rotate to align with external field (think compass)
Class 18

9. Concept Question: Dipole in Field
The current carrying coil above will feel a net force
upwards
downwards
of zero
I don’t know
Class 18

10. Concept Q. Answer: Dipole in Field
Answer: 2. Feels downward force. The forces shown produce a net downward force
Class 18

11. Concept Question: Dipole in Helmholtz
Week 08, Day 2
Week 08, Day 2Week 08, Day 2
Concept Question: Dipole in Helmholtz
A dipole pointing along the positive x-direction and located at the center of a Helmholtz coil will feel:
a force but not a torque.
a torque but not a force.
both a torque and a force.
neither force nor torque.
Class 19
Class 19Class 19 11

12. Concept Q. Answer: Dipole in Helmholtz
Week 08, Day 2
Week 08, Day 2Week 08, Day 2
Concept Q. Answer: Dipole in Helmholtz
Answer: 2. a torque but not a force. The Helmholtz coil makes a UNIFORM FIELD.
Dipole feels only torque (need gradient for force).
Class 19
Class 19Class 19 12

13. Concept Question: Dipole in Anti-Helmholtz Coil
Week 08, Day 2
Week 08, Day 2Week 08, Day 2
Concept Question: Dipole in Anti-Helmholtz Coil
A dipole pointing along the positive z-direction and located at the center of an anti- Helmholtz coil will feel:
a force but not a torque.
a torque but not a force.
both a torque and a force.
neither force nor torque.
Class 19
Class 19Class 19 13

14. Concept Q. Answer: Dipole in Anti-Helmholtz Coil
Week 08, Day 2
Week 08, Day 2Week 08, Day 2
Concept Q. Answer: Dipole in Anti-Helmholtz Coil
Answer: 1. A force because there is a non-gradient of the magnetic field but no torque because the magnetic field at the center is zero.
Class 19
Class 19Class 19 14

15. W09D1: Sources of Magnetic Fields: Ampere’s Law
Class 18

16. Ampere’s Law
Class 20

17. Concept Question: Ampere’s Law
Integrating B around the loop shown gives us:
a positive number
a negative number
zero
Class 20

18. C.Q. Answer: Ampere’s Law
Week 07, Day 2
C.Q. Answer: Ampere’s Law
Answer: 3. Total enclosed current is zero, so
Class 16

19. Concept Question: Ampere’s Law
Integrating B around the loop in the clockwise direction shown gives us:
a positive number
a negative number
zero
Class 20

20. C.Q. Answer: Ampere’s Law
Week 07, Day 2
C.Q. Answer: Ampere’s Law
Answer: 2.
Net enclosed current is out of the page, so field is counter-clockwise (opposite to circulation direction)
Class 16

21. W09D2: Faraday’s Law
Class 18

22. Concept Question: Loop in Uniform Field
Week 09, Day 1
Concept Question: Loop in Uniform Field
While a rectangular wire loop is pulled upward though a uniform magnetic field B field penetrating its bottom half, as shown, there is
a current in the loop.
no current in the loop.
I do not understand the concepts of current and magnetic field.
I understand the concepts of current and magnetic field but am not sure of the answer. 22
Class 21 22

23. Concept Q. Ans.: Loop in Uniform Field
Week 09, Day 1
Concept Q. Ans.: Loop in Uniform Field
Answer: 1. The motion changes the magnetic flux through the loop. The magnetic flux is decreasing in time as more of the loop enters a region of zero magnetic field. According to Faraday’s Law there is an induced current through the loop. 23
Class 21 23

24. Concept Q.: Loop in Uniform Field
Week 09, Day 1
Concept Q.: Loop in Uniform Field
While a rectangular wire loop is pulled sideways though a uniform magnetic field B field penetrating its bottom half, as shown, there is
a current in the loop.
no current in the loop.
I do not understand the concepts of current and magnetic field.
I understand the concepts of current and magnetic field but am not sure of the answer. 24
Class 21 24

25. Concept Q. Ans.: Loop in Uniform Field
Week 09, Day 1
Concept Q. Ans.: Loop in Uniform Field
Answer: 2. The motion does not change the magnetic flux through the loop. The magnetic flux is constant in time. According to Faraday’s Law there is no induced current through the loop. 25
Class 21 25

26. Concept Question: Loop
The magnetic field through a wire loop is pointed upwards and increasing with time. The induced current in the coil is
Clockwise as seen from the top
Counterclockwise
Class 22

27. Concept Question Answer: Loop
Answer: 1. Induced current is clockwise
This produces an “induced” B field pointing down over the area of the loop.
The “induced” B field opposes the increasing flux through the loop – Lenz’s Law
Class 22

28. Concept Question: Moving Loop
Week 9, Day 2
Concept Question: Moving Loop
A circuit in the form of a rectangular piece of wire is pulled away from a long wire carrying current I in the direction shown in the sketch. The induced current in the rectangular circuit is
Clockwise
Counterclockwise
Neither, the current is zero 28
Class 22 28

29. Concept Q. Answer: Moving Loop
Week 9, Day 2
Concept Q. Answer: Moving Loop
Answer: 1. Induced current is clockwise
B due to I is into page; the flux through the circuit due to that field decreases as the circuit moves away. So the induced current is clockwise (to make a B into the page)
Note: Iind dl x B force is left on the left segment and right on the right, but the force on the left is bigger. So the net force on the rectangular circuit is to the left, again trying to keep the flux from decreasing by slowing the circuit’s motion 29
Class 22 29

30. Concept Question: Faraday’s Law: Loop
A coil moves up from underneath a magnet with its north pole pointing upward. The current in the coil and the force on the coil:
Current clockwise; force up
Current counterclockwise; force up
Current clockwise; force down
Current counterclockwise; force down
Class 22

31. Concept Question Answer: Faraday’s Law: Loop
Answer: 3. Current is clockwise; force is down
The clockwise current creates a self-field downward, trying to offset the increase of magnetic flux through the coil as it moves upward into stronger fields (Lenz’s Law).
The I dl x B force on the coil is a force which is trying to keep the flux through the coil from increasing by slowing it down (Lenz’s Law again).
Class 22

32. W10D1: Inductance and Magnetic Field Energy
Class 18

33. Concept Question: Solenoid
A very long solenoid consisting of N turns has radius R and length d, (d>>R).  Suppose the number of turns is halved keeping all the other parameters fixed. The self inductance
remains the same.
doubles.
is halved.
is four times as large.
is four times as small.
None of the above.
Class 23

34. Concept Q. Ans.: Solenoid
Solution 5. The self-induction of the solenoid is equal to the total flux through the object which is the product of the number of turns time the flux through each turn. The flux through each turn is proportional to the magnitude of magnetic field which is proportional to the number of turns per unit length or hence proportional to the number of turns. Hence the self-induction of the solenoid is proportional to the square of the number of turns. If the number of turns is halved keeping all the other parameters fixed then he self inductance is four times as small.
Class 23