Chapter 20 Magnetism

Question 0

Two iron bars attract each other no matter which ends are placed close together. Are both magnets? Explain.

Answer 0

No, magnets would attract and REPEL.

Opposite charges attract and like charges REPEL.

Question 1

Draw the magnetic field lines around a straight section of wire carrying a current horizontally to the left.

Answer 1

X = in • = out
x x x x x x

《—《—《—
_____________

• • • • • •

Question 2

Suppose you have 3 iron rods, 2 of which are magnetized but the 3rd is not. How would you determine which 2 are magnetic without using any additional objects?

Answer 2

Put them close to each other. The 2 that repels are magnetized.

Question 3

A horseshoe magnet is held vertically with the North Pole on the left and South Pole on the right. A wire passes between the poles, equidistant from them, and carries a current directly away from you. In what direction is the force on the wire?

Answer 3

North Pole                South Pole 
     -----> magnetic field (B) right 
     xxxxxxx  current (I) in
      | Force (F) down 
     V

Question 4

A charged particle is moving in a circle under the influence of a uniform magnetic field. If an electric field that points in the same direction as the magnetic field is turned on, describe the path the charged particle will take.

Answer 4

The charged particle would spiral due to the magnetic field and accelerate in the direction of the electric field.

Question 5

A charged particle moves in a straight line through a particular region of space. Could there be a nonzero magnetic field in this region? If so, give two possible situations.

Answer 5

Yes,
1. If the charged particle (current) is move parallel or anti parallel to the magnetic field. F = q • V • B
V and B at 180 degrees = no F
2. If an electric field acts opposite of the magnetic field, canceling it out.

Question 6

If a negatively charged particle enters a region of uniform magnetic field which is perpendicular to the particle’s velocity, will the KE of the particle increase, decrease, or stay the same. Explain your answer. (Neglect gravity.)

Answer 6

The particle’s KE will stay the same b/c the magnetic field is perpendicular to the velocity. It only changes the direction not the value of the force.
KE = 1/2 m • V^2
If V is constant, KE is constant

Question 7

Charged particles move in the vicinity of a current carrying wire. For each charged particle the arrow indicates the direction of motion of the particle and the + or - indicated the sign of the charge. For each of the particles, indicate the direction of the magnetic force due to the magnetic field produced by the wire.

Answer 7

(a). + ->          F = down
• • • • • B
---------》 I
(b).  - ^           F = left
          |
• • • • • • B 
---------》 I
(c).  -

Question 8

Explain why a strong magnet held near a television screen causes the picture to become distorted. Also, explain why the picture sometimes goes completely black where the field is the strongest.

Answer 8

The electrons in the TV screen are depleted by the magnetic field so they don’t go where they are suppose to go. Instead of moving in a straight path, they move at an angle. Some areas will be completely black if the magnetic field is strong enough to turn the electrons or completely deflect it.

Question 9

In a particular region of space there is a uniform magnetic field B. Outside this region, B = 0. Can you inject an electron into the field perpendicularly so it will move in a closed circular path in the field?

Answer 9

No, a electron being injected perpendicularly will simply move in a semicircle, falling out of the region. Since the force is centripetal due to the perpendicular B and I, electrons can’t stay in the region unless they are already there to begin with.

Question 10

What factors determine the sensitivity of a galvanometer?

Answer 10

Strength of magnet and # of coils