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Motion in Electromagnetic Fields

Question 1

If we have 2 wires, with currents flowing parallel in them, what is the direction of the force between them?

  • A) The forces are pointing away from each other.
  • B) The forces are pointing towards each other.
  • C) There is no force between them.
  • D) One of the forces points at the other wire, the other points away from the wire.

Answers and Explanations

Question 2

What is by definition 1 Ampere?

  • A) It is the current that would cause a force of \(2 \cdot 10^{-7} \, \text{N}\) per meter between two long parallel wires separated by 1 m in a vacuum.
  • B) It is the rate of change of charge over time: \(\frac{Q}{t}\).
  • C) It is the current that flows when a 1 Volt battery with 0 internal resistance is connected across a \(1 \, \Omega\) resistor.
  • D) It is the current that would cause a force of \(2 \cdot 10^{-7} \, \text{N}\) per meter between two long anti-parallel wires separated by 1 m in a vacuum.

Answers and Explanations

Question 3

A negative point charge is stationary in a magnetic field, what is the magnitude of the resultant force on the particle?

  • A) \(F = B N e v\)
  • B) \(F = B I L\)
  • C) \(F = q v B\)
  • D) 0

Answers and Explanations

Question 4

A magnetic field is in the plane of the paper, pointing to the right. A positive point charge \(q\) is shot up and into the field with an angle of \(45^\circ\). What path is the particle going to follow?

  • A) The field has no effect on it, it will just pass through.
  • B) The field will eventually slow it down, bringing it to a stop.
  • C) It will move in a circle.
  • D) It will move with a helical path.

Answers and Explanations

Question 5

A magnetic field of strength \(B\) is pointing into the page, and in the same region of space an electric field is pointing vertically down with strength \(E\), what is the constant speed of an electron shot into this region, when moving from left to right?

  • A) \(v = \frac{E}{B}\)
  • B) \(v = \frac{B}{E}\)
  • C) \(v = \frac{F}{qB}\)
  • D) 0

Answers and Explanations

Question 6

We have a region bounded by the north pole of a magnet on the top, and the south pole on the bottom. We shoot an electron into the field from left to right, in what direction will it experience a force?

  • A) It won't experience any force since magnetic fields don’t create a force.
  • B) Out of the page.
  • C) Towards the north pole.
  • D) Towards the south pole.

Answers and Explanations

Question 7

A charged particle of mass \( m = 2.0 \times 10^{-3} \: \mathrm{kg} \) and charge \( q = +1.5 \: \mathrm{\mu C} \) enters a region with a uniform electric field of strength \( E = 5000 \: \mathrm{N/C} \). The particle is initially at rest.

What is the acceleration of the particle?

  • A) \( 3.75 \: \mathrm{m/s^2} \)
  • B) \( 2.75 \: \mathrm{m/s^2} \)
  • C) \( 4.75 \: \mathrm{m/s^2} \)
  • D) \( 3.25 \: \mathrm{m/s^2} \)

Answers and Explanations

Question 8

A charged particle of charge \( q = +2.0 \: \mathrm{\mu C} \) moves with a velocity of \( v = 300 \: \mathrm{m/s} \) in a uniform magnetic field \( B = 0.5 \: \mathrm{T} \). The velocity is perpendicular to the field.


What is the magnitude of the magnetic force acting on the particle?

  • A) \( 5.0 \times 10^{-4} \: \mathrm{N} \)
  • B) \( 2.0 \times 10^{-4} \: \mathrm{N} \)
  • C) \( 4.0 \times 10^{-4} \: \mathrm{N} \)
  • D) \( 3.0 \times 10^{-4} \: \mathrm{N} \)

Answers and Explanations

Question 9

A velocity selector has a uniform electric field \( E = 4000 \: \mathrm{N/C} \) and a uniform magnetic field \( B = 0.2 \: \mathrm{T} \), arranged perpendicularly. What is the speed of a charged particle that passes through the selector undeflected?

  • A) \( 10000 \: \mathrm{m/s} \)
  • B) \( 30000 \: \mathrm{m/s} \)
  • C) \( 20000 \: \mathrm{m/s} \)
  • D) \( 40000 \: \mathrm{m/s} \)

Answers and Explanations

Question 10

A wire of length \( L = 0.5 \: \mathrm{m} \) carries a current \( I = 10 \: \mathrm{A} \) in a uniform magnetic field \( B = 0.3 \: \mathrm{T} \). The wire is oriented such that the current is perpendicular to the field. What is the magnitude of the force acting on the wire?

  • A) \( 1.5 \: \mathrm{N} \)
  • B) \( 1 \: \mathrm{N} \)
  • C) \( 2.5 \: \mathrm{N} \)
  • D) \( 3.5 \: \mathrm{N} \)

Answers and Explanations

Question 11

A proton of charge \( q = 1.6 \times 10^{-19} \: \mathrm{C} \) and mass \( m = 1.67 \times 10^{-27} \: \mathrm{kg} \) moves perpendicular to a uniform magnetic field of strength \( B = 0.2 \: \mathrm{T} \) with a velocity \( v = 1.0 \times 10^6 \: \mathrm{m/s} \). What is the radius of the circular path taken by the proton?

  • A) \( 0.124 \: \mathrm{m} \)
  • B) \( 0.10 \: \mathrm{m} \)
  • C) \( 0.05 \: \mathrm{m} \)
  • D) \( 0.15 \: \mathrm{m} \)

Answers and Explanations

Question 12

A charge of \( q = 2.0 \: \mathrm{\mu C} \) is moved a distance \( d = 0.5 \: \mathrm{m} \) parallel to a uniform electric field of strength \( E = 1000 \: \mathrm{N/C} \). What is the work done by the field on the charge?

  • A) \( 0.005 \: \mathrm{J} \)
  • B) \( 0.1000 \: \mathrm{J} \)
  • C) \( 0.010 \: \mathrm{J} \)
  • D) \( 0.001 \: \mathrm{J} \)

Answers and Explanations

Question 13

A positive charge \( q = +1.0 \: \mathrm{\mu C} \) is moving with a velocity \( v \) to the right in a magnetic field \( B \) directed into the page. What is the direction of the magnetic force on the charge?

  • A) Up
  • B) Down
  • C) Into the page
  • D) Out of the page

Answers and Explanations

Question 14

A wire of length \( L = 0.8 \: \mathrm{m} \) carries a current \( I = 5 \: \mathrm{A} \) in a uniform magnetic field \( B = 0.4 \: \mathrm{T} \). The angle between the current and the field is \( 30^\circ \). What is the magnitude of the magnetic force acting on the wire?

  • A) 3.8 N
  • B) 0.8 N
  • C) 1.8 N
  • D) 2.8 N

Answers and Explanations