Q: An alpha particle (charge $$q = 3.2 \times 10$$^{-19}$$ C, mass m = 6.64$$ \times $$10^{-27} kg) $$enters a $$1.10 T $$magnetic field at $$35.6 km/s $$perpendicular to the field. What is the diameter of its circular path?

$$d = 2 \times \dfrac{mv}{|q|B}$$

Q: A straight conductor of length $$l$$ carries a current $$I$$ and is placed in a uniform magnetic field $$B at an $$angle $$\phi. $$What is the vector expression for the force on the conductor?

$$\vec{F} = I \vec{l} \times \vec{B}$$

Question 1

Which of the following statements about magnetic field lines is CORRECT?

Accepted Answer

Magnetic field lines are tangent to the magnetic field vector $$\vec{B} at $$every point.

Question 2

A charged particle moves with velocity $$\vec{v}$$ perpendicular to a uniform magnetic field $$\vec{B}. $$What is the radius $$R of $$its circular path?

Accepted Answer

$$R = \dfrac{mv}{|q|B}$$

Question 3

Which of the following best describes the force on a charged particle moving in a magnetic field?

Accepted Answer

The force is always perpendicular to both the velocity and the magnetic field.

Question 4

A straight wire carries a current $$I to $$the right. What is the direction of the magnetic field directly above the wire?

Accepted Answer

Out of the page

Question 5

Which equation correctly expresses the magnetic flux $$\Phi_B$$ through a surface?

Accepted Answer

$$\Phi_B = \int \vec{B} \cdot d\vec{A}$$

Question 6

According to Gauss’s law for magnetism, what is the total magnetic flux through any closed surface?

Accepted Answer

$$\int \vec{B} \cdot d\vec{A} = 0$$

Question 7

A proton enters a region with both a uniform electric field $$\vec{E}$$ and a uniform magnetic field $$\vec{B}$$, both perpendicular to its velocity. If the proton passes through undeflected, what must be true?

Accepted Answer

$$v = E/B$$

Question 8

A current-carrying wire of length $$l is $$placed in a uniform magnetic field $$\vec{B}$$, making an angle $$\phi$$ with the field. What is the magnitude of the force on the wire?

Accepted Answer

$$F = IlB \sin \phi$$

Question 9

A rectangular coil of area $$A$$ and current $$I is $$placed in a uniform magnetic field $$B. $$What is the maximum torque $$	au_{max}$$ experienced by the coil?

Accepted Answer

$$	au_{max} = IAB$$

Question 10

An alpha particle (charge $$q = 3.2 	imes 10$$^{-19}$$ C, mass m = 6.64$$ 	imes $$10^{-27} kg) $$enters a $$1.10 T $$magnetic field at $$35.6 km/s $$perpendicular to the field. What is the diameter of its circular path?

Accepted Answer

$$d = 2 	imes \dfrac{mv}{|q|B}$$

Question 11

A charged particle moves in a helical path in a uniform magnetic field. Which component of its velocity remains unchanged?

Accepted Answer

The component parallel to the magnetic field

Question 12

Which of the following is TRUE about the work done by a magnetic field on a moving charged particle?

Accepted Answer

The magnetic field does no work on the particle.

Question 13

A current-carrying wire is placed in a uniform magnetic field. If the current is doubled and the length of the wire is halved, how does the magnetic force change?

Accepted Answer

It is doubled

Question 14

A rectangular coil of wire with area $$A$$ and current $$I is $$rotated in a uniform magnetic field $$B. $$What is the potential energy $$U of $$the coil when its magnetic moment $$\vec{\mu}$$ makes an angle $$\phi$$ with $$\vec{B}$$?

Accepted Answer

$$U = -\mu B \cos \phi$$

Question 15

Which of the following is a correct statement about the period $$T of a $$charged particle in uniform circular motion in a magnetic field?

Accepted Answer

$$T$$ depends only on the mass, charge, and magnetic field.

Question 16

A straight conductor of length $$l$$ carries a current $$I$$ and is placed in a uniform magnetic field $$B at an $$angle $$\phi. $$What is the vector expression for the force on the conductor?

Accepted Answer

$$\vec{F} = I \vec{l} 	imes \vec{B}$$

Question 17

A DC motor uses a coil in a magnetic field. What is the purpose of the commutator in the motor?

Accepted Answer

To reverse the direction of current in the coil every half turn

Question 18

If a particle with charge $$q$$ and mass $$m$$ moves with velocity $$v$$ perpendicular to a magnetic field $$B$$, what is the cyclotron frequency $$f$$?

Accepted Answer

$$f = \dfrac{B|q|}{2\pi m}$$

Magnetic Fields and Magnetic Forces: Study Notes

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