Q: A wire loop of radius 0.1 m carries a current of 2 A. What is the magnetic field at the center of the loop?

$$B = \frac{\mu_0$ I}{2R} = \frac{4\pi \times 10$$^{-7}$$ \times 2}{2 \times 0.1} = 2\pi \times 10$$^{-6}$$\ \mathrm{T}$$

Question 1

Which of the following statements best explains why a compass needle points north?

Accepted Answer

The north pole of the compass needle is attracted to the Earth's geographic north, which is actually a magnetic south pole.

Question 2

A long, straight wire carries a current $$I$$ upward. What is the direction of the magnetic field produced by the wire at a point directly east of the wire?

Accepted Answer

Into the page

Question 3

Which equation gives the magnitude of the magnetic field at a distance $$r$$ from a long, straight wire carrying current $$I$$?

Accepted Answer

$$B = \frac{\mu_0$ I}{2\pi r}$$

Question 4

A proton moves to the right in a region where the magnetic field points upward. What is the direction of the magnetic force on the proton?

Accepted Answer

Out of the page

Question 5

A 2-meter wire carries a current of 4 A perpendicular to a 3 T magnetic field. What is the magnitude of the force on the wire?

Accepted Answer

$$F = BIL = (3)(4)(2) = 24\ \mathrm{N}$$

Question 6

Which of the following best describes the field lines of a bar magnet?

Accepted Answer

They emerge from the north pole and enter the south pole outside the magnet.

Question 7

A charge $$q$$ moves with velocity $$v$$ perpendicular to a magnetic field $$B. $$What is the magnitude of the force on the charge?

Accepted Answer

$$F = qvB$$

Question 8

A solenoid is constructed with $$N$$ turns, length $$L$$, and carries current $$I. $$What is the magnetic field inside the solenoid?

Accepted Answer

$$B = \mu_0$ \frac{N}{L} I$$

Question 9

Two parallel wires carry currents in the same direction. What is the nature of the force between them?

Accepted Answer

They attract each other.

Question 10

A mass spectrometer uses both electric and magnetic fields to select particles of a specific velocity. If the electric field is $$E$$ and the magnetic field is $$B$$, what velocity $$v$$ will allow a particle to pass through undeflected?

Accepted Answer

$$v = \frac{E}{B}$$

Question 11

A circular loop of radius $$R$$ carries current $$I. $$What is the magnetic field at the center of the loop?

Accepted Answer

$$B = \frac{\mu_0$ I}{2R}$$

Question 12

A 3 C charge moves at 4 m/s perpendicular to a 5 T magnetic field. What is the magnitude of the force on the charge?

Accepted Answer

$$F = qvB = (3)(4)(5) = 60\ \mathrm{N}$$

Question 13

If a wire loop with area $$A$$ and $$N$$ turns carries current $$I in a $$uniform magnetic field $$B$$, what is the maximum torque it can experience?

Accepted Answer

$$	au = N B A I$$

Question 14

A toroidal solenoid has $$N$$ turns, carries current $$I$$, and has mean radius $$R. $$What is the magnetic field inside the toroid?

Accepted Answer

$$B = \frac{\mu_0$ N I}{2\pi R}$$

Question 15

Which of the following is true about magnetic monopoles?

Accepted Answer

They have never been observed; only magnetic dipoles exist.

Question 16

A 2 kg, $$-3 C $$charge is accelerated through a potential difference of 4 V and enters a velocity selector with $$E = 5 N/C $$and $$B = 2 T. $$What is the speed of the charge as it enters the selector?

Accepted Answer

$$v = \frac{E}{B} = \frac{5}{2} = 2.5\ \mathrm{m/s}$$

Question 17

Which of the following best describes the force between two parallel moving charges of the same sign and direction?

Accepted Answer

They attract each other due to the magnetic force.

Question 18

A wire loop of radius 0.1 m carries a current of 2 A. What is the magnetic field at the center of the loop?

Accepted Answer

$$B = \frac{\mu_0$ I}{2R} = \frac{4\pi 	imes 10$$^{-7}$$ 	imes 2}{2 	imes 0.1} = 2\pi 	imes 10$$^{-6}$$\ \mathrm{T}$$

Magnetic Fields and Forces: Study Notes

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