Q: A 1.0 m long rod of mass 0.5 kg is levitated 7.0 cm below another rod carrying equal current. What is the required current in each rod?

$$I = \frac{2\pi r mg}{\mu_0$ L \sin\theta}$$

Q: A voice coil in a loudspeaker has diameter 0.025 m, 55 turns, and is placed in a 0.10 T magnetic field with 2.0 A current. What is the magnetic force on the coil?

$$F = (2.0)(55\pi \times 0.025)(0.10) = 0.86 N$$

Question 1

In a mass spectrometer, ions are accelerated by a potential difference $$V$$ and then enter a region with a uniform magnetic field $$B. If $$the radius of the ion's path is $$r$$, which equation correctly relates the mass $$m of $$the ion to its charge $$q$$, velocity $$v$$, and the magnetic field?

Accepted Answer

$$m = \frac{qBr}{v}$$

Question 2

The mass spectrum of naturally occurring neon shows three peaks at atomic masses 20, 21, and 22. What does the height of each peak represent?

Accepted Answer

The relative abundance of each isotope

Question 3

A wire carrying a current $$I is $$placed in a magnetic field $$B at an $$angle $$	heta to $$the field. What is the magnitude of the force on a length $$L of $$the wire?

Accepted Answer

$$F = ILB\sin	heta$$

Question 4

A horizontal wire 2.0 m long carries a current of 20 A towards the east. The Earth's magnetic field points north with a magnitude of $$5.0 	imes 10$$^{-5}$$ T. What is the magnitude of the force on the wire?

Accepted Answer

$$2.0 	imes 10$$^{-3}$$ N

Question 5

Which right-hand rule is used to determine the direction of the force on a current-carrying wire in a magnetic field?

Accepted Answer

Point your thumb in the direction of current, fingers in the direction of magnetic field, and the palm points in the direction of force.

Question 6

A 1000 kg boat uses MHD propulsion with a 1.0 T magnetic field and a current of 100 A across plates separated by 2.0 m. What is the electric power required?

Accepted Answer

$$P = IV = 2000 W$$

Question 7

What is the time required for the MHD boat to accelerate to 10 m/s if the force is 200 N and the mass is 1000 kg?

Accepted Answer

$$t = \frac{v}{a} = 50 s$$

Question 8

The magnetic field at a distance $$r$$ from a long, straight wire carrying current $$I is $$given by which equation?

Accepted Answer

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

Question 9

A particle with charge $$q$$ moves with velocity $$v$$ parallel to a wire carrying current $$I. $$What is the force on the particle due to the wire's magnetic field at distance $$r$$?

Accepted Answer

$$F = qvB = qv \frac{\mu_0$ I}{2\pi r}$$

Question 10

Two parallel wires carry equal currents in opposite directions and are separated by distance $$r. $$What is the nature of the force between them?

Accepted Answer

The wires repel each other

Question 11

A 1.0 m long rod of mass 0.5 kg is levitated 7.0 cm below another rod carrying equal current. What is the required current in each rod?

Accepted Answer

$$I = \frac{2\pi r mg}{\mu_0$ L \sin	heta}$$

Question 12

The magnetic field at the center of a circular loop of radius $$R$$ carrying current $$I is$$:

Accepted Answer

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

Question 13

A solenoid of length $$l$$ with $$N$$ turns and current $$I$$ produces a magnetic field inside given by:

Accepted Answer

$$B = \mu_0$ n I$$ where $$n = \frac{N}{l}$$

Question 14

A voice coil in a loudspeaker has diameter 0.025 m, 55 turns, and is placed in a 0.10 T magnetic field with 2.0 A current. What is the magnetic force on the coil?

Accepted Answer

$$F = (2.0)(55\pi 	imes 0.025)(0.10) = 0.86 N$$

Question 15

What is the acceleration of the loudspeaker coil and cone if the force is 0.86 N and the mass is 0.020 kg?

Accepted Answer

$$a = \frac{F}{m} = 43 m/s$^2$$

Question 16

The net torque $$	au on a $$current-carrying coil of $$N$$ turns, area $$A$$, current $$I in a $$magnetic field $$B at $$angle $$\phi is$$:

Accepted Answer

$$	au = NIAB\sin\phi$$

Question 17

In a galvanometer, the angular displacement $$\Delta\phi of $$the needle is proportional to:

Accepted Answer

The current $$I in $$the coil

Question 18

Which component in a DC motor ensures that the coil continues to rotate in the same direction?

Accepted Answer

Split-ring commutator

Lec 11

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