Q: According to the right-hand rule, what is the direction of the magnetic force on a positive charge moving in a magnetic field?

The force is in the direction of $$\vec{v} \times \vec{B}.$$

Q: What is the vector formula for the magnetic force $$\vec{F} on a $$straight wire segment of length $$\vec{l}$$ carrying current $$I in a $$magnetic field $$\vec{B}$$?

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

Question 1

Which statement best explains why a compass needle points north when placed on a map, as shown in the image?

Accepted Answer

The compass needle aligns with the Earth's magnetic field, which points toward the magnetic north pole.

Question 2

If two bar magnets are placed with their north poles facing each other, what will happen according to the diagram?

Accepted Answer

The magnets will repel each other.

Question 3

Why does an unmagnetized iron nail get attracted to either pole of a bar magnet, as shown in the image?

Accepted Answer

The nail is attracted because the bar magnet induces a magnetic moment in the nail.

Question 4

What happens when a bar magnet is broken in two, according to the diagram?

Accepted Answer

Each piece becomes a new magnet with both north and south poles.

Question 5

A compass placed near a wire carrying a current will deflect. What determines the direction of this deflection?

Accepted Answer

The direction of the current in the wire.

Question 6

What is the magnitude of the magnetic force on a charge $$q$$ moving perpendicular to a magnetic field $$\vec{B}$$ with velocity $$\vec{v}$$?

Accepted Answer

$$F = qvB$$

Question 7

A charge $$q$$ moves at an angle $$\phi to a $$magnetic field $$\vec{B}. $$What is the formula for the magnetic force?

Accepted Answer

$$F = qvB \sin \phi$$

Question 8

According to the right-hand rule, what is the direction of the magnetic force on a positive charge moving in a magnetic field?

Accepted Answer

The force is in the direction of $$\vec{v} 	imes \vec{B}.$$

Question 9

If a negative charge moves in a magnetic field, how does the direction of the magnetic force compare to that for a positive charge?

Accepted Answer

It is opposite to that given by the right-hand rule.

Question 10

A cathode-ray tube (CRT) uses magnetic fields to deflect electron beams. If the tube axis is parallel to the $$x-$$axis and the beam is deflected in the $$-z$$ direction, what is the direction of the magnetic field?

Accepted Answer

In the $$+y$$ direction.

Question 11

Which statement about magnetic field lines is correct?

Accepted Answer

Magnetic field lines are tangent to the direction of $$\vec{B} at $$each point.

Question 12

What is the formula for magnetic flux $$\Phi_B$$ through a surface?

Accepted Answer

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

Question 13

A charged particle moves in a uniform magnetic field at right angles to the field. What is the shape of its path?

Accepted Answer

A circle.

Question 14

A particle with velocity components both parallel and perpendicular to a magnetic field moves in what kind of path?

Accepted Answer

A helical path.

Question 15

What is the condition for a charged particle to pass through a velocity selector undeflected?

Accepted Answer

$$v = E/B$$

Question 16

What is the SI unit of magnetic field $$B$$?

Accepted Answer

Tesla (T)

Question 17

What is the vector formula for the magnetic force $$\vec{F} on a $$straight wire segment of length $$\vec{l}$$ carrying current $$I in a $$magnetic field $$\vec{B}$$?

Accepted Answer

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

Question 18

What is the vector formula for the torque $$\vec{	au} on a $$current loop with magnetic moment $$\vec{\mu} in a $$magnetic field $$\vec{B}$$?

Accepted Answer

$$\vec{	au} = \vec{\mu} 	imes \vec{B}$$

Question 19

In a direct-current motor, what causes the rotor to spin?

Accepted Answer

The magnetic torque on the current loop.

Question 20

The Hall effect can be used to determine whether charge carriers in a conductor are negative or positive. What is observed if the charge carriers are negative?

Accepted Answer

The Hall emf polarity is opposite to that for positive carriers.

Magnetic Field and Magnetic Forces: Study Notes

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