Question 1

A transformer is to be used to step down voltage from an alternating current source from 220 V to 110 V. If the primary has 120 turns, then the number of turns in the secondary is

Accepted Answer

The ratio of the number of turns in the primary to the number of turns in the secondary is equal to the ratio of the voltages. In this case:

220 V / 110 V = 120 turns / x

Solving for x:

x = (120 turns * 110 V) / 220 V = 60 turns

Therefore, the answer is B) 60.

Question 2

The south pole of a bar magnet is moved toward a short helical coil of wire (solenoid) along the axis of the coil. The coil has 100 turns and the ends of the coil are connected to form a closed circuit. If the coil is replaced with a single loop of the same type of wire, and the magnet is moved exactly as before, the current induced in the loop is

Accepted Answer

The induced current in a loop is proportional to the rate of change of magnetic flux through the loop. In the case of the coil with 100 turns, the change in flux through each turn adds up, effectively multiplying the induced emf (and thus the current, assuming resistance is constant) by the number of turns. When replaced with a single loop, the induced emf (and thus the current) would be 100 times smaller because the change in magnetic flux is only happening through one loop instead of 100.

Question 3

If an electron is placed at rest near a bar magnet, the electron will

Accepted Answer

Magnetic fields do not exert a force on stationary electric charges, so an electron placed at rest near a bar magnet will not move due to the magnetic field alone.

Question 4

A long straight wire carries a current. A positive charge moves toward the wire in a direction perpendicular to the wire. The direction of the force on the charge will be

Accepted Answer

The answer of A long straight wire carries a current....

Question 5

Imagine a magnetic field pointing into the plane of this page. An electron moves across the paper from left to right. The direction of the magnetic force on the electron will be

Accepted Answer

Using the right-hand rule for a negative charge, the thumb points in the direction of velocity (left to right), fingers point into the page (magnetic field), and the palm faces the direction of force, which is downward for an electron.

Question 6

In outer space, you observe a straight stream of electrons flowing past you at 14.5 miles per second, about the same speed that Mars moves on its orbit around the sun.  You maneuver your spacecraft to face the current so that the electrons are moving from left to right and so that you are 10 m from the stream. Now you fire your rockets and boost your speed so that you are also moving at 14.5 miles per second, exactly at the same speed and direction as the electrons. In what direction does a compass needle point now?

Accepted Answer

A compass needle points towards the magnetic north pole, which is caused by the Earth's magnetic field. The presence of a magnetic field would cause the compass to point in a particular direction, but since the question states that no magnetic field is observed, the correct answer is E.

Question 7

Two current-carrying wires are parallel to one another and separated by 1 cm. If the distance between them is increased to 2 cm the new force will be what factor times the original force?

Accepted Answer

According to the formula for the force between two parallel current-carrying wires, the force is inversely proportional to the distance between them. So, if the distance is doubled from 1 cm to 2 cm, the force will decrease by a factor of 2 (i.e. it will become half). Therefore, the new force will be half or 1/2 times the original force. Hence, the answer is B.

Question 8

The south pole of a bar magnet is moved toward a short helical coil of wire (solenoid) along the axis of the coil. The coil has 100 turns and the ends of the coil are connected to form a closed circuit. If the coil is replaced with a single loop of the same type of wire, and the magnet is moved exactly as before, the magnet experiences a force that is

Accepted Answer

The answer of The south pole of a bar magnet...

Question 9

A bar magnet is broken in half and the magnetic field around one piece is mapped out. What is wrong with this picture?

Accepted Answer

The answer of A bar magnet is broken in half...

Question 10

In outer space, you observe a straight stream of electrons flowing past you at 14.5 miles per second, about the same speed that Mars moves on its orbit around the sun.  You maneuver your spacecraft to face the current so that the electrons are moving from left to right and so that you are 10 m from the stream. In what direction does a compass needle point?

Accepted Answer

The answer of In outer space, you observe a straight...

Question 11

The magnetic pole near the Earth's north geographic pole actually has to be

Accepted Answer

The magnetic pole near the Earth's north geographic pole has to be an S pole because it attracts the N pole of a compass needle. This is because opposite poles attract each other in magnets.

Question 12

A loop of wire is oriented in the magnetic field as shown below (the plane of the loop is perpendicular to the direction of the magnetic field). Which of the following is true?

Accepted Answer

According to Faraday's law of electromagnetic induction, an induced EMF is generated in a conductor when it cuts magnetic field lines. The direction of the induced EMF is such that it opposes the change in flux. In this case, if the loop is moved upwards or downwards, it will cut the magnetic field lines and there will be a change in the flux. Therefore, an induced current will be produced in the loop, and the direction of the induced current will be such that it will oppose the change in flux. Hence, the correct answer is E.

Question 13

Two long bar magnets are aligned so that north poles face each other. The magnets are separated by 1 cm, and a repulsive force between the north poles is 0.04 N. When the separation is increased to 2 cm the force will be

Accepted Answer

The force between two magnetic poles is inversely proportional to the square of the distance between them. Doubling the distance (from 1 cm to 2 cm) decreases the force by a factor of 4 (2^2), so the new force is 0.04 N / 4 = 0.010 N.

Question 14

Two particles move into a magnetic field. They enter moving at the same speed and direction. The magnitude of the charge on each is known to be the same. One particle curls to the right and the other to the left upon entering the field, the one on the left moving in a larger circle than the one on the right. From this we can say that

Accepted Answer

The fact that the particles curl in opposite directions upon entering the magnetic field indicates they have opposite charges, due to the force exerted by the magnetic field being dependent on the charge's sign. The particle moving in a larger circle has more mass because, for a given charge and velocity, a more massive particle experiences less acceleration and thus moves in a larger circle according to the centripetal force equation $F = \frac{mv^2}{r}$, where $F$ is the magnetic force, $m$ is the mass, $v$ is the velocity, and $r$ is the radius of the circle.

Question 15

In the southern hemisphere, the north pole of a compass needle

Accepted Answer

In the southern hemisphere, the north pole of a compass needle points away from the south geographic pole because the Earth's magnetic south pole (which attracts the north pole of a magnet) is actually located near the Earth's geographic north pole. This is why compasses work similarly in both hemispheres, with the north pole of the compass needle pointing towards the Earth's magnetic north pole, which is in the vicinity of the Earth's geographic north pole.

Question 16

A transformer has 200 turns on the primary and 20 turns on the secondary. The primary is connected to a direct current source of 100 millivolts. The voltage in the secondary coil will be

Accepted Answer

The voltage in the secondary coil will be 10 millivolts because the transformer is a step-down transformer.

Question 17

The scientist credited with discovering that electric currents produce magnetic fields was

Accepted Answer

Hans Christian Oersted discovered that electric currents produce magnetic fields in 1820. Ampere contributed to the development of electromagnetic theory, but he did not make this particular discovery. Coulomb is known for studying the force between electric charges, not the interaction between electric currents and magnetic fields. Tesla made many contributions to the study of electromagnetism, but he did not discover the relationship between electric currents and magnetic fields. Faraday is known for his work on electromagnetic induction, but he did not make this specific discovery about the interaction between electric currents and magnetic fields.

Question 18

Think of a closed rectangular loop of wire around the edge of this page. A magnetic field perpendicular to the page points upward and increases in strength in a certain time. During this time the conventional current induced in the loop will be

Accepted Answer

According to Lenz's Law, the induced current will oppose the change in magnetic flux. Since the magnetic field is increasing upward, the induced current will create a magnetic field pointing downward, resulting in a clockwise current.

Question 19

The sketch below shows a bar magnet in a loop of wire with the bar perpendicular to the plane of the loop. The left-hand side shows the magnet being spun around its long axis while on the right side the magnet is turned end-over-end. Which case creates a larger induced current in the loop?

Accepted Answer

The answer of The sketch below shows a bar magnet...

Question 20

If an electron moves in a magnetic field,

Accepted Answer

The force on an electron moving in a magnetic field is given by the Lorentz force law, which states that the force is proportional to the cross product of the velocity of the electron and the magnetic field. This means that the electron only feels a force if its velocity vector is perpendicular to the magnetic field direction. If the velocity is parallel or antiparallel to the magnetic field, the cross product is zero, and no force is experienced.

Quiz 14: Magnets and Electromagnetism