AP Physics 2 : AP Physics 2

Study concepts, example questions & explanations for AP Physics 2

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Example Questions

Example Question #1 : Right Hand Rule For Charge In A Magnetic Field

An positively charged object moving west on the equator experiences a magnetic force pointing up to the sky. What is the direction of the dominant magnetic field at this location?

Possible Answers:

West

Northwest

East

North

South

Correct answer:

South

Explanation:

Use the right hand rule. The first vector, the velocity is represented by the thumb pointing to the west. The resultant vector, the middle finger (or palm) is pointed to the sky. The index finger is pointed south.

Example Question #1 : Right Hand Rule For Charge In A Magnetic Field

Simple circuit mag field

Which direction will the force be on a proton moving left at location ?

Possible Answers:

To the left

Up, towards the top of the screen

To the right

Down, to the bottom of the screen

None of these

Correct answer:

Down, to the bottom of the screen

Explanation:

Using the right hand rule for magnetic fields, it is seen that the magnetic field is point into the page at location . Using the right hand rule for force on a moving positively charged particle, it is seen that the force is acting down.

Example Question #1 : Right Hand Rule For Charge In A Magnetic Field

Simple circuit mag field

An electron at location is moving to the right. What will be the direction of the magnetic force?

Possible Answers:

Out of the page

Down, towards the bottom of the page

Into the page

None of these

Up, towards the top of the page

Correct answer:

Down, towards the bottom of the page

Explanation:

From the right hand rule for a current carrying wire, the magnetic field is pointing into the page. From the right hand rule for magnetic force, the force is pointed down on a negatively charged electron.

Example Question #1 : Right Hand Rule For Charge In A Magnetic Field

An electron is traveling into a screen at the same time another electron is traveling out of the screen. They are far enough away to not collide with each other. Will the magnetic forces between them be attractive or repulsive?

Possible Answers:

Attractive

Neither

None of these

Repulsive

The attractive and repulsive forces cancel each other out

Correct answer:

Attractive

Explanation:

Electrons in out page

Per conventional symbols, the "X" represents electrons traveling into the page, and the "dot" represents electrons coming out of the page. Picking either electron and using the right hand rule for current traveling, then reversing for the negative charge of the electron gives us the direction of the magnetic field at the other electron. Using the right hand rule again for moving charges in a magnetic field, then reversing for the negative charge of the electron shows that the charges will experience an attractive magnetic force.

Example Question #3 : Right Hand Rule For Charge In A Magnetic Field

Suppose that an electron is accelerating outward, within the plane of the page, in a counter-clockwise direction as shown in the picture below. In what direction would a magnetic field need to be pointing in order to cause the electron to move in this way?

Right hand rule

Possible Answers:

Down

Down and to the left

Into the page/screen

Up

Out of the page/screen

Correct answer:

Out of the page/screen

Explanation:

In this question, we're shown the path an electron takes as it moves outward in a counter-clockwise direction. We're asked to find the correct orientation of the magnetic field that will cause the electron to move this way.

There are a couple of things to note in this question. First, we know that the electron is traveling in a circular motion. This is true even though it is accelerating outwards. Thus, it must be experiencing some form of centripetal force to stay in its partially circular path.

Also, the source of that centripetal force must be the magnetic field. We know that it cannot be contributed by gravity, as the charge of an electron makes its gravitational mass insignificant.

Since the magnetic force is the source of the centripetal force, we can use this to determine how the magnetic field needs to be oriented. To do this, we use the right-hand rule. Although there is more than one way to use this rule, we can use one of them; the thumb points in the direction of the moving charge, the rest of the fingers point in the direction of the magnetic field, and the palm faces in the direction of the magnetic force vector. And remember, this only works with the right hand!

If we use our thumb to represent the motion of the electron, our fingers will point forward (pointing into the page). Moreover, our palm will always be facing towards the center of the circle. This is what we're looking for. This represents a situation where the magnetic force acts as the centripetal force by always pointing toward the center of the circular path of the electron.

And last but certainly not least, one crucial thing to remember is that we are following a negative charge! The right hand rule is meant to work for positive charges. To correct this, we can reverse the orientation of the magnetic field that we found. Originally, our fingers pointed into the page, suggesting that the magnetic field vector would point into the page. If we reverse this, then the magnetic field vector would point out of the page. Thus, the correct answer is that the magnetic field would need to be pointing out of the page in order to keep the electron moving in its partially circular path.

Example Question #41 : Magnetism And Electromagnetism

Suppose the induced electromotive force of a double loop wire has a magnitude of  when the change in magnetic flux is . How much time has elapsed for the flux change?

Possible Answers:

Correct answer:

Explanation:

Write the formula for induced electromotive force.

Since there are two loops, .

Solve for .

Example Question #2 : Induction

A magnetic rod is inside a coil of wire which is connected to an ammeter. If the rod is stationary, which of the following statements are true?

Possible Answers:

The rod loses its magnetic field

The rod induces a small current

There is no induced current

None of these

There is induced current relative to the strength of the rod's magnetic field

Correct answer:

There is no induced current

Explanation:

Current is induced in wire when the magnetic flux changes. When the magnetic rod is in motion, the flux is changing, so current is induced. If the coil were expanded or contracted with the rod still there, the flux would change and current would be induced. In our case, the rod is stationary and the coil isn't changing shape. Therefore, the flux is not changing, so there is no current being induced. Additionally, there's no reason for the rod to lose its magnetic property.

Example Question #3 : Induction

Rail system

 

A system consists of two rails, whose resistance is zero, a conductive bar, whose resistance is also zero, and a resistor whose resistance, 0.02 Ohms. The bar remains in contact with the rails as it slides to the right with a velocity . The distance from one rail to the other is 0.07m. The magnetic field, which is directed out of the page, has a magnitude of 0.0015T. What is the magnitude and direction of current in the resistor?

Possible Answers:

No current will flow in the resistor

 down toward the bottom of the page

 down toward the bottom of the page

 up toward the top of the page

 up toward the top of the page

Correct answer:

 down toward the bottom of the page

Explanation:

When a conductor moves through a magnetic field in such a way that it cuts through magnetic field lines, the mobile charge carriers separate due to the magnetic force on them, creating a potential . Since there is no resistance anywhere else in the circuit, all of this potential is lost in the resistor, so we can apply Ohm's law:

Because the positive charge in the rod feels an upward force due to the right-hand rule, the top of the rod has a greater potential than the bottom, and current flows counterclockwise around the circuit, resulting in a downward direction in the resistor.

Example Question #51 : Magnetism And Electromagnetism

Flux

 resistor is connected to a coil with  turns and a cross sectional area of . A magnet is lowered as shown in the figure. The magnetic field increases from  to  in . Find the magnitude of the current going through the resistor.

Possible Answers:

Correct answer:

Explanation:

There is a change in flux produced by the changing magnetic field which is given by

where  is the cross sectional area of the coil,  is the change in magnetic field, and  is the angle of the field lines relative to the normal of the cross section of the coil.

In this case the magnet is perpendicular to the cross section of the coil and so

The change in magnetic field is just the final given value minus the intial value. Faraday's Law says that an emf will be generated by a change in flux,

where  is the number of turns in the coil. Plugging in the change in flux gives

The change in time is just  since we can start our clock at zero. The current can be found using Ohm's Law where

Example Question #691 : Ap Physics 2

Suppose a  long solenoid has a current increase from zero to  in . The induced EMF magnitude is . Find the inductance.

Possible Answers:

Correct answer:

Explanation:

Write the formula for inductance and substitute the givens. The length of the solenoid has no effect in this problem.

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