Ch 20 Magnetism and Ch 21 EM Induction

The maximum torque on a current carrying loop occurs when the angle between the loop's magnetic moment and the magnetic field vector is

90°

What fundamental fact underlies the operation of essentially all electric motors?

A current-carrying conductor placed perpendicular to a magnetic field will experience a force.

A circular loop of wire is rotated at constant angular speed about an axis whose direction can be varied. In a region where a uniform magnetic field points straight down, what must be the orientation of the loop's axis of rotation if the induced emf is to be zero?

It must be vertical.

All of the following are units of magnetic flux except

T/V∙m.

A velocity selector consists of a charged particle passing through crossed electric and magnetic fields. The forces exerted by these fields are in opposite directions, and only particles of a certain velocity will move in a straight line. In the following, disregard the magnitudes of the fields. In a velocity selector, the particles move toward the east, and the magnetic field is directed to the north. What direction should the electric field point?

down

A charged particle moves and experiences no magnetic force. From this we can conclude that

either no magnetic field exists or the particle is moving parallel to the field

A charged particle is injected into a uniform magnetic field such that its velocity vector is perpendicular to the magnetic field vector. Ignoring the particle's weight, the particle will

follow a circular path.

A charged particle moves with a constant speed through a region where a uniform magnetic field is present. If the magnetic field points straight upward, the magnetic force acting on this particle will be maximum when the particle moves

in a plane parallel to the Earth's surface.

When placed askew in a magnetic field, a current carrying loop that is free to rotate in any direction will experience a torque until its magnetic moment vector

is aligned with the magnetic field vector.

Two long parallel wires placed side-by-side on a horizontal table carry identical current straight toward you. From your point of view, the magnetic field at the point exactly between the two wires

is zero.

A circular coil lies flat on a horizontal table. A bar magnet is held above its center with its north pole pointing down. The stationary magnet induces (when viewed from above)

no current in the coil.

Consider two current-carrying circular loops. Both are made from one strand of wire and both carry the same current, but one has twice the radius of the other. Compared to the magnetic field at the center of the smaller loop, the magnetic field at the center of the larger loop is

none of the given answers

A long straight wire carries current toward the east. A proton moves toward the east alongside and just south of the wire. What is the direction of the force on the proton?

north

A particle carrying a charge of +e travels in a circular path in a uniform magnetic field. If instead the particle carried a charge of +2e, the radius of the circular path would have been

one-half the original radius.

According to Lenz's law, the direction of an induced current in a conductor will be that which tends to produce which of the following effects?

oppose the effect which produces it

The direction of the force on a current-carrying wire in a magnetic field is described by which of the following?

perpendicular to both the current and the magnetic field

Two long parallel wires placed side-by-side on a horizontal table carry identical size currents in opposite directions. The wire on your right carries current toward you, and the wire on your left carries current away from you. From your point of view, the magnetic field at the point exactly midway between the two wires

points down.

A current carrying loop of wire lies flat on a table top. When viewed from above, the current moves around the loop in a counterclockwise sense. What is the direction of the magnetic field caused by this current, outside the loop? The magnetic field

points straight down

A current carrying circular loop of wire lies flat on a table top. When viewed from above, the current moves around the loop in a counterclockwise sense. What is the direction of the magnetic field caused by this current, inside the loop? The magnetic field

points straight up.

The magnetic field produced by a long straight current-carrying wire is

proportional to the current in the wire and inversely proportional to the distance from the wire.

In a mass spectrometer a particle of mass m and charge q is accelerated through a potential difference V and allowed to enter a magnetic field B, where it is deflected in a semi-circular path of radius R. The magnetic field is uniform and oriented perpendicular to the velocity of the particle. Derive an expression for the mass of the particle in terms of B, q, V, and R.

qB2R2/(2V)

if the north poles of two bar magnets are brought close to each other, the magnets will

repel

A vertical wire carries a current straight down. To the east of this wire, the magnetic field points

south

The Earth's geographic North Pole is magnetically a

south pole.

The SI unit of magnetic field is the

tesla

The force on a current-carrying wire in a magnetic field is equal to zero when

the current is parallel to the field lines.

The force on a current-carrying wire in a magnetic field is the strongest when

the current is perpendicular to the field lines.

) A circular wire loop lies in a horizontal plane on a table and carries current in a counterclockwise direction when viewed from above. At this point, the Earth's magnetic field points to the north and dips below the horizontal. Which side of the coil tends to lift off of the table because of the torque caused by the currents interaction with the magnetic field?

the south side

Faraday's law of induction states that the emf induced in a loop of wire is proportional to

the time variation of the magnetic flux.

Two long parallel wires are placed side-by-side on a horizontal table. If the wires carry current in the same direction,

the wires attract each other.

) Two long parallel wires are placed side-by-side on a horizontal table. If the wires carry current in opposite directions,

the wires repel each other.

An electron has an initial velocity to the south but is observed to curve upward as the result of a magnetic field. The direction of the magnetic field is

to the west.

A wire lying in the plane of the page carries a current toward the bottom of the page. What is the direction of the magnetic force it produces on an electron that is moving perpendicularly toward the wire, also in the plane of the page, from your right?

toward the bottom of the page

An electron moving along the +x axis enters a region where there is a uniform magnetic field in the +y direction. What is the direction of the magnetic force on the electron? (+x to right, +y up, and +z out of the page.)

-z direction

At double the distance from a long current-carrying wire, the strength of the magnetic field produced by that wire decreases to

1/2 of its original value.

Consider two current-carrying circular loops. Both are made from one strand of wire and both carry the same current, but one has twice the radius of the other. Compared to the magnetic moment of the smaller loop, the magnetic moment of the larger loop is

4 times stronger.

Two long parallel wires carry equal currents. The magnitude of the force between the wires is F. The current in each wire is now doubled. What is the magnitude of the new force between the two wires?

4F

A circular loop of wire is rotated at constant angular speed about an axis whose direction can be varied. In a region where a uniform magnetic field points straight down, what must be the orientation of the loop's axis of rotation if the induced emf is to be a maximum?

Any horizontal orientation will do.

1 T is equivalent to

N/A∙m.

The south pole of a magnet points toward the Earth's

North Pole

A coil lies flat on a table top in a region where the magnetic field vector points straight up. The magnetic field vanishes suddenly. When viewed from above, what is the sense of the induced current in this coil as the field fades?

The induced current flows counterclockwise.

Doubling the diameter of a loop of wire produces what kind of change on the induced emf, assuming all other factors remain constant?

The induced emf is 4 times as much.

Doubling the strength of the magnetic field through a loop of wire produces what kind of change on the induced emf, assuming all other factors remain constant?

The induced emf is twice as much.

Doubling the number of loops of wire in a coil produces what kind of change on the induced emf, assuming all other factors remain constant?

The induced emf is twice times as much.

Which of the following is correct?

When a current carrying wire is in your right hand, thumb in the direction of the current, your fingers point in the direction of the magnetic field lines.

A circular coil lies flat on a horizontal table. A bar magnet is held above its center with its north pole pointing down, and released. As it approaches the coil, the falling magnet induces (when viewed from above)

a counterclockwise current in the coil.

An electric current produces

a magnetic field.

As a coil is removed from a magnetic field an emf is induced in the coil causing a current to flow within the coil. This current interacts with the magnetic field producing a force which

acts in the direction opposite to the coil's motion.

If the south pole of one bar magnet is brought near the north pole of a second bar magnet, the two magnets will

attract

A charged particle moves across a constant magnetic field. The magnetic force on this particle

causes the particle to accelerate

A horizontal wire carries a current straight toward you. From your point of view, the magnetic field caused by this current

circles the wire in a counter-clockwise direction.

) A coil lies flat on a horizontal table top in a region where the magnetic field points straight down. The magnetic field disappears suddenly. When viewed from above, what is the direction of the induced current in this coil as the field disappears?

clockwise

A coil lies flat on a level table top in a region where the magnetic field vector points straight up. The magnetic field suddenly grows stronger. When viewed from above, what is the direction of the induced current in this coil as the field increases?

clockwise

A bar magnet falls through a loop of wire with the north pole entering first. As the north pole enters the wire, the induced current will be (as viewed from above)

counterclockwise

A long straight wire lies on a horizontal table and carries an ever-increasing current northward. Two coils of wire lie flat on the table, one on either side of the wire. When viewed from above, the induced current circles

counterclockwise in the east coil and clockwise in the west coil.

At a particular instant, an electron moves eastward at speed V in a uniform magnetic field that is directed straight downward. The magnetic force that acts on it is

directed to the south.

A charged particle is observed traveling in a circular path in a uniform magnetic field. If the particle had been traveling twice as fast, the radius of the circular path would be

twice the original radius.

If a bar magnet is divided into two equal pieces

two magnets result

A vertical wire carries a current straight up in a region where the magnetic field vector points due north. What is the direction of the resulting force on this current?

west