Magnetism (scary)

hey pooks idk if eberything is right but oh well soo la voo

If you cut a bar magnet in half, what happens to the poles?

If you cut a bar magnet in half, what happens to the poles?

the two resulting magnets are still magnets with 2 poles (magnetic monopoles DO NOT exist). when the magnet is cut in half, there are still domains, therefore there are polarized ends

what is the main source of magnetism in materials?

electron spin (charges in motion; if a charge is not moving, there is no magnetic field)

why are some objects not magnetic?

electron spins cancel out (electrons are almost always in pairs unless there is an odd amount of them)

domains

small areas/regions of aligned spins → alignment of these domains gives ferromagnetic materials their magnetic properties

magnetic domain

local region within material in which all unpaired electrons are oriented the same way

Ferromagnetism

occurs when magnetic field produce spins that do not cancel out completely in certain materials (e.g. iron, cobalt, nickel)

Paramagnetic

material weakly interacts with external field and temporarily lines up some of their domains. when field removes, domains assume random ordering.

Diamagnetic

uncommon; domains line up opposite to the external field; materials with this property are repelled by a magnet

how can magnetized items become unmagnetized

unalign domains by heating, cooling, or hitting

if there is magnetic force, then there is also…

magnetic field

where do magnetic fields start/end?

lines start at north and end at south OUTSIDE of bar magnet → start at south and end at north INSIDE bar magnet. forms a never-ending loop.

where is the Earth’s magnetic south pole located?

at geographic north

what does the range of the magnetic field depend on?

the object’s size

magnetic flux

the number of field lines that cross a certain area at right angles to that area; in Weber (Wb)

maximum and minimum flux

maximum flux when angle between magnetic field lines and area vector is zero (cos0 = 1)

When the coil is rotated between the pole pieces of a magnet as shown, during one complete rotation of the coil, how often will the magnetic flux linked with the coil be maximum and minimum?

maximum twice, minimum twice

(initially plane of coil is perpendicular to B field lines since the coils face the poles of the magnet → coil is flush to the magnetic poles kind of like holding a book flat in your hand, so it’s parallel → perpendicular → parallel)

what do current carrying wires produce

induced magnetic field

what type of shaped magnetic field does a long, straight current carrying wire create?

a cylindrical magnetic field

B relationship with distance

inversely proportional

B relationship with current

directly proportional

what is the curly right hand rule?

used to find direction of magnetic field → point your right thumb in the direction of the current in the wire and curl fingers

value of permeability of free space

measure of ability for magnetic field to propagate through a vacuum → 4π * 10^-7

symbol for out the page (towards you)

circle with dot in the middle

symbol for in the page (away from you)

circle with x in middle

solenoid

wire coiled into several different loops → creates strong magnetic field by combining several loops

solenoids create B fields that look like

bar magnet

magnetic field of solenoid is proportional to

current in wire + number of loops/turns of wire per unit length

What is the shape of the magnetic field produced by a straight current carrying wire

circle

does light need magnetism to exist?

yes, bc it is an electromagnetic wave

when the electric current flows INTO the page, the B field is

clockwise

when the electric current flows right, the magnetic field is

out of the page and into the page

An electric current flows in a counterclockwise direction around a circular loop of wire. What is the direction of the magnetic field?

out the page

What is the direction of the magnetic field at point P due to the current flowing into the page?

tangential to the circle, kind of in the clockwise direction

What is the direction of the magnetic field at the origin due to two current-carrying wires of equal magnitude? The currents are separated from the origin by the same distance, r.

up and right from origin

A long, straight wire carries an electric current I. What is the direction of the magnetic field at point P?

into the page

two long, straight wires carry equal currents directed into the page. What is the direction of the magnetic field at the midpoint between the wires?

The magnetic field is 0 (they cancel out)

when is magnetic force on a moving charged particle at max value

when a charged particle is moving perpendicular to the field

can magnetic force change the speed of the particle

NO → velocity and force are set quantities; only direction is changing

Right Hand Rule

thumb → velocity

fingers → B field direction

palm → force direction if charge is positive

back of hand → force direction if charge is negative

shape of path of charge moving through B field

circular

What is the magnetic force on the electron that is moving out of the page?

there is no magnetic force acting on the electron

Observe that the stream of electrons in a cathode ray tube interacts with a bar magnet. Why might the stream of electrons change direction when the magnet is held nearby?

due to interactions between moving electrons and B field of magnet → deflected from their og path

An electron moving to the right at 4.5*10⁴ m/s enters a 1.0 mT magnetic field pointed upward. What would the force be if the particle were a neutron?

0 N → neutrons don’t have a charge

mass spectrometer

device that is used to separate out atoms and molecules based on their mass - and is used to analyze the physical makeup of substances in terms of their relative concentrations of their constituent parts

velocity selector

device that consists of perpendicular electric (curves up) and magnetic (curves down) fields and is used against charged particles

How to calculate velocity of the particles that go straight through the velocity selector

Fe = Fb → qE = qvB → E/B = v

what type of particles can pass through a velocity selector?

particles with specific velocity of v = E/B

• when in B field, field exerts force perpendicular to motion that makes charges move in circle. all start w/ same velocity bc of velocity selector

• the movement of proton is depicted in the picture

• electrons would go the other way and curve more since electrons are lighter

• various masses separate out along diameter of mass selector. can find mass once you find r.

solve for the mass of particles

Fb = mv²/r

qvB = mv²/r

m = qBr/v

solving for radius

Fb = mv²/r

qvB = mv²/r

r = mv/qB

What describes the behavior of a charged particle that is injected into an area that just has a magnetic field?

speed remains the same (B field can change direction, but cannot change speed → force is perpendicular to velocity of particle)

right hand rule for current carrying wire

thumb → direction of current

fingers → B field direction

palm → force direction if charge is positive

back of hand → force direction if charge is negative

What is the direction of the magnetic force on the current carrying wire (green) in the magnetic field (red)?

current and B field are parallel → no force (A)

right hand rule to find B force on two current carrying wires

1. use curly RHR to find direction of B field on other wire

2. use RHR to figure out direction of force on that wire

• wires w/ current going in same direction attract

• wires w/ currents going in different direction repel

An electron moving North encounters a uniform magnetic field. If the magnetic field points East, what is the direction of the magnetic force on the electron?

out of the page

Two parallel wires have charges moving in the same direction. Is the force between them attractive or repulsive?

attractive

what is necessary to induce a current from magnetic field?

motion bc magnetic field needs to be changing (either circuit or magnet can move). magnetic flux through coil/loop changes → B field changes

where is induced emf the strongest? where is it weakest?

greatest when there is a lot of field lines

least at center of bar magnet

ways to induce current

1. Circuit moved into or out of the magnetic field

2. Circuit is rotated in the magnetic field

3. Intensity and/or direction of magnetic field is varied

A bar magnet is moved towards a circular conducting loop. what happens to the B field and current flow?

B field in loop increases (more field lines going through loop) and a current flows through loop

what will generate an emf in a conducting loop

emf in a conducting loop will be generated when magnet moves towards surface of loop

Lenz’s Law

magnetic field of the induced current is in a direction to produce a field that opposes the change causing it

(if the external field gets weaker, the induced current tries to replace the "missing" external field by generating a magnetic field to "replace" it. If the external field gets stronger, the induced current opposes the "extra" external field by generating a magnetic field opposing the change.)

apply Lenz’s Law

losing B field, need to add to field

induced current moves CCW

apply Lenz’s Law

adding B field, need to oppose it

induced current moves CW

A coil of wire is sitting on a table top. A magnet is held above it with the North Pole pointing downwards. The magnet is then pushed down towards the coil. What is the direction of the induced current in the coil of wire?

B field increases (in downward direction), needs to oppose it (needs something upward) → curly RHR → induced current is CCW

A coil of wire is sitting on a table top. A magnet is held above it with the North Pole pointing downwards. What is the direction of the induced current in the coil of wire?

no induced current

A permanent magnet is pushed into a stationary ring that is suspended from a vertical string. What happens to the ring? How can we use Lenz’s law to explain this experiment?

initially, ring and magnet repel since the induced current is north. as the magnet passes through the ring, the south end and ring attract

what is emf like

potential difference

units for emf

volts

how can magnetic flux be changed

1. The magnetic field strength

2. The area

3. The angle of orientation

generator

• converts mechanical energy into electrical energy (e.g. turbine)

• has constant B field

• faster = greater induced emf

formula for induced emf from a generator

ε = NABωsin(ωt)

max occurs when sinωt=1

alternating current

Since the emf generated goes from positive to negative, the output of the current is changing directions as well as the coil spins.

The rate at which the coil in the generator rotates determines the maximum generated emf.

Why would we want a wire to spin?

more practical to create spinning motion + want magnetic torque

torque equation

𝜏=NIABsinθ

θ = angle between B field and area

motors

do the opposite of generators; convert electrical energy to mechanical energy

transformers

• device that converts a small applied emf to a larger one, or a larger applied emf to a smaller one

• has two coils of wire wrapped around a core of soft iron

  • primary coil: connected to the primary AC source and has N₁ number of turns.

  • secondary coil: connected to a resistor and has N₂ number of turns

step up transformer

When N₂ is greater than N₁ → the secondary emf is greater than the primary emf

step down transformer

 When N₁ is greater than N₂ → the secondary emf is smaller than the primary emf

Can a magnetic field set a stationary electron in motion? If so, how?

NO

if stationary, v = 0 m/s, meaning force exerted on electron will also be zero

An electron passes through a magnetic field without being deflected. What can you conclude about the orientation between the magnetic field and the velocity of the electron?

the B field is parallel or anti-parallel to the velocity of electron (sin0 = 0)

Two straight wires are parallel to each other. If the currents in the wires are in the same direction, will the wires attract or repel each other? What if the currents are opposite?

If currents are in the same direction, wires attract. if currents in opposite directions, wires repel.

What five factors affect the magnitude of the induced emf in a coil of wire?

1. number of coils/turns

2. area

3. angle

4. B field strength

5. time

Does dropping a strong magnet down a long copper tube induce a current in the tube? If so, what effect will the induce current have on the magnet?

YES

as magnet falls through tube, magnetic field changes, inducing a current. the induced current will try to oppose the magnet, slowing the magnet’s speed through the tube.

A transformer is constructed so that the coil on the left has 60 turns and the coil on the right has 3 turns. If the input potential difference is across the coil on the left, what type of transformer is this?

step down transformer

Where is the north magnetic pole on the Earth?

The South pole of earth

What would happen if a positive side of a magnet is placed near the positive side of another magnet?

Repel

At what place do the magnetic field lines leave a magnet?

North pole

What is the unit of magnetic field?

Tesla

What is it called when materials can be permanently magnetized by the application of the field (all of their domains will line up in the same direction)?

Ferromagnetic

Which of the following will cause a change in flux through a loop?

 I.     A rotating loop within a static magnetic field

 II.     A static loop in a changing magnetic field

 III.     A loop moving parallel within a magnetic field

I and II → A rotating loop within a static magnetic field and  A static loop in a changing magnetic field

A loop sits next to a current carrying wire of current I. Will rotating a loop with respect to its center of mass induce a current?

NO

rotating the loop is not changing B, N, A, or angle

A loop of wire sits perpendicular to a magnetic field pointing into the page. The magnetic field begins to decrease. What is the direction of the induced current in the loop?

CW

A magnetic field is pointing upwards through a loop when it is shut off. What is the direction of the induced current in the loop?

CCW

What direction is the magnetic force?

Toward the center of the curved path

What is the direction of the magnetic field that would cause the deflection shown for the proton beam?

into the page