Magnetism, Electromagnetism, &

Electromagnetic Induction

Chapters 24-25

Magnetic Fields• The source of all magnetism is moving

electric charges.

• Iron is the element with the most magnetic properties due to its net electron spin of 4.

• Magnetic field lines are vectors with a direction from North to South.

• Magnetic field lines must not cross each other.

• Magnetic fields are measured in Teslas.

Earth’s Magnetic Field

The angle between the magnetic and geographic poles is called the magnetic variation.

Compasses

• Compass needles are magnetized and line up along magnetic field lines.

• The North magnetic pole of a compass points to the geographic north.

• Since opposites attract, the magnetic pole in the Northern Hemisphere is actually a South magnetic pole.

• The North pole of a compass points in the direction of the field lines.

Magnetic Field around a current-carrying wires

• A current moving through a wire creates a magnetic field around that wire.

• The magnetic field forms concentric circles around the wire.

• Use the right hand rule shown to predict the direction of the field.

Electromagnets• Electromagnets are

temporary magnets formed by wrapping wire around an iron core.

• The iron becomes magnetized when the current is flowing due to the magnetic field being concentrated inside the coil of wire.

To find the North pole of an electromagnet, wrap the fingers around the coil in the direction that the current is flowing and the thumb points to the North pole.

Force of a magnetic field on a charged particle

• A charged particle moving through a magnetic field will experience a force that will cause it to move in a circular path.

• The force is to both the velocity and the magnetic field direction. sinqvBF

F = force(N), q = charge(C), v = velocity(m/s),

B = mag. field strength(T), =angle between v & B

Force of a magnetic field on a current-carrying wire

• A conductor with a current flowing through it in a magnetic field will experience a force.

sinIlBF F = force(N), I = current(A), l = length of wire(m),

B = mag. field strength(T), =angle between l & B

Force between 2 current-carrying wires

• When a current flows through a wire a magnetic field is produced around it.

• When 2 wires carry current near each other there will be an interaction (force) between the magnetic fields produced by each individual wire.

Induced EMF (Voltage)• A conductor in a

changing magnetic field will have an EMF (voltage) induced .

• Either the conductor can be moving across field lines or the magnetic field can itself be changing.

BlvEMF EMF = electromotive force, voltage(V), B = magnetic field strength (T), v = velocity to l(m/s),

Induced Current

• When a EMF (voltage, or potential difference) is present in a closed loop of conducting material current will flow.

RV

REMFI

I=current(A), EMF = V = Voltage(V), R = resistance

Lenz’s Law• The motion of a

conductor through a magnetic field will induce a current in that conductor.

• That current will cause the wire to experience a force that opposes the motion of the wire.

Motors vs. Generators• Motors

– Electric current is changed to motion.

– A coil of wire with a current through it will be forced to turn in a magnetic field.

• Generators– Motion is changed

to electric current.– Turning a coil in a

magnetic field will induce an EMF (voltage), thus current is produced.

AC Generator• As the loop of wire is turned in the magnetic field, one side is moving up while the other is moving down, therefore a current is induced in opposite directions in the different sections of the loop.

• As the loop continues to turn, the sections of wire change places and so the current switches direction.

• This causes the current to change constantly as shown in the graph.

AC/DC• Alternating Current (AC)

– current that switches direction of flow on regular time intervals

– 60 Hz in US– created by EMF

induced in a coil of wire turning in a magnetic field

• Direct Current (DC)– current that flows in

only one direction through a circuit

– supplied by batteries or electrochemical cells

– created by a chemical reaction that produces a potential difference (voltage) between the two electrodes (terminals)

Effective vs. Maximum with AC Current

• DC values are comparable to Effective AC values.

• AC circuits do not get the effect of the maximum current and voltage produced

• The power equivalent of AC to DC voltage is half.

ACDC

maxeff

PP

PP

21

21

maxeff

maxeff

VV

II

707.

707.

Transformers• An alternating current flows through the

primary coil creating an alternating magnetic field.

• This changing magnetic field induces an EMF (Voltage) in the secondary coil and thus current flows.

• In an ideal transformer, Power in = Power out

To solve Transformer Problems

The ratio of voltages on the two coils is equal to the ratio of the number of turns in the coils.

p

s

p

s

NN

VV

sspp

outin

IVIVPP