Chapter Chapter 2020&&2121

Like Poles of a magnet repel; unlike poles attract.

Electricity and Magnetism – how are they related?

When an electric current passes through a wire a magnetic field is formed.

Right hand rule in a current carrying wire See image on page 770

Magnetic Field of a current loop See images on page 771

Solenoids – continuous loops of wire, they form strong magnets on the inside.

How can you tell North / South?

Deflection of a compass needle near a current carrying wire, showing the

presence and direction of the magnetic field

Right Hand Rule

B Magnetic Field 4 fingers V velocity of moving charge thumb F force on a positive charge palm

of hand A negative charge would have the opposite direction of force.

Arrows into a page are drawn as X’s, arrows out of the page are drawn as points.

Study Image on page 774 Answer questions on page 783 #35

B, to the right

V, up the page

What would be the direction of force on an electron? A proton?

XForce is into the page, Magnetic field is down the page.

What is the direction of velocity for a positive charge?What is the direction of velocity for a negative charge?

2 parallel wires carrying current, they attract if the current is in the same direction

2 current carrying wires repel if their currents run in opposite directions.

Loudspeakers work by this idea

F = ILB sinθ I = current (amps)

L= Length of wire(m) B = magnetic field (T)

Tesla, (G) Gauss F = Force on electric current in magnetic field

(N)

F = q vB sinθ q= particle charge,

coulombs (C) V = velocity of

charge (m/s) B = magnetic field

(T)

B = µo I/ 2πr Magnetic field due to a

straight wire

µo = permeability of free space 4 x10–7 Tm/A I = current (amps)

r = perpendicular distance to the wire

B = magnetic field (T) Teslas

(a) An unmagnitized piece of iron is made up of domains that

are randomly arranged. Each domain is like a tiny magnet;

the arrow represent the magnetization direction, with the arrowhead being the N pole. (b) In a magnet, the domains are preferentially

aligned in one direction and may be altered in size by the

magnetization process

Φm = BA CosθMag Flux

Φm = magnetic flux (Tm2) Tesla meter2

Weber(1wb=1Tm2)

B = magnetic field A = area of loop

EAVE = ΔΦm /ΔtFaradays Law of

Induction EAVE = EMF (Tm2/ sec) Φm = magnetic flux

(wb) Δt = time (sec)

E = BLVEmf Induced in a moving

Conductor B = magnetic Field

V = Velocity L = length (m)

True as long B,V,L are mutually perpendicular

Transformers

A device that changes one ac potential difference to a different ac potential difference.

Power companies increase voltages for long distance transmission, then they must decrease voltages before going into your home.

We say the voltage has been stepped up or it has been stepped down.

Transformers

They have 2 sides – primary and secondary

Primary is the side closest (wired) to the generator

Secondary is the side wired to the resistor or the consumer.

A soft Iron core connects both sides.

Transformer Equation

V2N1 = N2V1

V = voltage

N = number of turns or coils of wire

1 = Primary

2 = Secondary

Vs/ Vp = Ns/Np

V= Voltage N = # of Turns

Ns > Np step up transformer, increase

voltage Ns < Np step down transformer, decrease

voltage Power stays the same,

(almost)

What is a galvanometer?A galvanometer is an electromagnet that interacts with a permanent magnet. The stronger the electric current passing through the electromagnet, the more is interacts with the permanent magnet.

The greater the current passing through the wires, the stronger the galvanometer interacts with the permanent magnet.

Galvanometers are used as gauges in cars and many other applications.

Galvanometer

A simple instrument designed to detect electric current.

When calibrated to measure current, it is an ammeter.

When calibrated to measure voltage, it is a voltmeter.

Electromagnetic Induction The production of an emf (electro-

motive force – kinda like voltage) in a conducting circuit by a change in the strength, position, or orientation of an external magnetic field.

Faradays Law

The induced emf (electromotive force) in any closed circuit is equal to the time rate of change of the magnetic flux through the circuit.

It is the operating principle of transformers, inductors, many types of motors and generators.

(A) A current induced when a magnet is moved toward a coil. (B) The induced

current is opposite when the magnet is moved away from the coil. Note that the galvanometer zero is at the center of the

scale and the needle deflects to the left or right, depending on the direction of the

current. In (C) no current is induced if the magnet does not move relative to the coil.

Lenz Law

An induced current is always in such a direction as to oppose the motion or change causing it

Induction

Wires spinning in magnetic fields is what underlies all electric motors and electric generators.

Most of the rest of the chapter deals with applications of this. Electric motors, electric generators, and transformers will be as far as we go down this road.

What are electric motors?An electric motor is a device which changes electrical energy into mechanical energy.

Go to the next slide

How does an electric motor work?

Simple as that!!

We have seen how electricity can produce a magnetic field, but a magnetic field can also produce electricity! How?

What is electromagnetic induction?

Moving a loop of wire through a magnetic field produces an electric current. This is electromagnetic induction.

A generator is used to convert mechanical energy into electrical energy by electromagnetic induction.

Carefully study the next diagrams:

Direct current versus alternating current –

AC vs DC : What’s the difference?

Direct current is electrical current which comes from a battery which supplies a constant flow of electricity in one direction.

Alternating current is electrical current which comes from a generator. As the electromagnet is rotated in the permanent magnet the direction of the current alternates once for every revolution.

Go to this website and click the button for DC then for AC to visually see the difference between the two.

You can see that the DC source is a battery – current flows in one direction. The AC source is the generator and the current alternates once for each revolution.

Explanation of Fig. 21-17

(a) Schematic (simplified) diagram of an alternator. The input electromagnet current to the rotor is connected through continuous

slip rings. Sometimes the rotor is made to turn by a belt from the engine. The current in the wire coil of the rotor produces a magnetic

field inside it on its axis that points horizontally from left to right, thus making

north and south poles of the plates at either end. These end plates are made with

triangular fingers that are between them as shown by the blue lines. As the rotor turns,

these field lines pass through the fixed stator coils (shown on the right for clarity, but in

operation the rotor rotates within the stator) inducing a current in them, which is the

output

The emf is induced in the segments ab and cd, with

velocity components perpendicular to the field B are

v sin θ

Determining the flux through a flat loop of wire. This loop is

square, of side l and area A=l2

A current can be induced by changing the area of the coil. In both this case and that

of Fig. 21-6, the flux through the coil is reduced. Here the brief induced current

acts in the direction shown so as to try to maintain the original flux (Φ = BA) by

producing its own magnetic field into the page. That is. as the area A decreases, the

current acts to increase B in the original (inward) direction

http://www.youtube.com/watch?v=QPd963cCeec