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