Chin-Sung LinLesson 18

In 1831, Michael Faraday (England) and Joseph Henry (US) independently discovered that magnetism could produce current in a wire

Faradays law Electromagnetic inductionElectromagnetic induction any change in the magnetic field around a conductor induces a voltage (or emf)Faradays law The induced voltage (or emf) in a coil is proportional to the product of the number of loops and the rate of change of the magnetic field within those loops

How can we change the magnetic field around a conductor to induces a voltage (or emf)?

The change could be produced by relative motion of a wire with respect to the magnetic field

The change could be produced by moving the coil into or out of the magnetic field

The change could be produced by rotating the coil relative to the magnet

The change could be produced by changing the magnetic field strength

A magnet moving past a stationary conductor, or A conductor moving through a stationary magnetic field

The work done to the magnet is equal to the energy generated in the circuit to which the coil is connected

Wmechanical = Welectric

Induced voltage depends on:Speed of the wire traversing the magnetic field lines. Quicker motion induces a greater voltage (V ~ v)Number of loops of wire that moves in a magnetic field. The voltage is proportional to the number of loops (V ~ N)

If the coil does not form a complete circuit, what will happen?

Induced voltage without current, no work to plunge the magnet into the coil-+

If the coil forms a complete circuit, what is the direction of the induced current?

The induced magnetic field is repelling, the current will flow in a way to create such a repelling field

The more loops of the coil, the more voltage induced (V ~ N)The more voltage induced in the coil, the more current through the resistor in the circuit (I ~ V)The more current through the coil, the stronger the magnetic field it generated (B ~ I)The stronger the magnetic field generated, the stronger the repelling force acting back to your magnet (F ~ B)A coil with more loops is a stronger electromagnet and push back harder

What factors will affect the induced current?

Induced current depends on the induced voltage the resistance of the coil and thethe reactance of the coil

Reactancesimilar to resistance depends on the number of loops in the coil the frequency of the AC source

ReactanceThe counter-emf is the source of the opposition to current flow changeA constant DC current has a zero rate-of-change, and sees an inductor as a short-circuitAn AC current has a time-averaged rate-of-change that is proportional to frequency, this causes the increase in inductive reactance with frequency

Induced voltage of a moving conductor in a magnetic fieldV = v B L

The movement of a magnet is alternating, the induced voltage alternates on directionThe greater the frequency of the field change, the greater the induced voltage

The frequency of the induced alternating voltage equals the frequency of the alternating magnetic field within the loops High FrequencyLow Frequency

Generator a device that converts mechanical energy to electrical energyMotor a device that converts electrical energy to mechanical energy

When a closed conductor loop is moved in a magnetic field, an induced current flows through itThe direction of induced current is given by the Flemming's right hand generator rule

Whats the direction of the induced current?NS

Given by the Flemming's right hand generator ruleNS

As the number of magnetic field lines within the loop changes, the magnitude and direction of the induced voltage and current change

One complete rotation of the loop produces on complete cycle in voltage and current

The voltage induced by the generator alternates, and the current produced is alternating current (AC)The standard alternating current is 60 Hz

Hydro power generators

A a static device that transfers electrical energy from one circuit to another through inductively coupled conductorsA static device that transfers electrical energy to magnetic energy, and to electric energy againA device with which we can raise (for transmission) and lower (for use) the AC voltage in a circuitTransformer only works for AC

Primary and secondary coilsUse AC voltage source (primary coil)AC voltage is induced (secondary coil)Frequency AC voltage source = Frequency Induced AC voltage

Iron core (high permeability) is inserted into the coils to intensify the magnetic fieldIron core forms a complete loop to guide all magnetic field lines through the secondary

Transformer Symbol:

Np no. of turns of primary coilNs no. of turns of secondary coilVp voltage of primary coilVs voltage of secondary coilIp current of primary coilIs current of secondary coil

Step-up transformerNP < NS VP < VS Step-down transformerNP > NS VP > VS

PP = PS IP VP = IS VS

VP / NP = VS / NSIP VP = IS VSVP / VS = IS / IP = NP / NSVS = VP (NS / NP )IS = IP (NP / NS )

Faradays law: An electric field is created in any region of space in which a magnetic field is changing with timeThe magnitude of the created electric field is proportional to the rate at which the magnetic field changesThe direction of the created electric field is at right angles to the changing magnetic field

Maxwells law: A magnetic field is created in any region of space in which an electric field is changing with timeThe magnitude of the created magnetic field is proportional to the rate at which the electric field changesThe direction of the created magnetic field is at right angles to the changing electric field

In 1861 Scottish physicist James Clerk Maxwell discovered the theory of electromagnetismMaxwell united all previously unrelated observations and equations of electricity, magnetism and optics into a consistent electromagnetic field theory

German physicist Heinrich Rudolf Hertz was the first to satisfactorily demonstrate the existence of electromagnetic waves by building an apparatus to produce and detect VHF or UHF radio waves

A charge oscillates back and forth in empty space will produce electromagnetic waves in space where vibrating electric and magnetic fields regenerate each other

Electric field is perpendicular to the magnetic field, and both are perpendicular to the direction of the motion of the wave

No medium is required The speed the speed of lightThe wave is continuously self-reinforcing. The changing electric field induced a magnetic field. The changing magnetic field acts back to induce a electric field