Chapter 17Electromagnetic Induction

The prime link between electricity and magnetism is…

MOTION

Magnetism Creates Electricity Through MotionElectricity Creates Magnetism Through Motion

Anytime a charged particle moves, a magnetic field is generated

Magnetism Creates Electricity Through MotionElectricity Creates Magnetism Through Motion

Anytime a charged particle moves, a magnetic field is generated

If the direction of the current changes, the resulting magnetic field

reverses as well

If a conductor (wire) is passed through a magnetic field a voltage is induced across the conductor. (Current flows through the wire)

N SThis phenomenon is called electromagnetic induction

Left Hand Rule

The magnetic field with circular lines of forces is in a plane perpendicular to the current in the wire.

Determining Magnetic Polarity (N vs S)You can also determine magnetic

polarity using the Left Hand Rule.

Using your left hand, curl your fingers in the direction that current is flowing through a coil of wire.

Explaining Left hand rule vs Right hand ruleIn our class we use the left hand

rule because we use the electron flow model.

If we were using the traditional current flow model, the right hand rule would be used

It is useful for illustration purposes to denote when current is going toward you or away from you.

X means current is going into the board. A dot means is coming out.

Notice the direction of magnetic field follow the left hand rule.

Notice current going in the same direction aids each other’s magnetic field.

This is why current going through a coil makes a large electromagnet! (See next slide)

Remember! You can go from electricity to magnetism, or magnetism to electricity.

You can either move a magnet through a coil of wire inducing current, or…

You can move current through a coil of wire inducing a magnetic field.

If the magnet inside the coil of wire below was stationary, would an electromagnetic field be induced?

Faraday’s Law

“A voltage is induced by a changing or moving magnetic flux cutting the turns of a coil.”

When current is produced an induced voltage is also produced. This occurs when there is relative motion between a magnetic field and/or a current carrying conductor

3 factors effect the size of induced voltage

1. The number of turns in the coil2. The strength of the magnetic field (Number of flux lines)3. The relative speed between the coil and the magnetic field.

Example

A magnetic flux of 1000 uWb cuts across a coil of 2000 turns in 0.1 s. Calculate the induced voltage.

Lenz’s Law“The induced voltage in a circuit

causes a current to flow. This current’s magnetic field is in the opposite direction of motion of the original magnetic field, thus opposing it.”

If this law weren’t true, what would happen?

Lenz’s law in actionhttp://

www.youtube.com/watch?v=keMpUaoA3Tg

Magnetic Field of and ElectromagnetDepends on 3 things. The 3 C’s

◦Current◦Coils◦Core

AC GeneratorsConverts mechanical energy into

electrical energy (Faraday’s Law)What does the opposite?

(Converts electrical energy into mechanical)◦A Motor

How much voltage can a generator make?The amount of voltage depends on 4

things:1. The strength of the magnetic field2. The angle at which the conductor

cuts the magnetic field3. The relative change in speed

between conductor and magnetic field

4. The length of the conductor within the magnetic field.

How an AC generator works…

http://www.listenonrepeat.com/watch?v=uYfTzCa71SE

http://www.listenonrepeat.com/watch?v=gqA3WoOunEA&feature=related

See magnetic field handout

N S

How do DC Generators work?DC generators produce DC voltage. However this voltage is not a nice flat voltage, it is pulsating. See video on next slide.

Same as AC generator except slip rings are replaced with two piece commutator.

How do DC generators work?

http://www.listenonrepeat.com/watch?v=Xi7o8cMPI0E&feature=related

Generators are not perfectly efficientNot all 100% of the mechanical energy is converted

into electrical energy.The efficiency depends on 3 things.1. I2*R or copper losses in the winding

◦ (Energy is lost as heat, heat is reduced by using Small Awg wire)

2. Eddy current loss in the core material◦ Currents that go in the opposite direction as the current

being made. This can cause a circulating flow of electrons, or a current, within the body of the conductor. These circulating eddies of current create induced magnetic fields that oppose the change of the original magnetic field due to Lenz's law.

3. Hysteresis loss (magnetic friction)◦ (Magnetic dipoles constantly change directions, this

molecular friction produces heat. Heat goes into armature causing its resistance to increase.)

What about motors?Motors are opposite of

generators.Motors convert electrical energy

into mechanical energy.Note: The DVD movie after lunch

will do a good visual job of explaining how motors work

Types of DC motorsSeries Motors: Windings are connected in series with Armature

Starting torque is greater than other DC motorsSpeed varies widely with load.

Types of DC motorsShunt Motors: Windings are connected in parallel with

Armature. Starting torque is smaller than other DC motors. Speed does not vary widely with load.

Types of DC motorsCompound Motors: A mixture of series and shunt.

Stepper Motors: Motors that turn a specific amount of time. Used in printers, scanner, disk drives… (Micro-controllers drive stepper motors)

Brushless DC Motors

In a BLDC motor, the electromagnets do not move; instead, the permanent

magnets rotate and the armature remains static.