Electromagnetic Induction Physics La Cañada High School Dr.E

Electromagnetic Electromagnetic InductionInduction

PhysicsPhysics

La CaLa Caññada High Schoolada High School

Dr.EDr.E

• Electromagnetic InductionElectromagnetic Induction• Motors/Generators Motors/Generators http://science.http://science.howstuffworkshowstuffworks.com/motor4..com/motor4.htmhtm

• Faraday’s LawFaraday’s Law

• TransformersTransformers

• Power TransmissionPower Transmission

• SolenoidsSolenoids

• MRIMRI

The Magnetic FieldThe Magnetic Field• The electric field proves a useful concept to The electric field proves a useful concept to

explain the effects of charge at a distance explain the effects of charge at a distance (explains how one charge knows another (explains how one charge knows another charge is there).charge is there).

• Stationary charges produce only an electric Stationary charges produce only an electric fieldfield

• Moving charges (in flux) produce both an Moving charges (in flux) produce both an electric fieldelectric field and a and a magnetic fieldmagnetic field

• Electromagnetic InductionElectromagnetic Induction

• Motors/GeneratorsMotors/Generators http://science.howstuffworks.com/motor4.htm http://science.howstuffworks.com/motor4.htm

• Faraday’s LawFaraday’s Law• TransformersTransformers• Power TransmissionPower Transmission• SolenoidsSolenoids• MRIMRI

MotorsMotors• Motors work due to two major principlesMotors work due to two major principles

1.1. Opposite poles attract while like poles repelOpposite poles attract while like poles repel2.2. Current running through a coiled wire Current running through a coiled wire

creates a magnetcreates a magnet

TheoryTheory

MechanicsMechanics

SN SN

S

S

S

SSS

• Electromagnetic InductionElectromagnetic Induction• Motors/Generators Motors/Generators http://science.howstuffworks.com/motor4.htm http://science.howstuffworks.com/motor4.htm

• Faraday’s LawFaraday’s Law• TransformersTransformers

• Power TransmissionPower Transmission

• SolenoidsSolenoids

• MRIMRI

Faraday’s Law of InductionFaraday’s Law of Induction

The voltage induced in a coil The voltage induced in a coil is proportional to the number is proportional to the number of coils times the magnetic of coils times the magnetic flux (rate at which the flux (rate at which the magnetic field changes)magnetic field changes)

NN

Wrap Rule to find Magnetic FieldWrap Rule to find Magnetic Field1.1. Wrap your fingers in the direction of the currentWrap your fingers in the direction of the current2.2. The magnetic field points in the direction of the thumb (to The magnetic field points in the direction of the thumb (to

the left)the left)3.3. Since the field lines leave the left end of solenoid, the left Since the field lines leave the left end of solenoid, the left

end is the end is the NNorth poleorth pole

• An ammeter is connected in a circuit of a conducting An ammeter is connected in a circuit of a conducting looploop

• When a bar magnet is moved closer to, or farther from, When a bar magnet is moved closer to, or farther from, the loop, an electromotive force (emf) is induced the the loop, an electromotive force (emf) is induced the looploop

• The ammeter indicates currents in different directions The ammeter indicates currents in different directions depending on the relative motion of magnet and loopdepending on the relative motion of magnet and loop

• When the magnet stops moving, the current returns to When the magnet stops moving, the current returns to zero as indicated by the ammeterzero as indicated by the ammeter

• Electromagnetic InductionElectromagnetic Induction• Motors/Generators Motors/Generators http://science.howstuffworks.com/motor4.htm http://science.howstuffworks.com/motor4.htm

• Faraday’s LawFaraday’s Law

• TransformersTransformers• Power TransmissionPower Transmission

• SolenoidsSolenoids

• MRIMRI

Primary VoltagePrimary Voltage

# of 1# of 1o o turnsturns

Secondary VoltageSecondary Voltage

# of 2# of 2o o turnsturns

(Power IN)(Power IN) (Power OUT)(Power OUT)

(Voltage x Current)(Voltage x Current)primaryprimary(Voltage x Current)(Voltage x Current)secondarysecondary

• Electromagnetic InductionElectromagnetic Induction• Motors/Generators Motors/Generators http://science.howstuffworks.com/motor4.htm http://science.howstuffworks.com/motor4.htm

• Faraday’s LawFaraday’s Law

• TransformersTransformers

• Power TransmissionPower Transmission• SolenoidsSolenoids

• MRIMRI

Power TransmissionPower TransmissionPlant Plant

6000 V and 20Amps6000 V and 20Amps

Local and Homes Local and Homes 120 V and 100 Amps120 V and 100 Amps

Low Voltage Wires Low Voltage Wires 2200 V and 54 Amps2200 V and 54 Amps

High Voltage Wires in TownHigh Voltage Wires in Town120,000 V and 1 Amp120,000 V and 1 Amp

Ultra High Voltage Ultra High Voltage 400,000 V and 0.3 Amps400,000 V and 0.3 Amps

• Electromagnetic InductionElectromagnetic Induction• Motors/Generators Motors/Generators http://science.howstuffworks.com/motor4.htm http://science.howstuffworks.com/motor4.htm

• Faraday’s LawFaraday’s Law

• TransformersTransformers

• Power TransmissionPower Transmission

• SolenoidsSolenoids• MRIMRI

SolenoidsSolenoidsThe magnetic field of a solenoid is essentially identical to The magnetic field of a solenoid is essentially identical to

that of a bar magnet.that of a bar magnet.

The big difference is that we can turn the solenoid The big difference is that we can turn the solenoid onon and and off off ! It attracts/repels other permanent magnets; it ! It attracts/repels other permanent magnets; it attracts ferromagnets, etc.attracts ferromagnets, etc.

Solenoid ApplicationsSolenoid ApplicationsDigital [on/off]:Digital [on/off]:

– DoorbellsDoorbells

– Power door locksPower door locks– Magnetic cranesMagnetic cranes– Electronic Switch “relay”Electronic Switch “relay”

Close switch current magnetic field pulls in plunger closes larger circuit

Advantage:A small current can be used to switch a much larger one– Starter in washer/dryer, car ignition, …

Magnet off plunger held in place by springMagnet on plunger expelled strikes bell

Solenoid ApplicationsSolenoid ApplicationsAnalog (deflection Analog (deflection α α I I ):):

– Variable A/C valvesVariable A/C valves– SpeakersSpeakers

In fact, a typical car has over 20 solenoids!

Solenoids are everywhere!

• Electromagnetic InductionElectromagnetic Induction• Motors/Generators Motors/Generators http://science.howstuffworks.com/motor4.htm http://science.howstuffworks.com/motor4.htm

• Faraday’s LawFaraday’s Law

• TransformersTransformers

• Power TransmissionPower Transmission

• SolenoidsSolenoids

• MRIMRI

Thanks to

MRI /MRI / NMR NMRIf we “bathe” the protons in radio waves at a particular frequency, the protons can flip back and forth.

If we detect this flipping hydrogen!

The presence of other molecules can partially shield the applied magnetic field, thus changing the resonant frequency (“chemical shift”).

Looking at what the resonant frequency is what molecules are nearby.

If a strong magnetic field gradient is produced across the sample, can look at individual slices, with ~millimeter spatial resolution. B

Small Blow freq.

Bigger Bhigh freq.

Signal at the right frequency only from this slice!

BibliographyBibliography1. Magnetism: Examples of Magnetic Field Calculations, Innovations in Undergraduate

Physics Education at Illinois @ online.physics.uiuc.edu/courses/phys112/spring04/ Lectures/Lect15.ppt, 4/17/04