ELECTROMAGNETISM - Montgomery College

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Transcript ELECTROMAGNETISM - Montgomery College


The marriage of electricity and magnetism


Different aspects of the same force: Electromagnetic Force


• Luigi Galvani = Frog leg • Alessandro Volta = Voltaic pile (Battery sandwich) • Stacked copper and zinc together with a layer of wet paper between metals producing a weak current

Invention of the battery led to the understanding of electromagnetism and their symbiotic relationship • Modern batteries • Positive carbon rod • Surrounded by electrolytic paste • Housed in a negative zinc.

Invention of the battery led to the understanding of electromagnetism and their symbiotic relationship • Therefore a battery is a source of conversion of some form of energy (magnetic) into electric current • Battery is a source of EMF

Hans Oersted

• Discovered the relationship between magnetism and electric current • A magnetic field is produced by current running through the wire • Any charge in motion induces a magnetic field

“Hand” Rules

• Right Hand rule • Thumb indicates the direction of the current • Fingers indicate direction of magnetic field lines

What if????

• We take a wire • and form the wire into loops • Does resistance increase?

• Does the magnetic field increase? (see pg 91)

What if

• We place another coil (B) of wire (solenoid) next to the one that is producing the magnetic field? (A) • Will this produce an electric current. (see pg 91)

• Take a look at the solenoid marked A and compare it to solenoid marked B • Which has more coils?

• When Coil B is placed in the magnetic field of Coil A what will happen? a b

• Where would there be an increase in voltage. A or B?

• If there are more coils on the SECONDARY SIDE or to the right of the solenoid A, then would there be more voltage on the B side? a b

• Look on page 94 and find the term STEP-UP TRANSFORMER Compare that definition to the transformer on the left What type of transformer is this? a b

Now look at the circuit below Can you find the step up transformer?

Number 8 is the primary side of the step up transformer and numbers 9 are the secondary side of the step up transformer

Now try and find it on page 117.

If the primary side of the step up transformer has less coils than the secondary side on a step up transformer that would convert Voltage to……….what is it that we use to control the quality of x-rays???

Faraday Rule

• AKA: 1st law of Electromagnetism • A magnetic field cannot be still and produce a current. But if the magnet is moving a current is produced.

Furthermore: Induced current by a magnet depends on

• Strength of magnetic field • Velocity of magnetic field • Angle of the conductor to the Magnetic field • # of turns in a conductor

TO PRODUCE electric current

• Move a bar near a coil of wire • OR • Move coil of wire near stationary magnet • Current increases with increase in magnetic field


• The current going through the step-up transformers is AC…..Think about that…How does AC move?

AC pulses!!!

• Which is movement! That its why AC must be used in a step up transformer.

Numbers 15 and 14 are step down transformers-why?????

Look at it closely…note the number of coils on the primary side versus the secondary side (see arrows)

Where does the current go from this transformer?? What happens to the voltage?


• Most important thing to remember even if you do not understand this law is the following: • It is the basis for self induction • The autotransformer and choke coil in an x ray circuit both run on self induction


Lenz’s Law: Induced current flows in a direction such that it opposes the action that induces it