Magnetism Section 3

Section 3: Electric Currents from Magnetism

Preview• Key Ideas • Bellringer • Electromagnetic Induction• The Electromagnetic Force• Transformers

Magnetism Section 3

Key Ideas

〉What happens when a magnet is moved into or out of a coil of wire?

〉How are electricity and magnetism related?

〉What are the basic components of a transformer?

Magnetism Section 3

Bellringer

1. List as many items as you can that use DC current. (Hint: Batteries supply DC current.)

2. List as many items as you can that use AC current. (Hint: Standard wall outlets supply AC current.)

3. If you want to plug a CD player that normally uses batteries into a wall socket, an AC adapter is required. What is the function of the AC adapter?

4. Electric power that goes into a neighborhood must be stepped down, or decreased, in voltage before it goes into a home. Explain why this is necessary.

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Electromagnetic Induction

〉What happens when a magnet is moved into or out of a coil of wire?

〉Moving a magnet into and out of a coil of wire causes charges in the wire to move.

• electromagnetic induction: the process of creating a current in a circuit by changing a magnetic field

• Faraday’s law states:

An electric current can be produced in a circuit by a changing magnetic field crossing the

circuit.

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Electromagnetic Induction, continued

• As the loop moves in and out of the magnetic field of the magnet, a current is induced in the circuit.

• Rotating the circuit or changing the strength of the magnetic field will also induce a current in the circuit.

• Electromagnetic induction obeys conservation of energy.– Pushing a loop through a magnetic field requires

work.

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Electromagnetic Induction, continued

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Visual Concept: Ways of Inducing a Current in a CircuitClick the button below to watch the Visual Concept.

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Electromagnetic Induction, continued

• The magnetic force acts on moving electric charges.

– The force is at its maximum value when the charge moves perpendicularly to the field.

– As the angle between the charge’s direction and the direction of the magnetic field decreases, the force on the charge decreases.

• The magnetic force acts on wires carrying a current.

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Electromagnetic Induction, continued

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Electromagnetic Induction, continued

• Generators convert mechanical energy into electrical energy.

• generator: a machine that converts mechanical energy to electrical energy

• alternating current (AC): an electric current that changes direction at regular intervals

– For each half rotation of the loop, the current produced by the generator reverses direction.

• AC generators produce the electrical energy you use in your home.

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AC Generator

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Induced Current

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Visual Concept: Function of a Generator

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The Electromagnetic Force

〉How are electricity and magnetism related?〉Electricity and magnetism are two aspects of a

single force, the electromagnetic force.

• The energy that results from these two forces is called electromagnetic (EM) energy.

• Light is a form of electromagnetic energy.

• EM waves are made up of oscillating electric and magnetic fields that are perpendicular to each other.

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Visual Concept: Electromagnetic Waves

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Transformers〉What are the basic components of a transformer?

〉In its simplest form, a transformer consists of two coils of wire wrapped around opposite sides of a closed iron loop.

• transformer: a device that increases or decreases the voltage of alternating current

• primary coil: wire attached to a source of alternating current

• secondary circuit: wire attached to an appliance

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Transformers, continued• Current in the primary coil, creates a changing

magnetic field that magnetizes the iron core.

• The changing magnetic field of the iron core then induces a current in the secondary coil.

• Transformers can increase or decrease voltage.

– The voltage induced in the secondary coil of a transformer depends on the number of loops, or turns, in the coil.

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Transformers, continued• In a step-up transformer, the primary coil has fewer

turns than the secondary coil does.

– The voltage across the secondary coil is greater than the voltage across the primary coil.

• In a step-down transformer, the secondary coil has fewer loops than the primary coil does.

– The voltage across the secondary coil is lower than the voltage across the primary coil.

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Transformers, continued• Transformers must obey the law of conservation of

energy.– The current in the secondary coil of a step-up

transformer is always less than the current in the primary coil.

• Transformers are used in the transfer of electrical energy.

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Transformers, continued

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Visual Concept: Transformer

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