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5.14 MAGNETIC FIELDS AND ELECTRIC CURRENT

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A magnetic field is created whenever an electric current flows through a conductor.

This is electromagnetism. The conductor can be a:

“straight-line conductor” (uncoiled wire) or

coiled wire (a.k.a solenoid)

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A Solenoid:

Electromagnet

When a current flows through the wire ….

a magnetic field is generated

Similar to that of a bar magnet.

Is a wire wrapped around a core.

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Cores: A Core is the object that is inserted

into the solenoid, creating an electromagnet. easy to align or misalign the domains

Different ferromagnetic metals can be used for the core: iron, nickel or cobalt. Iron is most commonly used because

when you turn off the electricity it demagnetizes!

Steel is usually not used because it is difficult to demagnetize.

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The “Hand” Rules There are two sets of “hand” rules that can be

used to determine the direction of the magnetic field around an electrical conductor

Your choice of which set of rules to use depends on which system you use to describe electric current:Electron Flow or Conventional Current

Both systems are correct, and both work, but you must be careful not to mix them up!

The “Left Hand Rules” are covered in slides 6 to 11 The “Right Hand Rules” are covered in slides 12 to 17

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Left Hand Rule

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This rule will help you determine the direction of the magnetic fields created by an electric current. The 1st Left Hand Rule is used with a

straight wire (uncoiled). The 2nd Left Hand Rule is used with a

solenoid (or coil) Remember electrons flow from – to + !!!!

5.15 The Left Hand Rules

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Straight Line Conductors (Uncoiled)

1. Find the + and - ends of the wire

2. Point your thumb in the direction of the electron flow (towards +end)

3. Your fingers are pointing in the direction of the field

-

+

Direction of the electron flow

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Conventional current

(right hand rule)

Electron flow (left hand rule)

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+

Solenoids (or coils)

1. Find the +and - ends of the wire

2. Draw arrows on the front of the coil pointing in the direction of the electron flow (towards +).

3. Place your hand on the page with your fingers pointing in the direction of the electron flow.

4. Your thumb will point towards the north end of the solenoid.

-

SN

Up behind the core andDown in front of the core.

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The Left Hand Rules will help you with all of the electromagnetic problems …got it?

Jump to slide 18

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The Right Hand Rules

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Conventional Current

If you use “conventional current” to describe the direction of the current, it is said to flow from the positive to the negative terminal

You must use the “Right Hand Rules”

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There are two “hand rules” that will help you to determine the direction of the magnetic fields created by electric currents.

The 1st Right Hand Rule is used with a straight line or uncoiled wire.

The 2nd Right Hand Rule is used with a solenoid (or coil)

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Straight Line Conductors (Uncoiled Conductors)

1. Find the positive and negative ends of the wire

2. The conventional current flows from positive to negative

3. Point thumb of your right hand in the direction of the conventional current

4. Wrap your fingers around the wire

5. Your fingers will point in the direction of the magnetic field

-

+Direction of the Conventional Current

Direction of themagnetic field

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+

Solenoids (or coils)1. Find the positive and negative

ends of the wire2. The conventional current flows

from positive to negative3. Draw (or imagine) arrows on

the front of the coil pointing in the direction of the conventional current.

4. Wrap your right hand around the solenoid with your fingers pointing in the direction of the electron flow (in the direction of the arrows).

5. Your thumb will point towards the north end of the solenoid.

-

S N

Up behind the core andDown in front of the core.

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The Right Hand Rules will help you with all of the electromagnetic problems …got it?

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Solenoid Example - # 11. Find the positive and

negative ends of the wire

2. Determine the direction of the electric current

3. Wrap your fingers around the coil pointing in the same direction as the current

4. Thumb points “N”. Other end is “S”.

5. Lines of Magnetic force run N to S .

+-

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Solenoid Example # 21. Find the positive

and negative ends of the wire

2. Determine the direction of the electric current

3. Wrap your fingers around the coil pointing in the same direction as the current

4. Thumb points “N”. Other end is “S”

5. Lines of Magnetic force run N to S

-+

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Determining the Direction of the Magnetic Field around a Straight Line Conductor

+

+

+

+

-

-

-

-

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Effects of a Straight Line Conductor on a Compass

- +

Compass arrowPoints in the same

direction as theMagnetic Filed

Compass

Determine thedirection of theMagnetic Field

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The magnetic field flows in a continuous circle around the wire perpendicular to it.

Determine the direction of the magnetic field

Compasses placed around the wire will point in the direction of the magnetic field

+

- +

-

The Continuous Magnetic Field Around a Straight-line Conductor

If we reverse the terminals, the magnetic field will flow in the opposite direction

And the magnets will point the other way…

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Straight Line ConductorExamples

-

-

-

- +

+

+

+

A B

C D

Determine the direction that the compass would point if it were placed on top of the wires as illustrated.

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Hand Rule for Straight Line Conductors:Your thumb points in the direction of the

electric currentYour fingers will point in the direction of

the magnetic fieldThere is no North or South Pole. The magnetic field flows in a continuous

circle around the wire.The compass points in same direction as

the magnetic field.

5.16 Electromagnetic Induction

Is the process where you use a moving magnetic field to produce an electrical current. (p171)

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• This is achieved by:

• Moving a conductor perpendicularly inside the magnetic field

• Moving a magnet around a conductor

• eg. Hydro electricity

• Electric generators

How it works.

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Finish for homework if needed

Textbook p174 21 to 25