1 magnetic fields and electric current electromagnetism: when an electric current flows through a...

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MAGNETIC FIELDS AND ELECTRIC CURRENT

Electromagnetism: when an electric current flows through a wire a magnetic field is created.

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Conductors

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

The conductor can be a “straight-line” conductor (i.e. an uncoiled wire)

Or it can be a coiled wire called a solenoid or a coil

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Solenoid

Wire wrapped around a core (tube)

Have a current flowing through them

Have a magnetic field Their magnetic fields

look like the magnetic field around a bar magnet.

Electromagnet

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Cores

A Core is the object that is inserted into the solenoid, creating an electromagnet.

Different materials influence the strength of the electromagnet.

Different metals can be used for the core: iron, steel, nickel or cobalt.

Iron is most commonly used because when you turn off the electricity it demagnetizes.

Metals like steel remain magnetized thus creating a permanent magnet.

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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 Flowor 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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The Left Hand Rules

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Electron Flow

If you use “electron flow” to describe the direction of the current, it flows from the negative to the positive terminal

You must use the “Left Hand Rules”

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

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

The 2nd Left 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 electrons flow from negative to positive

3. Point the thumb of your left hand to + with hand flat on page

4. Your knuckles will be on the NORTH

5. Your fingertips will be on the SOUTH -

+

Direction of the electron flow

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+

Solenoids (or coils)

1. Find the positive and negative ends of the wire

2. The electrons flows from negative to positive

3. Draw (or imagine) arrows on the front of the coil pointing in the direction of the electron flow.

4. Place your hand on the page 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.

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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?

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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

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+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 (Nerd to Sexy).

+-

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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 (Nerd to Sexy).

-+

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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

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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

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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

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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 current

Your fingers will point in the direction of the magnetic field

North and South are never on the ends of the wire

There is no North or South Pole. The magnetic field flows in a continuous circle around the wire.

The magnetic field flows in the same direction as the compass points.