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8.3 Force on a Conductor in a Magnetic Field. T he Principle of Electromagnetism : Oersted , 1820 Whenever an electric current flows in a conductor, a magnetic field is created around the region of the conductor.

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T he Principle of Electromagnetism : Oersted , 1820

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8.3 Force on a Conductor in a Magnetic Field

The Principle of Electromagnetism: Oersted, 1820

Whenever an electric current flows in a conductor, a magnetic field is created around the region of the conductor.

Question 1: If electrons are flowing up as shown in the straight conductor, we can conclude that:

A) There is no magnetic field around the conductor

B) The magnetic field forms concentric circles in the CCW direction

C) The magnetic field forms concentric circles in the CW direction

D) The magnetic field extends radially, pointing inward

e-

Note: Current flowing towards you (out of page)

Note: Current flowing away from you(into page) X

Interaction of a magneticfield around a Conductor with a surrounding magnetic field:

Consider positive current flowing up in a straight conductor (top view)

B

Now consider what happens when there is a surrounding uniform magnetic field surrounding the conductor (B)

FIELD LINES

CANCEL

FIELD LINES

The two magnetic fields interact to combine or cancel creating a strong field on one side and weaker field on the other!

Small B field

Large B field

Net Force

Motor Principle: A current carrying conductor in the presence of an external magnetic field will experience a force that is perpendicular to both the current (I) and the magnetic field (B) directions.

The magnetic force, FM, is given by:

FM=ILBsin

FM

I

FM=magnetic force in Newtons

I=current in Amperes (Coulombs/second)

L=length of conductor in magnetic field (m)

B=external magnetic field (Tesla)

= angle between current direction (I) and magnetic field direction (B)

B

Maximum force occurs when I and B are at an angle of

_________!

90 

Units Check: FM=ILBsin

RS (A) (m) (T)

LS N

Thus: 1(N) = (1A)(1m)(1T)

Or 1T = N/A●m

Base units of Tesla

Right Hand Rule for Motor Principle

B

Thumb positive current direction (I)

Fingers magnetic field direction (B)

(N to S)

I

Palm direction of force (FM)

FM

Example 2: What is the direction of the force on the wire shown below?

Up

Down

Into the page

Out of the page

There is no force on the wire

Example 3: What is the direction of the force on the wire shown below?

Up

Down

Into the page

Out of the page

There is no force on the wire.

Example 4:The conductor shown (length 5.0 cm) carries a conventional (positive) current of 6.0 A through a uniform magnetic field of 0.40 T. Find the magnitude and direction of the magnetic force on the wire.

up

N

wire

E

W

down

S