Magnetism and Magnetic Forces: Understanding the Interaction of Moving Electric Charges, Slides of Applied Chemistry

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1

The Magnetic Field “B”

 Magnetic Field B

 Magnetic Force on a Moving Charge

 Calculating Charges

 Magnetic Force on a Current

 Torque on a Current Loop

 The Magnetic Dipole

2

Introduction

 TV Picture tubes, Loud speakers, Computer disk drivers, and Computer printers etc…… all uses magnetic forces

 The alignment of compass needle with the earth magnetism is an example of magnetic interaction

 The fundamental nature of magnetism is the interaction of moving electric charges

 Electric interactions are caused by the charges whether they are moving are not while

 Magnetic interactions are only caused by the moving charges

4

Magnetism

Moving charges – Create magnetic field

 The magnetic field exerts magnetic force on

moving charges

 We will look at both the aspects….

 Effect of magnetic field on moving charge

 How moving charges create magnetic field

Current is just another name for moving charges

5

Bar Magnet

• Bar magnet ... two poles: N and S

Like poles repel; Unlike poles attract.

• Magnetic Field lines: (defined in same way as electric field lines, direction and density)
• Does this remind you of a similar case in electrostatics? S N

The Electric Dipole

7

Magnetic Monopoles

 Perhaps there exist magnetic charges, just like electric charges. Such an entity would be called a magnetic monopole (having + or - magnetic charge)

 How can you isolate this magnetic charge?

Try cutting a bar magnet in half:

S N S^ N^ S N

Every individual part has a magnetic “dipole”! Magnetic monopoles have never been found

8

Geomagnetism

• If we hang a bar magnet in air
• One end of the magnet will point to the geographical north of the earth
• Can you name a place at geographical north pole?
• The other end will point towards geographical south of earth

10

Magnetic Force on a Moving Charge

 Review of electric interaction

 A distribution of electric charge creates an electric field E in the surrounding space

 The electric field exerts a force F = qE on any other charge that is present in the field

 We described the magnetic interaction in a similar way

 A moving charge creates a magnetic field in the surrounding space (in addition to the electric field)

 The magnetic field exerts a force F on any other moving charge or current that is present there

11

Magnetic Force on a Moving Charge

 Like electric field, magnetic field is also a vector quantity

 We will use the symbol B for magnetic field

 At any position the direction of magnetic

field B is that in which the north pole of a

compass needle tends to points

 The direction of magnetic field B points out of the North pole and into the south pole

13

The magnetic field B can apply magnetic force on electric charge only if:

• The charge is moving
• The velocity vector has a component perpendicular to B

Magnetic Force on a Moving Charge

FB =^ qv x B

14

Magnetic Force on a Moving Charge

 Characteristics of magnetic force on a moving charge are

 Magnitude of magnetic force is directly proportional to the magnitude of the charge

 Magnitude of force is proportional to the strength of field B

 Magnitude of force is also proportional to the velocity of the charge

 Magnetic force is perpendicular both to the velocity and magnetic field B

16

Right Hand Palm Rule

17

Magnetic Field

The magnitude of magnetic filed near the earth surface is 10-4^ Tesla which also know as 1 Gauss

Therefore 1 Tesla = 10^4 Gauss

Tesla

Am

N

C m s

N

qv

F

B =^ B^ = = =

FB =^ qv x B

19

Magnetic Force on a Moving Charge

• Three points are arranged in a uniform magnetic field. The B field points into the screen
• A positively charged particle is located at point A and is stationary. The direction of the magnetic force on the particle is

no magnetic force is experienced

20

Magnetic Force on a Moving Charge

A positive charge moves from point A towards B: Left

A positive charge moves from point B towards C: Up

A positive charge moves from point A towards C:

What is the direction of the magnetic force

on the particle in each case?

Up and left