1

N S

Magnetic field lines ALWAYS run north to south.

The number of magnetic field lines denotes the strength of the magnetic field (B). The more lines the stronger the magnet

B = number of field lines or flux per square metre in units called Tesla (T) after Nikoli Tesla

2

geographic north

geographic north

magnetic north

s

N

3

1820Professor Oersted was demonstrating an experiment for students when he accidentally discovered that a compass needle moved when it was close to a wire connected to a battery.

Professor Hans Christian Oersted

4

B

I

Oersted noticed that if the current was running in one direction the red part of the needle pointed toward the wire.

5

I

B

However if the current flowed in the opposite direction then the red part of the needle pointed way from the wire

6

RIGHT HAND GRIP RULE:

I

From this observation a rule was developed

Thumb = direction of current.

Fingers = direction of mag field lines

7

current outwards current inwards

-

Current is coming out of the page

Current going into the page

Use thumb and fingers of right hand grip rule to work out direction of field.

8

length l

I

THE SOLENOID

The size of the magnetic field can be increased by the following ways:

1. Increase the number of turns.

2. Place an iron core in the centre.

3. Increase the current.

The field lines of each wire interact to increase the overall strength of the field. Again use thumb and fingers of the right hand grip rule to calculate which way the filed runs. NORTH always has field lines coming out of the solenoid.

9

N S

Interaction between a magnet and the field due to a current

N S

10

N S

F

The interaction of 2 fields. One form the magnet and one from the current carrying wire interact. Where they are going in the same direction they add together in strength. Where they go in opposite directions they cancel out. A force is then applied in the direction of the greatest force. In this case upwards.

11

FLEMING’S LEFT HAND MOTOR RULE

FOREFINGER = FIELD

THUMB = MOTION

SECOND FINGER = CURRENT

To determine the direction of force the following rule needs to apply:

12

FLEMING’S LEFT HAND MOTOR RULE

FOREFINGER = FIELD

THUMB = MOTION

SECOND FINGER = CURRENT

13

N S

I

B

F

F = B I L

Where L = length of conductor in the field

14

S N

HAIR SPRINGS CONTROL MOVEMENT AND ALLOW CURRENT TO ENTER AND LEAVE THE COIL

RADIAL SOFT IRON POLE PIECES

COIL

PERMANENT MAGNET

MOVING COIL

METER

SOFT IRON CYLINDER

This is seen in any analogue meter:GalvanometerAmmeterVoltmeter

15

S N+

+

This shows the rotation of the coil and hence the needle as the current flows through the wires of the solenoid and produces a magnetic field which interacts with the static magnet. The more current the more rotation as the greater the magnetic field interaction.

16

N

A

B

D

C

I

S

brushes

EVERY TIME THE COIL PASSES THROUGH THE VERTICAL POSITION, THE COMMUTATOR REVERSES THE CURENT IN THE COIL, IN ORDER TO

SUSTAIN THE ROTATION

DC MOTORC

A

B

SN

D

I

Split ring commutator

17

LOUDSPEAKER

N

S

N

Solenoid on paper cone is forced to vibrate - left hand rule

magnet

S

N

N N

N

magnet

AC signalcoil

18

19

+ I

FF THE FORCE IS ALWAYS

PERPENDICULAR TO THE MOTION

THE PARTICLE MUST MOVE

ALONG AN ARC OF A CIRCLE

+

Charged particle moving through a magnetic field

20

x x x x x x x x x x x xx x x x x x x x x x x xx x x x x x x x x x x x

x x x x x x x x x x x x x x x x x x x x x x x x

B inwards

+

If the magnetic field is of a large enough area then the effect on the charged particle would be to produce a circular motion. If not then the particles trajectory will be diverted. What determines how much is how long it remains in the magnetic field area.

21

Do Exercises fromDo Exercises from

Pages: 177 - 190Pages: 177 - 190

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