ELECTROMAGNETISMNCEA LEVEL 2

PHYSICS

• Magnets and fields• Mag fields and current• Right hand grip rule• Solenoids• Motor Effect• DC Motor• Force on a charge• Induction

geographic north

geographic north

magnetic north

s

N

MAGNETS AND FIELDS

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 in units called Tesla (T) after Nikoli Tesla

N S

Professor Hans Christian

Oersted

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.

MAG FIELDS AND CURRENT

B

I

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

I

B

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

RIGHT HAND GRIP RULE:

I

From this observation a rule was developed

Thumb = direction of current.Fingers = direction of mag field lines

RH GRIP RULE

current outwards current inwards

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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.

Complete exercises

from Page 180 – 181

Rutter

3 things effect size of magnetic field

How do we increase the magnetic field generated by the

current?

SOLENOIDS

Answer: Use more wires

This led to the production of the SOLENOID

This combined the magnetic field produced in each wire together producing a larger

field

1. More current2. More wires3. Iron core inserted

into coil

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 field runs. NORTH always has field lines coming out of the solenoid.

length l

I

Do Exercise Page 182 Rutter

What happens if two magnetic fields

interact?

MOTOR EFFECT

Answer: Unbalanced forces are produced

Hang on unbalanced force must produce

motion. Newton told us this!F=ma

Can we get things to move by using two

magnetic fields?Answer: Of course we can

12

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Interaction between a magnet and the field due to a current

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N S

F

The interaction of 2 fields. One from 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.

How do I know which way the force is acting?

RIGHT HAND SLAP RULE

Palm = Force direction

Thumb = I directionFingers = field direction

Right Hand Slap Rule

Try the exercises on page 183 –

184Rutter

D.C. MOTOR

NA

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

C

A

B

SN

D

I

Split ring commutator

The DC Motor

How big is the force?

This can be calculated using the following equation:

F = BILsin

Where:F = size of the force (N)B = magnetic field strength (T)I = current (A) = angle made between two mag fields

Note: F = 0

F = max

Now try pages 185 – 8

Rutter

How to build a simple motor

FORCE ON A CHARGEWe have two types of charges:1. Positive and2. negative

Thus we use different slap rules for each when a charge enters a magnetic field1. Positive = RH slap rule2. Negative = LH slap rule

Note:If the field is large enough the charge will travel in a complete circle. F= mv2/r can be used

Two equations can be utilised to determine the force acting on each charge

F = Bvq

F = Bev

Where:B = mag field strength (T)v = velocity (ms-1)q = charge (C)e = charge of electron (C)

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FF

Charged particle moving through a magnetic field

THE FORCE IS ALWAYS PERPENDICULAR TO

THE MOTION

THE PARTICLE MUST MOVE

ALONG AN ARC OF A CIRCLE

Try Pages 188 – 190 Rutter

ELECTROMAGNETIC INDUCTION

If using electricity and magnetism can generate

movement can it work the other way?

ANS: Yes, we call it a generator and it works by electromagnetic induction

ANS: If we change the movement of charge to kinetic movement then the charges are still moving and so a magnetic field is still generated. This means two sets of fields will interact. This means a force will be produced which acts on the only thing that can move, the charge along the wire.

So what is different?

Demo of electromag induction

Hey, Mr Lees how much voltage is generated

when a magnet moves through a coil?

The variables that are involved are:

1. Size of the magnets magnetic field (B)

2. Speed of movement (v)3. Amount of wire (l)

This results in: V = Bvl Where:V = Voltage (V)B = magnetic field strength (T)l = length of wire (m)

The major use of this phenomenon is in hydro-electric dams in the production of electricity.

How a hydroelectric

dam uses induction to

make electricity?

BENMORE DAM NZ

Use the diagram opposite to help you describe how New Zealanders have embraced the Physics principles of “Electromagnetism” to generate electricity from water?

OPEN ENDED ESSAY QUESTION

Try Pages 190 – 193 Rutter

IDEAS THAT CAN BE USED FOR THIS QUESTION

The Solenoid

Induction

Water Height

Inside a generator