# ELECTROMAGNETISMx - MVPYX

Urban and Civil

Nov 16, 2013 (4 years and 6 months ago)

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ELECTROMAGNETISM

NCEA 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

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

1820

Professor
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

-

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

This led to the production of
the
SOLENOID

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

1.
More current

2.
More wires

3.
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

Hang on unbalanced
force must produce
motion. Newton told
us this!

F=ma

Can we get things to
move by using two
magnetic fields?

12

N

S

Interaction between a magnet and the field due to a current

N

S

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 direction

Fingers =

field direction

Right Hand Slap Rule

D.C. MOTOR

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

C

A

B

S

N

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)

㴠慮杬攠浡摥d扥瑷敥渠瑷漠

䘠㴠=

䘠㴠=ax

8
Rutter

How to build a simple motor

FORCE ON A CHARGE

We have two types of charges:

1.
Positive and

2.
negative

Thus we use different slap rules for each
when a charge enters a magnetic field

1.
Positive = RH slap rule

2.
Negative = LH slap rule

Note:

If the field is large enough the
charge will travel in a complete
circle.

F= mv
2
/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)

I

F

F

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
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