Transcript Do now! - MrSimonPorter

ALL magnets have two poles
NORTH seeking pole
SOUTH seeking pole
Breaking a magnet produces two
magnets!
N
N
S
N
S
S
N
S
Opposite poles attract and like poles repel
Magnetic materials
Iron (steel), Cobalt
and Nickel
Magnetic induction
When a magnetic material is close to a
magnet, it becomes a magnet itself
magnet
S
N
S
We say it has induced magnetism
N
Soft Magnetism
Pure iron is a soft magnetic material
before
S
N
S
after
N
S
NN
Not a
magnet
Iron nail
It is easy to magnetise but loses its
magnetism easily
Hard Magnetism
Steel is a hard magnetic material
before
S
after
N
S
S
N
NN
S
N
It’s a
magnet!
Steel paper
clip
It is harder to magnetise, but keeps its
magnetism (it is used to make magnets!)
Magnetic fields
Magnets (and electric currents) produce
magnetic fields around them.
In the magnetic field, another magnet or
magnetic material will experience a magnetic
force.
Magnetic field lines
We can represent the magnetic field
around a magnet using field lines.
Magnetic field lines
The arrows show the direction a compass
needle would point at that point in the field.
The closer the field lines
are, the stronger the
magnetic force felt
Mr Porter will
put these
slides on a
loop and you
are going to
mind-map
them
(Colours,
drawings,
single words)
Mind-map
Plotting magnetic fields
1. Two magnets, opposite poles facing each
other 6 cm apart
2. Two magnets, like poles facing each other
6 cm apart
3. Two magnets along side each other, 5 cm
apart, opposite poles opposite each other.
Field around a bar magnet
Two bar magnets
Strong uniform field
Two bar magnets
no field!
Earth’s Magnetic Field
Remember the
North of a compass
needle points to the
geographic north
pole (i.e. the
geographic North
pole is a magnetic
south pole!)
Moving charges (currents)
Moving charges (electric currents) also
produce a magnetic field
Conventional current –
electrons flow in the
opposite direction
http://www.sciencebuddies.org
Magnetic field around a straight wire
You can use the right
hand screw rule to
decide which direction
the field goes
Stronger field closer to
wire
To make an electromagnet
stronger
The 3 C’s
• More Coils
• More Current
• Use an iron Core
Field around a coil
A side view
Field around a solenoid (coil)
The Motor Effect
When a current is placed in a magnetic
field it will experience a force (provided the
current is not parallel to the field). This is
called the motor effect.
Can you
copy this
sentence
into your
books
please.
The Motor Effect
Can you copy
this please?
WITH
DIAGRAM!
The direction of the force on a current in a
magnetic field is given by Flemming’s left
hand rule.
Thumb = Motion
First finger = Field direction
(N to S)
Centre finger = Conventional Current
(+ to -)
Sample question
In this example, which way will the wire be
pushed? (red is north on the magnets)
Sample question
In this example, which way will the wire be
pushed? (red is north on the magnets)
Current
Field
Building a loudspeaker
These also use the motor effect
Coil in here
magnet
D.C.Motor
Commutator ensures that every
half rotation the current
direction reverses in the coil
brushes
D.C.Motor
Electromagnetic induction
If a magnet is
moved inside a
coil an electric
current is
induced
(produced)
Electromagnetic induction
A electric current is induced because the
magnetic field around the coil is changing.
Generator/dynamo
A generator
works in this way
by rotating a coil
in a magnetic
field (or rotating a
magnet in a coil)
Motor = generator
If electric energy enters a motor it is
changed into kinetic energy, but if kinetic
energy is inputted (the motor is turned)
electric energy is produced!
Transformers
A transformer consists of 3 main parts
Transformers
A changing current in the primary coil
produces a changing magnetic field in the
core. This changing magnetic field induces
a changing current in the secondary coil.
Changing voltages
The ratio of the number of turns on the
coils equals the ratio of the voltages
# of turns = Np
# of turns = Ns
Vp/Vs = Np/Ns
Changing voltages
A transformer where the voltage rises is
called a step-up transformer
Changing voltages
A transformer where the voltage falls is
called a step-down transformer
No energy for nothing
Power in = power out
VpIp = VsIs
a.c. only
Because transformers rely on induction
(changing fields), they only work with
alternating current (a.c.)
Why do we need transformers?
Current flowing through wires causes them
to get hot and energy is lost.
Why do we need transformers?
Keeping the current
low means
electricity can be
transported long
distances without
losing too much
energy.
Why do we need transformers?
However, in order to send a large quantity
of energy, high voltages are needed.
Transmission of Electricity
Step-up
Step-down