ALL magnets have two poles NORTH seeking pole SOUTH seeking pole.
Transcript of ALL magnets have two poles NORTH seeking pole SOUTH seeking pole.
ALL magnets have two poles
NORTH seeking pole
SOUTH seeking pole
Breaking a magnet produces two magnets!
N S
N S
NN SS
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
We say it has induced magnetism
NS
NSmagnet
Soft Magnetism
Pure iron is a soft magnetic material
It is easy to magnetise but loses its magnetism easily
NS
before after
Iron nail
SN
NS
Not a magnet
N
Hard Magnetism
Steel is a hard magnetic material
It is harder to magnetise, but keeps its magnetism (it is used to make magnets!)
NS
before after
Steel paper clip
NNS
It’s a magnet!
N
S
S N
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 closer the field lines are, the stronger the magnetic force felt
The arrows show the direction a compass needle would point at that point in the field.
Mind-mapMr Porter will put these slides on a loop and you are going to mind-map
them
(Colours, drawings,
single words)
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
N
S
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
http://www.sciencebuddies.org
Conventional current – electrons flow in the opposite direction
Magnetic field around a straight wire
Stronger field closer to wire
You can use the right hand screw rule to decide which direction the field goes
To make an electromagnet strongerThe 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
The direction of the force on a current in a magnetic field is given by Flemming’s left hand rule.
Centre finger = Conventional Current (+ to -)
First finger = Field direction (N to S)
Thumb = Motion
Can you copy this please?
WITH DIAGRAM!
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 coilbrushes
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
Vp/Vs = Np/Ns
# of turns = Ns# of turns = Np
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