Magnetism And Electromagnetism Flashcards
How do you plot magnetic field lines
[bar magnet )
Place the compass at one of the poles
Make a mark where the arrow points north
Continue this process tracing the line until it brings you back to the magnet .
What is an induced magnet
Magnetic materials that turn into magnets when they are placed into a magnetic field .
The forces between permanent and induced magnets are always attractive
How do you increase the strength of a magnetic field of a wire
Wrap the wire into a coil called a solenoid
It causes the magnetic lines to line up resulting in lots of field lines pointing in the same direction that are very close together .
To increase the strength even more place an iron block in the centre of the solenoid , this block becomes an induced magnet itself
Uses of electromagnets
Can be used in scrap yards to pick up individual items . They allow you to switch them off and on .
Also electromagnets can be used in circuits . When the switch in the circuit is closed it turns on an electromagnet which attracts an iron contact on a rocker . The rocker pivots and closes the contacts turning on a motor
What is needed to find the force that is applied to an object which is in an electromagnetic field
Magnetic flux density
Force = magnetic flux density x current x length of the wire
How does a DC motor function
A DC current flows through coil
Current induces a magnetic field around coil
Magnetic field of the coil interacts with the magnetic field of the permanent magnet
This induces a force that rotates the coil
How does an AC motor function
A handle is turned
The coil rotates in the magnetic field
The magnetic field induces a current through the coil
The output is AC because the coil rotates
How do loudspeakers work ?
An alternating current is sent through a coil of wire attached to the baseif a paper cone.
The coil surrounds one pole of a permanent magnet and is surrounded by the other pole . The current causes a force on the coil making it move .
When the current reverses the force acts on the opposite direction which causes the cone to also move in the opposite direction.
Variations in the current cause the cone to vibrate which makes the air around the cone vibrate .
What do alternators do
Generators rotate a coil in a magnetic field
As the coil spins , a current is induced in the coil. This current changes direction every half turn.
Instead of a split ring commutator AC generators have slip rings and brushes so the contacts don’t swap every half turn meaning they produce an alternating P.D
How do microphones work?
Sound waves hit a flexible diaphragm that is attached to a coil of wire wrapped around a magnet.
This cause the coil to move in the magnetic field generating a current .
The movement of the coil depends on the property of the sound wave
How does a transformer work
A transformer is a soft iron core joining 2 coils of wire . (The primary and the secondary)
When an alternating pd is applied across the primary coil the core magnetises and demagnetises . If the secondary coil is part of a complete circuit then a current is induced
Difference between step down and step up transformers
Step up have more turns on the secondary coil than the primary coil whereas step down transformers have more coils on the primary coil rather than the secondary coik.
The force between the north poles of 2 bar magnets is …
Repulsive
To experience the full force . What angle does the wire have to be tp the magnetic field .
90 degrees
If the wire runs parallel to the magnetic field lines then no force will act upon it
Equation to find the force in a magnet
Magnetic flux x length x current
