Magnets And Electromagnets Flashcards
What is repulsion?
If the poles are the same they repel
What is attraction?
If the poles are opposite they attract
Is magnetism a contact force?
No
What is the force between a magnet and a magnetic material?
Attractive
What are the magnetic materials?
Iron, cobalt, nickel, steel
What is a permanent magnet?
Produces its own magnetic field which is always there
What is an induced magnet?
On object that becomes magnetic when it is placed within a magnetic field
What is the force between an induced magnet and a permanent magnet?
Always attractive
What happens if an induced magnet is removed from the magnetic field?
It will quickly lose most or all of its magnetism
What is a magnetic field?
The region around a magnet where another magnet or magnetic material will experience a force due to the magnet
Represented by magnetic field lines which show the direction of the force that would act on a North Pole at that point, pointing from north to south
Field strength is greatest at the poles
The closer together the field lines, the stronger the field
How do you plot a magnetic field?
Small compases placed around a bar magnet will point in the direction of the field lines
If not near a magnet, the compass will align with Earths magnetic field
As a compass points towards a South Pole, the magnetic pole near the Earths geographic North Pole is actually a south pole
What is electromagnetism?
If an electric current flows through a wire, it will produce a magnetic field around the wire
The field strength increases with greater current and closer to the wire
Reversing current reverses the direction of the field
The field around a wire takes the shape of concentric circles at right angles to the wire
Fingers curl in direction of magnetic field if thumb points in the direction of the current
What is a solenoid?
Cylindrical coil of wire
Bending a current carrying wire into a solenoid increases the strength of the magnetic field
The shape of the magnetic field is similar to that of a bar magnet
Inside the solenoid, the magnetic field is strong and uniform which means it ah the same strength and direction at all points
Electromagnets are often solenoids with an iron core
What are advantages of electromagnets?
Turned on and off
Strength can be increased or decreased by adjusting current
What is a scrap yard cane?
Heavy objects containing magnetic materials can be lifted using an electromagnet
How does an electric bell operate?
Switch is pressed to turn electromagnetic on
The iron armature is attracted towards the electromagnet, making the hammer strike the gong
The circuit is broken so the the electromagnet stops working and the armature springs back
Circuit is complete again and the cycle starts again when the switch is pressed
What is a circuit breaker?
The circuit is held closed by a spring, but if the current becomes too large the electromagnet becomes strong and pulls the switch into an open position to turn the current off
What is the motor effect?
When a current carrying wire is placed in a magnetic field, it experiences a force
The force is due to the interaction between the field created by the current in the wire and the magnetic field in which the wire is placed
The magnet producing the field will experience an equal sized force in the opposite direction
The direction of the force is reversed if current or magnetic field are reversed
What is Flemings left hand rule?
Thumb is motion
First finger is field
Second finger is current
What is magnetic flux density?
A measure of the strength of a magnetic field
F = BIl
B is magnetic flux density in Tesla
What is an electric motor?
A current carrying coil of wire in a magnetic field tends to rotate
Each side of the coil experiences a force due to being a current carrying conductor in a magnetic field
The forces on each side of the coil are in opposite directions
What does the split ring commutator do?
Keeps the motor spinning in the same direction by swapping contacts with the power supply every half turn so current always flows in the same direction relative to the magnetic field.
Faster by increasing current or magnetic field strength
Reverse current or magnetic field
How do loudspeakers work?
Motor effect converts changes of currents in a coil of wire to changes of pressure in sound waves
A coil of wire is placed inside a permanent magnet and is attached to a diaphragm
When a current flows through the coil it experiences a force due to the motor effect
This causes the diaphragm to move
When the current changes direction, the force on the coils also changes direction, causing the diaphragm to move in the opposite direction
Variations make the coil and diaphragm vibrate
These vibrations create variations of pressure in air which form a sound wave
The frequency of sound wave is the same as the frequency of the ac supplied
Lenz’s Law
The induced potential difference always opposes the change that produces it. The coil will apply a force to oppose the magnet being pulled away from the coil. Therefore, the end of the coil closest to the magnet will become a south pole. This means it will attract the north pole of the magnet.
