P15 Electromagnetism Flashcards
What objects can be magnetised? (4)
Iron, steel, cobalt and nickel
What are permanent magnets usually made of and why?
Steel, because magnetised steel does not lose its magnetism easily
What is a magnetic field?
The region around a magnet in which a magnetic material experiences a force
What is induced magnetism?
When an unmagnetised magnetic material is placed in a magnetic field and becomes magnetised
What is induces a magnetic field around a wire?
Electric current passing along it
What does the corkscrew rule do?
Give the direction of the magnetic field for each direction of the current
What is a solenoid?
A long coil of insulated wire
How are the magnetic field lines inside a solenoid placed?
Parallel to the axis of the solenoid and all in the same direction
What does reversing the direction of current do to a single wire and solenoid?
Reverse the direction of the magnetic field
What does increasing the current do to a single wire and solenoid?
Increases the strength of the magnetic field
What is an electromagnet?
A solenoid in which an insulated wire is wrapped around an iron bar
How could you cause the iron bar in an electromagnet to lose its magnetism?
Switching the current through the solenoid off
Why are electromagnets useful?
They allow a magnetic field to be switched on and off by switching the current on and off
What devices are electromagnets used in? (4)
Scrapyard cranes, circuit breakers, electric bells and relays
How do electromagnets work in a scrapyard crane?
A current is switched on to attract the steel cars to the electromagnet in the crane. The current can then be switched off to release it
How do electromagnets work in a circuit breaker?
Too great a current makes the electromagnet open a switch, which cuts the current off
How do electromagnets work in an electric bell?
When the bell is pushed on, an electromagnet attracts an iron armature, which hits a bell. Because the iron armature is pulled away from the make-and-break switch, the electromagnet loses is magnetism and the armature springs back. This process repeats itself rapidly
How do electromagnets work in a relay?
The small current through the coil of an electromagnet is used to attract an iron armature, which switches on a much larger current in another circuit
What is the motor effect?
When a wire carrying an electric current inside a magnetic field experiences a force
During a motor effect, when is the force at its maximum?
When the wire is at an angle of 90 to the magnetic field
During a motor effect, when is the force at zero
When the wire is parallel to the magnetic field
What is magnetic flux density and what is its unit?
A measure of how strong a magnetic field is, Tesla
When would the direction of force on a wire be reversed?
If either the direction of the current of the magnetic field is reversed
When would the force on a wire be increased?
If either the size of the current or the strength of the magnetic field is increased
What makes the coil spin in an electric motor?
When a current passes through the coil a force acts on each side of the coil, in opposite directions due to the motor effect
What does a split-ring commutator do to that direction of the current around a coil?
It reverses the direction every half turn so that the coil is always pushed in the same direction
