Magnetism and electromagnetism Flashcards
Where are the magnetic forces strongest for a magnet
the poles
A permanent magnet produces its own _______
magnetic field
What is an induced magnet?
a material that becomes a magnet when it’s placed in a magnetic field
it loses magnetism when removed from the magnetic field
Induced magnetism always causes a ________
force of attraction
Magnetic field definition
the region around a magnet where a force acts on another magnet or/and magnetic material
The force between a magnetic and a magnetic material is _________
always one of attraction
What does the force of the magnetic field depend on?
the distance of the field from the magnet
What is the direction of the magnetic field lines?
North to south pole
How do we know about the Earth magnetic field?
A magnetic compass contains a small bar magnet.
The compass needle points in the direction of theEarth’s magnetic field.
What happens when current flows through a conducting wire?
a magnetic field is produced around the write
How to increase the strength of a magnetic field?
- increase the amount of current through the wire
- the decrease the distance from the wire
- shape the wire into a solenoid
-add an iron core
Describe the magnetic field lines inside a solenoid?
strong and uniform
How can you experiment the effects of “the size of current” and the “distance” from a wire
by placing magnetic compass at different points along the wire and turning the power supply on and off
Why does a solenoid increase the magnetic field strength?
- it concentrates a longer piece of wire into a smaller area
- the lopped shape means that the magnetic field lines around the wire are all in the same direction
What is the motor effect?
When a conductor carrying a current is placed in a magnetic field, the magnet producing the field and the conductor exert a force on each other
What is the motor effect caused by?
the field created by the current interacting with the magnetic field
How can the force in the motor effect be increased?
increasing :
- the size of the current
- the length of conductor in the magnetic field
- the flux density (density of the field lines around a magnet)
What is the formula to find the force of something when you have flux density?
F= BIL
Force = magnetic flux density x current x length (of wire within the field)
What would happen to the direction of the force, if the direction of the current or magnetic field is reversed?
it will reverse
How can the direction of the force on the conductor be found?
with Flemings left-hand rule
What is an electric motor?
A coil of wire carrying a current in a magnetic field and rotates
Why do electric motors rotate?
because the current going up one side of the coil is in the opposite direction to the current coming back down the other side, so one side moves up and the other moves down
What do the carbon brushes in electric circuits do?
allow the electric current to pass onto the split-ring commutator
Why does a coil stop when it reaches 90 degrees (vertical position) if it didn’t have a split-ring commutator?
because the current would continue to flow through each side of the coil in the same direction (opposite directions), so the direction of the forces exerted on both sides wouldn’t change.
this means the tip od the coil will be pulled upwards and the bottom downwards so the coil will stop rotating
What is a split-ring commutator?
a device used to reverse the direction of flow of an electric current in motors and generators
How is electric current passed onto the split-ring commutator?
the spilt ring is connected to conducting brushes
How do coils rotate continuously?
each time the coil passes the vertical position, the direction in which current flows around the coil reverses
this reverses the direction of the force exerted on each side of the coil which in turn cause continued rotation in the clockwise rotation
How do you make the coil rotate faster?
How do you make the coil turn in the opposite direction?
increasing the current or the magnetic field
reversing the current or the magnetic field
What do headphones and loudspeakers use the motor effect for?
to convert variations in current in electrical circuits to the pressure variations in sound waves
it converts electrical energy into kinetic and sound energy
How do you find the direction of field lines?
right-hand grip
How do you find the north pole of a solenoid
right-hand grip:
- fingers represent the direction of current flow
- thumb will point to the north pole of the solenoid
How do Loudspeakers work?
1) when a current flows through the coil, both the coil and the speaker core experience a force due to the magnetic field. The speaker core moves
2) Because the current is alternating when the direction of the current through the coil reverses, so too does the direction of the force exerted on it and the speaker core
3) this causes the speaker core to vibrate back and forth, producing a sound wave
4) the air around the core also vibrates creating variations in pressure that cause a sound wave
How do scrapyards work?
Scrap vehicles are lifted in a scrap yard using powerful electromagnets attached to cranes
when the current is turned on, the electromagnet attracts magnetic material below it
when the current is turned off, there is no more magnetic force
How do circuit breakers work?
this is a switch in series with an electromagnet, held by a spring
when the current is too large, the switch is pulled open by the electromagnet and it stays open until it is reset manually
How do electric bells work?
1) When the switch is pushed, the electromagnet is magnetised
2) The electromagnet attracts the armature
3) The hammer strikes the gong and breaks the circuit
4) the armature springs back, completing the circuit again and remagnetising the electromagnet
5) the cycle repeats for as long the button remains pushed
How do relays work?
Relays are used to switch an electrical machine on or off
1) a small current through the coil of the electromagnet magnetises the iron core
2) this pulls the armature onto the electromagnet, whose other end pushes the contacts together, completing the circuit.
3) When the current is switched off, the contacts open again, switching the circuit off.
