P7 | magnetism & electromagnetism Flashcards
what are the two poles on a magnet?
north and south
where is the magnetic force felt the most strongest in a magnet?
at the ends - poles
what type of forces are magnetic forces?
non-contact forces
give 3 magnetic elements
iron, cobalt and nickel
what are some features of permanent magnets?
- produce their own magnetic field
- can’t be turned on or off
what are some features of induced magnets?
- doesn’t produce its own magnetic field
- becomes a magnet when placed in another magnet’s magnetic field
what can you use to plot magnetic field lines?
iron filings or a compass - must always put arrows to show direction of magnetic field
the current travels from the north pole to the south pole
what direction does the magnetic field go around a magnet?
from the north pole to the south pole
define flux density
the density of field lines which indicate the strength of the magnetic field
what happens when two magnets are brought close to each other?
they exert a force on each other
what type of force is exerted if two like poles of a magnet are brought near each other?
a repulsive, non-contact force
what type of force is exerted if two unlike poles of a magnet are brought near each other?
an attractive, non-contact force
what type of force does induced magnetism always cause?
a force of attraction
what is a magnetic field?
the region surrounding a magnet where another magnet or magnetic material experiences a non-contact force
what is the force like between a magnet and magnetic material?
it is always attractive
how does the strength of a magnetic field alter as you move further away from the magnet producing it?
the magnetic field strength decreases the further you move away from the magnet
what does a magnetic compass contain?
a small bar magnet that points in the direction of the earth’s magnetic field
what is produced when current flows through a conducting wire?
a magnetic field is produced around the wire
what determines the strength of the magnetic field around a current-carrying wire?
- the magnitude of the current flowing through the wire
- the distance from the wire
what is a solenoid?
a coil of wire which when current passes through creates a strong magnetic field
describe the magnetic field found inside a solenoid
strong and uniform
what is an electromagnet?
- a solenoid with an added iron core
- adding the iron core increases the strength of the magnetic field
what is the motor effect?
when a force is exerted between a magnetic field and a current-carrying conductor placed in that field
what rule is used to determine the force experienced due to the motor effect?
fleming’s left hand rule
what does the thumb represent in fleming’s left hand rule?
movement
what does the first finger represent in fleming’s left hand rule?
the direction of the magnetic field (points from N to S)
what does the second finger represent in fleming’s left hand rule?
the direction of the current flow in the conductor (points from + to -)
what factors affect the strength of the electromagnet?
- current in the coil - the greater the current, the greater the strength
- no of turns in the coil - the more turns, the greater the strength
what factors affect the size of the force on a current-carrying wire in a magnetic field?
- the magnitude of the current flowing through the conductor
- the strength of the magnetic field that the conductor is placed in
if the direction of current in a current-carrying wire placed in a uniform magnetic field is reversed, what happens to the force?
the direction of the force is reversed
if the strength of the current in a current-carrying wire placed in a uniform magnetic field is increased, what happens to the force?
the strength of the force is increased
what criteria must be met for the equation linking force, magnetic flux density, current and length to hold?
the conductor must be at right-angles to the magnetic field it is placed in
what is the unit used for magnetic flux density?
tesla, T
in a calculation question, what do you do when asked to work out the angle?
- measure angle with protractor
- after using the equation (F = BIL) add sin (angle measured by protractor) to find out the angle
what are three ways to increase the force in a motor effect?
- increase the current
- increase the magnetic field strength
- increase the length of the conductor
how do electric motors work?
- direct current is passed through wire
- each side of the coil experiences opposite forces
- coil rotates
what does a split ring commutator do?
changes the current direction every half turn - forced in the same direction every half turn as the sides swap
what does it mean when the field lines are closer together?
the field is stronger
what factors does the magnetic field strength depend on?
- size of the current
- distance from the wire
what effect does the twisting a wire into a solenoid have?
it increases the magnetic field strength around the wire
the magnetic field inside a solenoid is…
strong and uniform
how can you further increase the strength of the magnetic field in a solenoid?
by adding an iron core
how is a solenoid different from an electromagnet?
an electromagnet is a solenoid with an iron core - it is a magnet that can be switched on and off
what is the magnetic field like outside if a solenoid?
- just like a bar magnet - ends of a solenoid act like the north pole and south pole of a bar magnet
- you can work out which side is which using the right-hand screw rule
how can the north pole of the solenoid be found?
wrap hands around the solenoid with fingers pointing in the direction of the current - the thumb points in the direction of the north pole
what are factors impacting strength of magnetic field?
the current and distance from the wire
how can the motor effect be increased?
- increasing the current
- increasing the length of the conductor
- increasing the flux density
what will happen if the direction of the current or magnetic field is reversed?
the direction of the force will reverse
what is a motor?
a current carrying conductor which rotates in a magnetic field
why do motors rotate in magnetic fields?
the current going up one side of the coil is in the opposite direction to the current going down the other side of the coil
why will a wire move when you put it in a magnetic field?
- the wire has a current flowing through it, causing it to have a magnetic field
- the magnetic field around the wire interacts with the magnetic field around the permanent magnet
- because of the interaction of those two magnetic fields there is a force acting on that wire, causing it to move
explain how fleming’s left hand rule can be used to predict the direction in which the copper rod will move
- hold all the fingers at right angles to each other
- thumb - direction of the force
- first finger - field
- second finger - current
what changes to equipment would increase the force on the copper rod?
- decrease the resistance of the variable resistor
- use stronger magnets - a stronger force will be exerted
if it has a solenoid, why would a door unlock when the switch is closed?
- the solenoid is a coil of wire with current travelling through it
- this creates a magnetic field
- which attracts the bolt
give two ways the resultant force on the bolt could be increased
- adding more turns to the solenoid
- increasing the current
explain why the iron rods move apart when the switch is closed
- the rods become induced magnets
- they have the same polarity
what links the force, current and field in fleming’s left hand rule?
they are all directly proportional to each other
e.g if the force reverses, so will the current
if the magnetic field is reversed, what happens to the force?
it is reversed
if the current is increased, what happens to the force?
it increases
describe the motion of the coil until it comes to an end
- the coil will continue to rotate slowly
- opposing forces will eventually bring it to rest
what three features of an electromagnet control the strength of the magnetic field obtained?
- the number of turns in the coil
- the current flowing through the coil
- the material of the core
what can make copper rollers move faster?
- increasing the current
- increase the speed of the motor
closing the switch creates a force that acts on the wire XY
explain why a force acts on the wire XY when the switch is closed
- the current flows through it - this means there is a magnetic field produced
- this means the magnet and the wire will create a magnetic field
- this is the motor effect
what happens 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
- this is called the motor effect
if a battery is replaced with a low frequency alternating current (a.c.) power supply, what would the movement of the wire be like? why?
- the magnetic field of the wire will change, causing the wire to move back and forth
- the direction of the magnetic force acting on it changes continuously
what changes would increase the force acting on the wire?
- increasing the current
- using stronger magnets so a stronger force is exerted
what two changes would reverse the direction of the force acting on the wire?
- reversing the direction of the current
- reversing the magnetic field
why would an electromagnet would be used rather than a permanent magnet?
- an electromagnet can be switched on and off
- a permanent magnet can’t be switched on and off
what happens if you place a wire between two poles of magnets?
- the magnets each have their own magnetic field
- this will cause a reaction and a force being produced on the wire (pushing it out the wire)
what happens if the wire is not at 90°?
it won’t experience as much of a force
