UNIT 4 Electricity and magnetism Flashcards
magnetic forces are due to
due to interactions between magnetic fields
Two bar magnets can repel or attract
left-hand = find force
right hand = find current
exam tip
- adding arrows pointing away from the north pole and towards the south pole
- making sure the magnetic field lines are further apart as the distance from the magnet increases
the relative strength of a magnetic field is represented by
represented by the spacing of the magnetic field lines
Properties of magnets
types of magnets
- ends of a magnet are called poles
- have two poles: a north and a south
- Permanent magnets
Induced (temporary) magnets
Permanent Magnets
- made out of permanent magnetic materials, for example steel
- will produce its own magnetic field
It will not lose its magnetism
Temporary (Induced) Magnets
- When magnetic material is placed in a magnetic field, the material can TEMPORARILY be turned into a MAGNET: INDUCED magnetism
steel obj -> can be magnetised and will remain magnetic for a while
- other obj, such as electromagnets or transformers (;;soft iron) will be demagnetised as soon as the cause of the induced magnetism is removed
When magnetism is induced on a material:
- One end of the material will become a north pole, the other end south pole
- Magnetic materials will always be attracted to a permanent magnet; so end of the material closest to the magnet will have the OPPOSITE pole to magnetβs pole closest to the material
- When magnetic material removed from magnetic field it will lose most/all of its magnetism quickly
there are positive and negative charges
what repels what
FOR MAGNETIC materials (that arenβt magnets) - attracted to either pole, doesnβt matter if N or S
positive charges repel other positive charges,
negative charges repel other negative charges,
but positive charges attract negative charges
LIKE REPEL. OPP ATTRACT
define magnetic material
Only a magnet can ____ another magnet
[Non-magnetic materials do not experience a force when placed in a magnetic field]
- experience a force when placed in a magnetic field
- attracted to a magnet when unmagnetised
- can be magnetised to form a magnet
only magnet can repel
electrical charge, Q, measured in
charge is measured in
Coulomb (C)
charging of solids by friction involves
ONLY
only a transfer of negative charge (electrons)
electric field as β¦
- shown by electric field lines
TOWARDS NEG. CHARGES
a region in which an
electric charge experiences a force
- charged object creates an electric field around itself; experiences electrostatic force
- Fields lines always point AWAY from positive charges and towards negative charges
direction of an electric field at a
point is β¦
the direction of the force on a positive
charge at that point
[i.e. βField lines show the direction that a positive charge would experience if it was at that pointβ]
- in demonstrations it is always electrons (negative charges) which are free to move according to that force
- strength of an electric field depends on the distance from the object creating the field:
[!!] field is strongest close to the charged object - shown by the field lines being closer together
[!!!] the field becomes weaker further away from the charged object - shown by the field lines becoming further apart
Electric Field Patterns
- objs in electric field experience an electrostatic force [force=vector β΄ the direction of this force depends on whether the charges are the same or opposite]
The force is either ATTRACTIVE or REPULSIVE,
meaningβ¦
- if charges are same (both pos./both neg.), this force = repulsive & the second charged object will move away from the charge creating the field
- if the charges are the opposite (negative and positive), this force will be ATTRACTIVE and the second charged object will move TOWARD the CHARGE CREATING THE FIELD
size of the force depends on the strength of the field at that point
so FORCE BECOMES
- STRONGER as the distance between the two charged objects DECREASES
& weaker - as the distance between the two charged objects increases
[relatshp between strength of the force and the distance applies to both the force of attraction and force of repulsion, i.e.
two negative charges brought close together will have a stronger repulsive force than if they were far apart]
complete recap:
- field lines around a POINT CHARGE
- field lines - Between Two Oppositely Charged Parallel Conducting Plates [uniform electric field]
- field lines - Around a Charged Conducting Sphere [demoβed using a Van der Graaff Generator, using streamers, small pieces of paper, polystyrene beads, aluminium foil containers]
- field lines going towards neg. away pos., evenly spaced, with arrow showing direction (towards neg)
- field lines are:
Directed from the positive to the negative plate
Parallel
Straight lines
- field lines are:
- field lines are - symmetrical, as with a point charge
- bc the charges on the surface of the sphere are evenly distributed.
charges are the same, so they repel.
the surface is conducting, allowing them to move
so just arrows pointing away from pos. evenly spaced around circle
electric current is related to the
happens bcβ¦
flow
of charge
two oppositely charged conductors are connected together (by a length of wire), charge will flow between the two conductors
direct current (d.c.) - straight horizontal line
and alternating current (a.c.) - parabola up and down dipping below x-axis too
difference?
voltage y-axis, time x-axis
direct: charge flow in 1 direction while in alternating the direction of charge flow changes regularly
dc
ELECTRONS. NEG TO POS
[state in circuit whether current/electron flow is clockwise or anticlockwise]
produced when using dry cells and batteries (and sometimes generators, although these are usually ac)
The electrons flow in one direction only, from the negative terminal to the positive terminal
ac
typically comes from mains electricity and generators
- needed for use in transformers in the National Grid
The direction of electron flow changes direction regularly
A typical frequency for the reversal of ac current in mains electricity is 50 Hz
Define electric current
I = Q/t
Q = It
the [amount of] charge passing a
point [in a circuit] per unit time [per second]
charge/second CHARGE PER SECOND
current eqn
I = Q/t
Current (A) = charge (C)/time (s)
current = power [of appliance] / [mains] voltage