Electricity and Magnetism Details Flashcards
Types of charge
-Positive(+) charge
-Negative(-) charge
Like charges…
repel
Unlike charges…
attract
The closer the charges…
the greater the force between them
Examples of good conductors
-Metals
-Carbon
Examples of poor conductors
-(Pure) water
-Earth
Examples of semi-conductors
-Silicon
-Germanium
Examples of insulators
-Plastics
-Glass
-Rubber
-Dry air
Relationship between charged and uncharged objects
A charged object will attract any uncharged object close to it
SI unit of charge
coulomb(C)
(referring to charge)Field lines always point…
from positive to negative charge
Where the electric field is strongest
Where the charges are most concentrated and the field lines are closest together(usually the sharpest curve of the conductor)
Conventional current direction
from positive to negative terminal
Direction of flow of electrons
from negative to positive terminal
SI unit of current
ampere(A)
SI unit of p.d.(potential difference)
volt(V)
If the p.d. across a cell is 2V, then…
2 joules of potential energy are given for each coulomb of charge
If charge flows at 3 coulombs per second, the current is…
3 amperes
Factors affecting resistance
-Length(proportional to resistance)
-Cross-sectional area(inversely proportional to resistance)
-Material
In metal conductors, resistance(in relation to temperature)…
increases with temperature
In semiconductors, resistance(in relation to temperature)…
decreases with temperature
There is a heating effect whenever current flows in a resistance(T/F)
True
SI unit for resistance
ohm(Ω)
Ohm’s law
the current is proportional to the p.d.(voltage)
Features of lamps connected in series
-The lamps share the same voltage from the battery(so each glows dimly)
-If one lamp is removed, the other one goes out too as the circuit is broken
Features of lamps connected in parallel
-Each lamp gets the full voltage from the battery as both are directly connected to it(so both glow brightly)
-If one lamp is removed, the other still keeps working as it is still part of an unbroken circuit
Features of resistors(or other components) in series
-The current in each of the components is the same
-The total p.d.(voltage) across all the components is the sum of the p.d.s across each of them
Features of resistors(or other components) in parallel
-The p.d.(voltage) across each of the components is the same
-The total current in the main circuit is the sum of the currents in the branches
(In terms of watts)1 kilowatt(kW) is…
1000 watts(W)
(In terms of joules)1 kilowatt-hour(kWh) is…
3,600,000J
Potential hazards of electricity
-Damaged insulation
-Overheating cables
-Damp conditions
-Excess current(from overloading of plugs, extension leads, single and multiple sockets)
Function of a fuse
Overheats and melts if the current rises above a set amount, breaking the circuit in the process(and stops the current flowing)
Two poles of a magnet
-North pole
-South pole
Where the magnetic field is strongest
Where the fields are closest together
In a magnet, the field lines run from…
north pole to south pole
Ways to demagnetize a magnet
-Hammering the magnet
-Heating the magnet to a high temperature
-Pulling the magnet out of a solenoid through which an alternating current is passing
Features of a magnetic field created by a current
-The magnetic field lines are circles
-The field is strongest close to the wire
-Increasing the current increases the strength of the field
Right-hand grip rule(the one you use to find the field direction)
If you grip the wire with your right hand(so that your thumb points in the conventional current direction), your fingers will point in the same direction as the field lines
Features of a magnetic field created by a current-carrying coil
There are magnetic poles at the ends of the coil
Right-hand grip rule(the one used to find which way round the poles are in a current-carrying coil)
If you grip the coil with your right hand(so that your fingers point in the conventional current direction), your thumb will point towards the North pole of the coil
How to make a permanent magnet
-Place a bar of a hard magnetic material into a solenoid
-Pass current through the solenoid(the hard magnetic material becomes magnetized)
-Even when the current is switched off it will stay magnetized
Ways to increase the strength of the magnetic field of an electromagnet
-Increasing the current
-Increasing the number of turns of the coil
Difference between an electromagnet and an ordinary magnet
An electromagnet can be switched on and off while an ordinary magnet cannot
Type of magnetic materials that can be suitable as the core of an electromagnet
Soft magnetic materials
Why hard magnetic materials cannot be the core of an electromagnet
They will stay magnetized(so they can’t be switched on or off)
How to reverse the direction of the magnetic field in an electromagnet
Reverse the current
Difference between a fuse and a circuit breaker
Circuit breakers can be reset(turned on again) after it has tripped(turned off)
Ways to increase the force produced by a magnetic field
-Increasing the current
-Using a stronger magnet
-Increasing the length of the wire in the field
Finger representations for Fleming’s left-hand rule
Thumb - Force
First finger(from thumb) - Field(N pole to S pole)
Second finger(from thumb) - Current(conventional current direction)
Conditions for Fleming’s left-hand rule to apply
The current and field directions are at right angles
Function of the commutator
Changes the direction of the current in the coil when the coil is nearly vertical(and the forces cannot turn it much further), allowing the forces to change direction and keep the coil turning
Ways to increase the turning effect of the coil
-Increasing the current
-Using a stronger magnet
-Increasing the number of turns on the coil
Ways to increase the induced electromotive force(and the current)
-Moving the coil faster
-Using a stronger magnet
-Increasing the length of wire in the magnetic field(eg. increasing the number of turns in the coil)
Ways to reverse the induced electromotive force and the current
-Moving the wire in the opposite direction
-Turning the magnet round(so that the field direction is reversed)
-Pulling the magnet out of the coil
Lenz’s law
The induced current always flows in a direction such that it opposes the change which produced it
Finger representations for Fleming’s right-hand rule
Thumb - Motion
First finger(from thumb) - Field
Second finger(from thumb) - Current
Situations when Fleming’s left-hand rule applies
When a current causes a motion
Situations when Fleming’s right-hand rule applies
When a motion causes a current
Ways to increase the maximum electromotive force(and the current)
-Increasing the number of turns in the coil
-Increasing the area of the coil
-Using a stronger magnet
-Rotating the coil faster
Ways to increase the induced electromotive force at switch-on or switch-off un mutual induction
-The core of the electromagnet goes right through the second coil
-The number of turns on the second coil is increased
Features of step-up transformers
-Have more turns on their output coil than their input coil
-Their output voltage is more than their input voltage
Features of step-down transformers
-Have fewer turns on their output coil than their input coil
-Their output voltage is less than their input voltage
Benefits of using alternating current over direct current for the mains
-Can be stepped up or down using transformers
-Can be generated more efficiently than direct current on a large scale
Advantages of transmitting power at a high voltage over low voltage
-Less power is lost
-Less energy is wasted
-Thinner, lighter and cheaper cables can be used
Resistance and length are…
directly proportional
Resistance and cross-sectional area are…
inversely proportional
