Electricity and Magnetism

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  • Created by: rachmhw
  • Created on: 12-04-19 10:30
what is magnetism?
the non-contact force that affects magnets and magnetic materials
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what is a magnet?
an object that has a magnetic field of its own
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what is a magnetic field?
the region of space around a magnet where it can have an effect
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what is a magnetic material?
a material that is affected by the magnetic field of nearby magnets
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where is the magnetic field of a bar magnet the strongest?
at the ends of the magnet
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what are the ends of the magnet called?
poles (north and south)
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how do you draw the magnetic field?
with magnetic field lines (from north to south)
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how does iron act as a magnetic material?
it is a soft magnetic material - it can be made into a magnet but it does not stay magnetised
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how does steel act as a magnetic material?
it can be magnetised but it does keep its magnetism - a permanent magnet
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when is magnetism induced in a soft magnetic material?
when it is brought near a magnet (magnetic material is temporarily magnetised by the magnetic field of a magnet)
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how can magnetic materials be magnetised?
stroking them in one direction with a magnet OR placing them inside a coil that has a direct current in it OR hammering the magnet when it is in a magnetic field
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how can magnetic be demagnetised?
heating the magnet OR placing the magnet inside a coil that has an alternating current in it OR hammering the magnet when it is not in a magnetic field
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what causes magnetic forces?
the interaction between magnetic fields
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how can you identify the pattern of magnetic field lines in an experiment?
by sprinkling ion filings near a magnet, use a plotting compass (mark position/direction of needle in many places = field lines and direction)
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what happens when a north pole is next to a north pole (or south - south)?
they repel
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what happens is a north pole is brought next to a south pole?
they attract
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how are electromagnets created?
made by wrappubg a wire carrying an electrical current around a soft iron core = a magnetic field is created around the coil
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what happens because a soft iron core is used for an electromagnet?
the magnetism is not permanent
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where can electromagnets be used due to their not permanent magnetism?
in scareyards for moving old cars around, useful in an electrical swicth called a relay
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what kind of magnetic field can a single, straight wire carrying an electric current cause?
it produces a magnetic field around it - a circular pattern around the wire
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what shows where field lines are strongest?
when they are close together (nearer the wire)
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what can change the strength of the field in circuit?
the current (increase current = increased strength of field)
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what would a diagram with field lines going to the left show about where the current is directed?
it is directed towards you out of the page (vise versa with field lines to the right)
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what are electric effects caused by?
by forces between the positively charged nucleus and negatively charged electrons
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how do like charges react?
they repel each other
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how do opposite charges react?
they attract each other
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what are electrical conductors?
materials that allow electric charges to move through them easily
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what materials are usually good conductors?
metals
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what are electrical insulators?
materials that do not allow electric charges to move through them
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what materials are usually good insulators?
wood and plastic
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what unit is charge measured in?
coulombs, C
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what is the structure of a metal like?
a lattice of positive ions with a sea of electrons - forces between positive nucleus and negative electrons hold the metal together
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how is an electrostatic charge created?
by adding or removing electrons
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how can you create an electromotive charge?
by rubbing a plastic rod with a cloth
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how can electrically charged objects be detected?
by their ability to attract small pieces of paper or by deflecting a stream of water running from a tap
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what is an electric field?
the region around a charge where it can exert a force
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how are electric fields described?
by using field lines
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what is the direction of an electric field at a point due to? what do field lines show?
the direction of the force on a positive charge at that point
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what are the field lines like around a positive point of charge?
more away from it (outwards)
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what are the field lines like around a negative point of charge?
more towards it (inwards)
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what are the field lines like between two parallel plates?
move perpendicular to plates from positive to negative
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examples of an object charged by induction
a balloon being rubbed on clothing and then sticking to the wall
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why does a ball stick to a wall due to induction?
if the balloon has a negative charge it will repel electrons from the surface of the ceiling if it is placed close to it = surface of ceiling becomes slightly positive (induced charge)
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what is an electric charge?
moving charge from one place to another
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what is an electric circuit?
the pathway of moving charge
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what carry the charge around a circuit?
electrons
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what is used to measure the electric current?
an ammeter, measured in amps, A
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what are the two types of ammeters? How do they work?
digital - gives a display directly as a number, analogue - a needle moves across a scale
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what causes the current in a metal?
it is due to the flow of electrons
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what is the equation for electric charge?
I = Q / t (I - current, Q - charge, t - time taken)
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what is the conventional current of a circuit?
current flows from positive terminal to negative terminal
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what is the direction of flow of electrons in a circuit?
flows from the negative terminal to the positive terminal
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when is energy transferred?
when charge flows around an electric circuit
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what does the electromotive force (e.m.f.) measure?
how much energy is transferred for each unit of charge that flows
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what does an e.m.f. or p.d. of 1 volt indicate?
that 1 joule of energy is transferred for each coulomb of charge that flows ( 1 V = 1 J/C)
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what does e.m.f. describe?
the energy transferred into electrical energy by the supply
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what does p.d. describe?
the energy transferred from electrical energy to other forms (e.g. heat , light) across a component
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what is e.m.f. and p.d. measured in?
volts with a voltmeter
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how do you calculate energy transferred in a circuit?
E = I x V x t (E - energy transferred in J, I - current A, V - p.d. in V, t - time taken s)
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how is power calculated in a circuit?
P= I x V
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what unit is used for power?
Watts, W
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what is resisitance?
how difficult it is for an electric current to flow through a component
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how do you calculate resistance in a circuit?
R = V / I
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how do you measure resistance in?
ohms, Ω
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what happens to resistance if the current is decreased?
the resistance increases
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describe an experiment used to determine resistance
measure the current and potential difference for several settings of the variable resistor, plot a graph of p.d. against current = resistance is equal to the gradient of the graph
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how are current and potential difference linked in a component?
in the current-voltage characteristic for the component
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what is the graph like for the current-voltage characteristics of a resistor?
the current is proportional to the p.d. and the resistance stays constant
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what is the graph like for the current-voltage characteristics of a filament lamp?
the graph curves, resistance increases at higher currents. because the temp rise = more difficult for electrons to pass through (the lattice of ions vibrate more vigorously)
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what physical dimension affects the resistance of a wire?
resistance increases as length increases, resistance increases as diameter decreases
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what are ohmic resistors?
resistors that follow Ohm's law that current is proportional to p.d.
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how is resistance linked to length?
they are proportional
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how is resistance linked to cross-sectional area?
they are inversely proportional
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where do electrical circuits transfer energy from and to?
it comes from the battery or power source to the circuit components then into the surroundings
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Common electric circuit symbols
-
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what is a series circuit like?
all the components are connected one after another
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what is a parallel circuit like?
the components are connected side by side, in parallel lines
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what is the brightness of lamps in a series circuit?
both lamps are the same brightness, they are both dim
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what is the brightness of lamps in a parallel circuit?
both lamps are the same brightness, they are both bright
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what is the current like in a series circuit?
current is the same at all points
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what is the current like in a parallel circuit?
the current from the source > the current in each branch AND the current from the source is the sum of the current in each branch
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advantages/disadvantages of lighting a lamp in a series circuit
lamps cannot be switched off independently, one lamp 'blows' = circuits is incomplete so both go out BUT the battery will last longer
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advantages/disadvantages of lighting a lamp in a parallel circuit
lamps can be switched off separately, one lamp 'blow' = other still works BUT battery will drain more quickly
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how do you find the combined resistance of a series circuit?
combined resistance = sum of resistance of each component
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how do you find the combined resistance of a parallel circuit?
combined resistance is less than the resistance of any component on its own
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in a series circuit, how do you find the e.m.f.? Why does this work?
by adding up the p.d. around the circuit. due to conservation of energy
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how do you find the total e.m.f of separate batteries in a series circuit?
add the e.m.f. of each battery
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how do you calculate the combined resistance of resistors in parallel?
1 / R = 1 / R1 + 1 / R2
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how do you calculate the combined resistance of just 2 resistors?
(product of the resistance) / (sum of the resistances)
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what does a potentiometer do?
a variable resistor that can be used to split a voltage into 2 parts, making a potential divider
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what does a thermistor do?
a component where the resistance decreases as its temperature increases
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what does a light-dependent resistor (LDR) do?
a component where the resistance decreases as the light intensity increases
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what does a relay do?
a switch where a coil is magnetism by an electric current, attracting the switch contacts together
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what does a diode do?
allows electrons to flow in one direction only. this is used in the rectifier
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how is a diode used in a rectifier?
it is used to convert alternating current (a.c.) to direct current (d.c.)
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what does the diode do in a rectifier?
it doesn't allow the electrons to pass in the 'reverse' direction, only half the cycle passes through. this is called half-wave rectification
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what 2 ways can electrical signals be sent?
using analogue - voltages have a value (continuous variation) OR digital coding - simply high/1 or low/0 states
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what are logic gates?
simple circuits that operate with digital signals and are designed to provide an output signal that depends on the combination of the input signals
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how is the operation of a logic gate described?
using truth tables
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Symbols of logic gates
-
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what are typical hazards of electricity?
damaged insulation cables - expose wires = electric shock OR overheating cables - wrongly rated cable/ coiled up cable = fire hazard OR damp conditions - water conducts = risk of connection increased
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what features can be added to reduce the risk of hazards?
fuses, circuit breakers, earth wires
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what does a fuse do?
protects a circuit from the effects of the current being too high
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what does a fuse do if the current is too high?
first thing to overheat and melt = circuit is broken and current switched off
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what rates should a fuse have?
a rate that is the lowest value about the required current e.g. a 3.5A appiliance would have a 5A fuse
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what do circuit breakers do?
same as fuse, but they trip a switch to stop the circuit working
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what are advantages of circuit breakers?
act more quickly AND they can be reset once the fault has been corrected (a fuse must be replaced)
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what do earth wires do?
protect the user from electric shock. only required when the appliance has a metal case (can conduct to user)
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what does the earth wire do if a fault occurs and the metal case becomes 'live'?
earth wire provides a low-resistance path to the earth = large current flows along that path = causes fuse to overheat and melt = switching off appliance so safe to touch
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what happens when there is motion between a conductor and a magnetic field?
an e.m.f. is induced across the ends of the conductor
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how can you increased the magnitude of the e.m.f. induced?
increase strength of magnetic field OR increase speed of motion OR wrap the conductor into a coil shape
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what does Flemming's right-hand rule say?
fingers at 900 degrees, thumb = direction of motion, first finger = magnetic field (N to S), second finger = direction of e.m.f.(positive to negative)
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what kind of current does a battery produce?
direct current (d.c.) - electrons move in a steady stream from negative to positive terminal
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what is an alternating current?
the electrons move backwards and forwards (alternate direction)
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why is alternating current used in mains electricity?
because there is less energy wastage when transferring over long distances
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describe how a simple a.c. generator works
when coil spins = wire cuts through magnetic field = e.m.f induced. slip rings are contacts at end of coil that rest against brushes that connect to an external circuit
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what is a voltage output- time graph for a simple a.c. generator like?
like a wave. peaks = when coil is horizontal between magnetic pole - coil is cutting through field lines the fastest. induced voltage is 0 when coil is vertical
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what are transformers?
electrical devices that are designed to increase or decrease voltages
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what do transformers consist of?
2 coils of insulated wire wrapped around a soft iron core
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describe the principle of the transformer? (how does it work)?
alternating voltage is applied to the primary coil = creates a changing magnetic field around it. this field cuts through the secondary coil = inducing a voltage. Soft iron core magnetises to increase the strength of the field
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what does a step-up transformer look it?
secondary coil is bigger than primary
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what does a step-down transformer look like?
primary coil is bigger than secondary
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what is the relationship between the turns and voltage?
primary coil voltage / secondary coil voltage = number of primary turns / number of secondary turns
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where are transformers used?
in high-voltage electricity transmissions
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what is an advantage of high-voltage transmissions?
it wastes less energy
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what equation can be used for 100% efficiency in a transformer?
primary coil voltage x primary coil current = secondary coil voltage x secondary coil current
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what does the equation for 100% efficiency in transformers mean?
sending electricity at high voltages means sending it at low current. lower current means less resistance = less energy wasted as heat
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what happens when a wire carrying an electric current is placed in a magnetic field (at 90 degrees to it)?
wire experiences a force at right angles to both the field and the direction of the current
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how can the direction of the force be reversed on a current-carrying conductor?
change direction of current OR change direction of magnetic field
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when do you use Flemming's left-hand rule?
to describe the direction of the force caused during the motor effect
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when do you use Flemming's right-hand rule?
to describe the direction of the e.m.f caused during the electromagnetic induction
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what does Flemming's left-hand rule say?
fingers at 90 degrees, first finger = magnetic field (N to S), second finger = direction of current (pos to neg), thumb = direction of force (movement)
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what experiment can be used to show the force on beams of charged particles?
use a electron beam to produce a cathode, it is deflected by the magnetic field produced by the coils
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what does a d.c. motor do?
a current-carrying coil in a magnetic field experiences a turning effect causing the motor coil to spin
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how can you increase the turning effect in a d.c. motor?
by increasing number of turns on the wire, increase the current, increase the strength of the magnetic field
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why does the d.c. motor experience a turning effect?
because the 2 sides of the motor coil experience forces in opposite directions (due to current being opposite on either side), split-ring commutator allows coil to disconnect from power supply for each half turn
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what does having split-ring commutators do for a d.c. motor?
connecting leads aren't tied in a knot, force spins coil in the same direction
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how can you make the motor coil of a d.c. motor spin the other way?
reverse direction of the magnetic field OR reverse polarity of the power supply
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what is a solenoid?
a magnetic field produced around a coil
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Card 2

Front

what is a magnet?

Back

an object that has a magnetic field of its own

Card 3

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what is a magnetic field?

Back

Preview of the front of card 3

Card 4

Front

what is a magnetic material?

Back

Preview of the front of card 4

Card 5

Front

where is the magnetic field of a bar magnet the strongest?

Back

Preview of the front of card 5
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