4. Electricty (16-21) Flashcards
Describe the forces between magnetic poles and between magnets and magnetic materials
magnetic forces are due to interactions between magnetic fields.
magnets have two poles (ends)- north and south.
like poles repel and unlike poles attract (force).
magnets experience attraction and repulsion.
Describe induced magnetism
when a magnetic material is only magnetised when placed in a magnetic field (brought close to the pole of a permanent magnet).
A permanent magnet can attract or repel another permanent magnet. it can also attract other unmagnetised magnetic materials. ex- a bar magnet can attract steel pins, fridge magnet, paper clips. these are made of magnetic materials but are not magnetised all the time.
when the permanent magnet is removed the pin will return to its unmagnetised state or it may retain a small amount of magnetism.
properties of temporary magnets and the properties of permanent magnets
- temporary magnets (made of soft iron)- easy to magnetise and demagnetise- readily loses its magnetism.
cores for electromagnets, transformer, radio aerials
- permanent magnets (made of steel)- difficult to magnetise and demagnetise- retains magnetism well.
permanent magnets, compass needle, loudspeaker magnets
difference between magnetic and non-magnetic materials
MM- experience a force when placed in a magnetic field.
are attracted to a magnet and can be magnetised
NM- no experience of force when placed in a magnetic field.
not attracted by a magnet and cannot be magnetised
Describe a magnetic field and state what determines drawing magnetic field lines
- a region in which a magnetic pole experiences a force.
- the direction of a magnetic field at a point is “the direction of the force on the N pole” of a magnet at that point- N to S with arrows.
- the relative strength of a magnetic field is represented by the spacing of the magnetic field lines-
MF stronger at poles- MFL closest together.
become weaker as distance from magnet increases as lines are getting further apart.
equal spacing bw MFL- MF has same strength
What happens when two magnets are placed near each other
when two bar magnets are placed close together, their magnetic fields interact & produce a new line of magnetic lines of force. The pattern indicates whether the magnets will attract or repel.
Describe the plotting of magnetic field lines with a compass or iron filings and the use of a compass to determine the direction of the magnetic field
Iron filings- place a piece of paper under a magnet. sprinkle iron filings on top of paper & tap it for it settle. they will line up in field.
Plotting compass- Use a permanent magnet kept on paper.
mark point of compass needle.
move compass to other side of dot.
align needle with dot & put a dot on other side of needle.
start again from another position next to magnet.
MF- north to south and arrows
Describe the uses of permanent magnets and electromagnets
Permanent magnet- compass, fridge magnet, school lab, toy
Electromagnet- electric doorbells, loudspeakers, electric motors, relays, transformers
Principles of charges
positive charges repel other positive charges, negative charges repel other negative charges, but positive charges attract negative charges.
charge is measured in coulombs
How is charging caused
charging of solids is caused by friction & it involves only a transfer of negative charge (electrons). Positive charge is not transferred as they are in bound to nucleus so can’t move freely.
if polythene rod becomes negatively charged then electrons are transferred from cloth to rod.
gain e- negative
lose e- positive
Describe an electric field
an electric field as a region in which an electric charge experiences a force.
the direction of an electric field at a point is the direction of the force on a positive charge at that point.
a charged object can affect both charged and uncharged objects, without touching them as it exerts a force on it.
it creates an electric field around itself. force of attraction and repulsion will become stronger as the distance between two charge object decreases & vice versa.
Describe simple electric field patterns
(a) around a point charge- arrow pointing outwards
(b) around a charged conducting sphere- arrows pointing outwards
(c) between two oppositely charged parallel conducting plates- +ve to -ve arrow parallel straight and curved at edges
why can charge MOVE through electrical conductors but not insulators
in insulators, e are tightly bound to their atoms and not easily removed. in conductors, e are free to move bw atoms.
insulators become charged as if on end becomes charged and the other one will be uncharged as e- can’t move.
conductors do not become charged as e- are free to move so they will flow through the object, through hand, through earth. it can only be charged if it is held by an insulating handle- the charge will spread evenly through conductor.
Describe an experiment to distinguish between electrical conductors and insulators
connect the cell and lamp in a simple circuit to make the lamp light.
make a gap in the circuit by removing a wire.
using the crocodile clips, attach a material into the gap in the circuit.
if the lamp lights, it is a conductor and if it does not, it is an insulator.
Define electric current
the charge passing a point per unit time/
the rate at which electric charge passes a point in a circuit
I = Q/t
Describe the use of ammeters (analogue and digital) with different ranges
analogue- has a needle, which moves across a scale. you have to make a judgement of the position of the needle against the scale. 0.1-1A & 1-5A
digital- gives a direct read-out in figures. there is no judgement involved in taking a reading. can measure small current
Describe electrical conduction and conventional electrons
conventional current is from positive to negative and that the flow of free electrons is from negative to positive
difference between direct current (d.c.) and alternating current (a.c.)
(d.c.)- electric current that flows in the same direction all the time. dry cells and batteries and sometimes generators
(a.c.)- electric current that periodically changes direction. mains electricity and generators, transformers
Define electromotive force (e.m.f.) & potential difference (p.d.)
the electrical work done by a source in moving a unit charge around a complete circuit. It is measured in volts (V).
E = W/Q
the work done by a unit charge passing through a component. p.d. between two points is measured in volts (V)
V = W/Q
Describe the use of voltmeters (analogue and digital) with different ranges
analogue- has a needle, which moves across a scale. you have to make a judgement of the position of the needle against the scale. range- 0.1-1V & 1-5V
digital- gives a direct read-out in figures. there is no judgement involved in taking a reading. range- can measure small p.d.
explain the current–voltage graphs for a resistor of constant resistance, a filament lamp and a diode
voltage on x-axis & current on y-axis
- ohmic resistor- straight line- directly proportional because resistor has constant resistance.
- filament lamp- at low voltage I-V directly proportional. at higher voltages, the graph starts to curve over because current increases slowly- if I inc, e- flow inc through wire. no. of collision bw e- and lattice inc. kinetic energy transferred to internal energy. more lattice vibration so more resistance.
- diode- acts as a switch & allows current to flow in one direction only.
arrow indicates direction of conventional current.
is a semiconductor- behaves like an insulator until enough p.d. given to make it behave as a conductor
State, qualitatively, the relationship of the resistance of a metallic wire to its length and to its cross-sectional area
(a) resistance is directly proportional to length
(b) resistance is inversely proportional to cross-sectional area
How is energy transferred from electric circuits
electric circuits transfer energy from a source of electrical energy, such as an electrical cell or mains supply, to the circuit components and then into the surroundings
Equation for electrical power and electrical energy
P = IV
E = IVT
Define the kilowatt-hour (kWh)
energy transferred in one hour at a rate of transfer of 1 kW
circuit components—
cells, batteries, power supplies, generators,
potential dividers, switches,
resistors (fixed and variable), heaters, thermistors (NTC only),
light-dependent resistors (LDRs),
lamps, motors, ammeters, voltmeters, magnetising coils,
transformers, fuses and relays, diodes and light-emitting diodes (LEDs)
Current and p.d. in series and parallel circuit
- current at every point in a series circuit is the same
the sum of the currents entering a junction in a parallel circuit is equal to the sum of the currents that leave the junction. for a parallel circuit, the current from the source is larger than the current in each branch
- the total p.d. across the components in a series circuit is equal to the sum of the individual p.d.s across each component. for a series circuit, the voltage from the source is larger than the voltage in each branch
the p.d. across an arrangement of parallel resistances is the same as the p.d. across one branch in the arrangement of the parallel resistance
resistance in series and parallel circuit
S- R= R1+R2+R3
P- combined resistance of two resistors in parallel is less than that of either resistor by itself.
1/R= 1/R1+1/R2- effective resistance for TWO resistors in parallel
State the advantages of connecting lamps in parallel in a lighting circuit
lamps in parallel circuit have normal brightness or are brighter than lamps in series circuit
if one lamp fails, the other lamp is still lit (because each lamp is provided with its own switch so it can be operated separately).
Describe the action of a variable potential divider AND equation for two resistors used as a potential divider
Part of a circuit consisting of two resistors connected in series to obtain a smaller voltage than supplied.
Splits emf of a power source between two components/resistors connected in series in proportion to resistances of the two components or resistors.
R1/R2 = V1/V2
State the hazards when using a mains supply
(a) damaged insulation- emit poisonous fumes, catch fire, lethal shock
(b) overheating cables- insulation melt, fumes
(c) damp conditions- water conducts electricity, electrocution risk, short circuit
(d) excess current from overloading of plugs, extension leads, single and multiple sockets- heat up and catch fire
Describe all types of wires and explain in which a switch must be connected to
a mains circuit consists of a live wire- carries current from wall socket to electrical appliances, a neutral wire- carries current back to socket and an earth wire- safety- protects the circuit & cabling for double-insulated appliances.
a switch must be connected to the live wire for the circuit to be switched off safely- current could still pass into the appliance if the switch was off & lead to to fire or someone being electrocuted if they touched a faulty appliance.
use and operation of double-insulation, earthing, trip switches and fuses
- double insulation- no need of earthing as electrically insulating material- so electric current can’t pass through it. a live conductor will not touch the outer case
- fuse- contains a thin section of wire, designed to melt and break the circuit if the current gets above a certain value.
- trip switch- includes a switch that opens (trips) when a current exceeds a certain value
- earthed- the earth wire provides a low resistance electrical path to ground and reduces the chances of a fatal electric shock.
a fuse without an earth wire protects the circuit and the cabling for a double-insulated appliance
Magnetic effect of a current
Describe the pattern and direction of the magnetic field due to currents in straight wires
and in solenoids- right hand grip rule
Describe how the magnetic effect of a current is used in
relays- switches that open and close via action of electromagnets. electrical circuits contain an electromagnet who’s magnetic field is induced and attracts switch on a second circuit near to it
loudspeakers- electrical to sound due to motor effect- has a coil wrapped around one pole of magnet. ac passes so direction of mf changes, force exerted on coil so it oscillates creating sound waves.
State the qualitative variation of the strength of the magnetic field around straight wires and
solenoids- closer and stronger near wire and poles and further and weaker
no of coils
size of current
add iron core
AC generators and em induction
-a conductor moving across a magnetic field or a changing magnetic field can induce an e.m.f. in the conductor
converts mechanical energy to electrical energy.`
direction of an induced e.m.f. opposes the change causing it
RHR- directions of force, field and induced current
e.m.f. against time- sin graph
peaks- horizontal
troughs- horizontal- 180 degree
zeros of the e.m.f.- vertical
factors affecting the magnitude of an induced e.m.f.-
(a) the number of turns on the coil
(b) size of coil
(c) the strength of the magnetic field
a.c. generator (rotating coil or rotating magnet)
slip rings- allows current to flow from to and from coil of ac generator
and brushes- rub against slip rings so have same emf as coil
DC motors
-a force acts on a current-carrying conductor in a magnetic field
converts electrical energy to mechanical energy.
reversing direction of force: (a) the current, (b) the direction of the field
LHR- force, magnetic field and current
- Know that a current-carrying coil in a magnetic field may experience a turning effect and that the
turning effect is increased by increasing:
(a) the number of turns on the coil
(b) the current
(c) the strength of the magnetic field
split-ring commutator- reverses direction of current in coil every half turn so it rotates in same direction
brushes- transmits current from battery to coil without breaking wires
transformer- what, how, why
alternating current in (primary coil)
(current in primary generates) changing magnetic field
iron core concentrates (magnetic) field OR iron core transfers (magnetic) field (to secondary coil)
secondary coil is in alternating / changing (magnetic) field OR secondary coil cuts (magnetic) field
e.m.f. induced (in secondary coil)
- step up, step down- increase or decrease voltage
Vp/Vs = Np/Ns
100% efficient same power- IpVp = IsVs
State the advantages of high-voltage transmission
P=I^2R
to explain why power losses in cables are smaller when the voltage is greater
