P4 - electricity & magnetism Flashcards
properties of magnets
-always strongest at its poles (closer the magnetic lines, stronger the attraction)
-points to North when freely suspended
-attracts magnetic materials by inducing them
note:
temp. temporary
per - permanent
mag - magnet
properties of magnetic field
-lines never cross
-N to S
-closer the lines, stronger the attraction
-Cross North Close, CNC
law of magnetism
-like poles repel
-unlike poles attract
define magnetic field
-a region where a magnetic pole experiences a force
-density of lines = strength of mag
-magnetic lines loop through mag
-need arrows from N to S when drawing
define magnetic material
-materials that can be magnetized
eg. iron, cobalt, nickel
define non- magnetic material
-materials that cannot be magnetized
eg. aluminum, copper, lead, brass, tin
properties of temp mag
-made of soft iron
-loses magnetism unless near a per mag
-dont retain magnetism for a long time
-eg. electromag
properties of a per mag
-made of ferromagnetic material eg. steel, iron, nickel
-retains magnetism
-poles dont change
define electromag
-soft iron turning into a magnet when an electrical current passes through the wire
-direction is current changes poles
-a temporary magnet
-magnetic field can be controlled by turning current on & off
ferrous metals
iron, steel, cobalt, nickel
ferrous magnets
hard - steel, alloys
soft - iron, mumetal
how does electromag work
-current flows through wire & creates a magnetic field
-winding wires in a coil makes magnetic field concentrated & amplifies magnetic field strength so soft iron is magnetized
-temp mag
-mag field: in circles, strongest close to wire, more current = more mag field
-right hand grip rule:
-(wire) thumb = current direction, finger = filed lines
-(solenoid) thumb = N pole
how to increase magnetic field strength in an electromag
-more no. coils
-more current
-use a material with high magnetic permeability
how to demagnetize a mag (temp/ permanent)
-heating
-dropping magnet frequently
-hammering
-alternating direction current
(domains are forced out of alignment)
define magnetic induction
-when a magnetic material is magnetized by the presence of a per mag near it
-the side of material near the magnet is opposite the pole of the per mag
-loses magnetism when per mag is gone
-steel becomes a per. mag, iron becomes a temp. mag
-eg (per)SN SN(mag. material)
what are mag keepers made of & why
-soft iron because it’s highly magnetic
-keepers complete mag circuit to preserving per mag strength - avoid self demagnetization
note: needles in compass are inverted
-S on needles shows N pole of earth
differences btw electromag & per mag
electro:
-made of soft iron
-loses magnetism
-magnetic field can be turned on & off
-poles change depending on current direction
-magnetic field can change
per:
-made of steel
-retains magnetism
-magnetic field cannot be turned off
-fixed poles
-constant magnetic field
ways to magnetize
-induced magnetism (temp.) - temp
-using current (temp.) - temp & per depending on material
-stroking - temp & per depending on material
how does stroking work
-stroke a per. mag to a magnetic material
-pole at end of magnetic material is opp. pole of per mag
diff btw steel & iron
steel:
-hard to magnetize
-retains magnetism for long time
iron:
-easy to magnetize
-retains magnetism for short time
drawing of mag. filed
-like poles: 1 neutral middle point, magnetic fields cancel
-unlike pole: no neutral points, magnetic fields combine
how do magnets work
-electrons spin & orbit nucleus, causing a magnetic effect
-usually the effect cancels but some don’t
-unmagnetized material, electrons/ atomic mag point in random directions
-magnetized material, atomic mag align
how to show magnetic field
-drop iron fillings
-use compass & follow the needle
uses of permanent magnets
-needles of compasses
-fridge door seal
-loudspeaker
uses of electromagnets
-circuit breaker
-relay
-electric bell
uses of temp magnets
-generators
-circuit breaker
define static electricity
-build up of charged particles in 1 place which causes a small shock when discharged
-results from transfer of only electrons by friction
define current
-flow of charges
-driven by a pd
-measures in amperes (A)
relationship btw current & charge
more current, more charge
relationship btw charges
-like charges repel
-unlike charges attract
define electric field
region in where an electric charge experiences a force
-nearer the charge, stronger the repulsion/ attraction
direction of electric field
direction of an electric field at a point is the direction of the force on a positive charge at that point
direction of mag field
direction of a magnetic field at a point is the direction of the force on the N pole of a magnet at that point
note: pd = potential difference
difference btw static & current
static - build up of charges in place
current - flow of charges
eg of insulating materials & how to charge them
eg. polythene, perspex
-friction
-rubbing it with a dry woolen cloth
-neutral materials gain opp. charge of the charged material
-electrons are transferred = imbalance in charge
-charge is localized (no free electrons to move freely)
relationship btw charge & distance
-closer the charges, the greater the distance btw them
what moves in a current
-electrons only ( - )
uses of static electricity
1)electrostatic smoke precipitators detectors:
-as the smoke particles rise up, they’re attracted to the charges plates on the side of the chimney
2)Spray painting:
-nozzle is connected to + terminal = drops are ( + )
-drop repel each other & spread out evenly
-car is connected to earth (- )
- + drops are attracted to - car
3)photocopiers:
-use static to attract black tonner to paper
is water an electrical conductor
-depends on salinity (minerals containing)
define electric I
-charge passing a point per unit time;
flow of charge around a circuit.
define electric I in metals
flow of electrons around a circuit.
what is needed of electric current to flow
delocalized electrons to move
define conductoers
-material that allows charge to pass through
-has delocalized electron to carry charge
eg. copper, carbon
define insulator
-material that don’t allows charge to pass through
-no delocalized electron to carry charge
-but electrons can be transferred by static electricity/ friction
eg. plastic, glass
what is PVC
-Poly Vinyl Chloride
-insulator around wires
define semi conductors
-electrical conductivity is btw conductor & insulator
-poor conductor when cold
-good conductor when hot
eg. silicon, germanium
direction of convectional current flow & flow of electrons
convectional current flow - ( + ) ( - )
flow of electrons - ( - ) ( + )
circuit drawing
-always mention direction of current flow
-switches open unless stated
-the longer line in battery is ( + )
how is charged measured
-in coulombs C
-1 volt = 1 joule of potential energy for each coulomb of charge
-each charge has potential energy
define ammeter
-device used to measure current in amperes (A)
-connected in series when measuring current
-current in each branch depends on resistance of component; current in diff branches add up to total current (parallel)
-components get voltage depending on resistance
define voltmeter
-device used to measure voltage (pd) in volts (V)
-connected in parallel when measuring voltage
-pd in diff in components/ resistance add up to total source voltage (series)
-all component get full voltage
define series
-components are connected in a loop with no branches
define parallel
-components connected in branches
diff btw series & parallel
series:
-voltage varies
-current is constant
-if bulb fuses, open circuit
-more bulbs, dimmer
parallel:
-voltage is constant (PVC - Parallel Voltage Constant)
-current varies in each branch depending on component resistance
-if bulb fuses, current still passes
-more bulbs, same light - take full voltage
define EMF
-Electro Motive Force eg, batter, generator
-electrical work done by source to moving an unit charge through the circuit
-max pd btw 2 point in a circuit when no current flows
-measure in V
-when cell is connected to circuit, pd drops because of energy wastage in cell
define pd
-work done by a unit charge passing between two points in a circuit
-pd btw 2 points is measured in V
define battery
series of joined cells
formula for momentum
-velocity x mass
how to hinder flow of electrons
-uses resistors
Ohm’s law
V = IR
voltage = current x resistance
-current is directly proportional to voltage when R is constant
note: R = resistance
Resistance
-hinders current flow = less current
-measure in Ohms Ω
why is copper used in wires
-good conductor
-little resistance to electrons = current passes easily
-less expensive
-bad resistor
why is nichrome used in heating devices
-bad conductor
-lots resistance to electrons = less current flow
-expensive
-good resistor
factors affecting R
-↑ length of wire, ↑ R (directly proportional)
-↑ cross section area of wire, ↓ R (inversely proportional)
-↑ temp in metal conductor, ↑ R, & ↑ temp in semi conductor, ↓ R
-depends on resistivity constant to materials
resistivity constant unit
rho ( ρ )
how is heat created in wires & uses
-when current flows, R causes heat
-atoms vibrate more, more heat
eg. bulbs, toaster
R relationship with length & cross sectional area of a wire
-resistance is directly proportional to length
-resistance is inversely proportional to cross-sectional
area
formula to get resistance from ρ, length & cross-sectional area
R = ρ x (length/ area)
types of resistors
resistor - keep I & V at the levels needed to work properly
1) fixed resistor - fixed R
eg. amplifiers
2) variable resistor - components with varying R
eg. fan, volume, heater
3) thermistor - high R in cold, low R in hot
eg. digital thermometer, fire alarm
4) light dependent resistor LDR - high R in dark, low R in light
eg. controlling light switches, security light circuit
5) diode - controls flow of current; high R in opp. direction of current
rate where energy is transferred
-power
-unit = Watt
-1 W = 1 J of energy transferred per second
-diff electrical components need diff power
energy transfer in circuit
-chem energy in cell
-potential energy in electrons
-thermal & electrical energy in bulb
formula for power with voltage & current/ energy
-P = IV
-p = E/ t (energy/ time)
difference btw ammeter & galvanometer
ammeter - how much current flows
galvanometer - if current flows or not
what doe electrical circuits do
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
use of fuse
-blows & opens circuit when high V/I
-like a resistor but opens circuit
uses of diodes
-allows current to flow in 1 direction only
-cathode after line, anode before line
-forward bias = anode has higher V than cathode, low R
-reversed bias = cathode has higher V than anode, high R
-rectification
V, I, R calculation in series
-R:
Rt = R1 + R2 + R3
-I: same in all places
-V: depends in resistor/ component (use V= IR)
V, I, R calculation in parallel
-R:
1/ RT = 1/R1 + 1/R2 + 1/R3
-V: same in all places/ when it reaches back to cell, V should be 0
-I: depends in resistor (use V= IR)/ is constant until its split off
(combined resistance of 2 resistors is less than that of the resistor by itself)
uses of potential dividers
-divide V in circuit so that each component only receive V requires
-2 or more resistors in series across a power supply
how to remove charge from insulator
-heating:
air ionizes & produce charges which neutralizes the charge in insulator
define relay
-device which uses a low current circuit to switch a high current circuit on or off
1) in cars to switch starter motor:
-when low current circuit switch is closed, electromag. on to attract iron armatures
-armature pivots & closes switch contracts in high current circuit
-when low current circuit switch is opened, electromag stops attracting armature so high current circuit is open
note: direction of a magnetic field at a point = direction of the force on the N of mag that point
note: direction of an electric field at a point = direction of force on a + charge at that point
define voltage of the source
- is cause of I in circuit
I & V graph when R is constant
-I is directly proportional to V
diff btw magnetic, magnetized & magnetic, unmagnetized
magnetic, magnetized - temp mag, poles change
magnetic, unmagnetized - per mag, poles don’t change
formula to get I from charge & time
I = Q/ t
current = charge/ time
amps, C, s
define voltage
energy given to charge to move through the circuit
resistance effects for V & I
-series
-parallel
-reduces I & affects voltage distribution
-affects I distribution
types of components
-cell
-battery
-filament lamp
-switch
-heater
-fixed resistor
-variable resistor
-light dependent resistor (LDR)
-thermistor
-fuse
-ammeter
-voltmeter
-galvanometer
-relay
-transformer
-dc power supply
-ac power supply
-diode
-light emitting diode (LEDs)
-motor
-generator
uses of rectification
-diode is used to convert ac current to dc current
-forward part of ac is let through, but not backward part
-can been seen through cathode ray oscilloscope (CRO)
define transuder
electronic device that converts energy into different forms
how does thermistor in fire alarm work
-at room temp, high R = low V
-as temp rises, lower R = more V to transistor, alarm if switched on
how does light dependent resistor work in security light circuit
-daylight, low R & low share of battery V = not enough V to turn on transistor
-dark, high R & enough share of battery V = transistor on & bulb lights up
hazards of electricity
1) water leak in plug/ socket, water conducts electricity, person gets electrocuted
2) frayed wire could have some wire with high R at one point = heats through insulation & causes fire
3) extension lead overheat when used coiled up = heat in wire form current has less room to escape
1)damaged insulation = exposed wire will cause electrical shock when touched
2)overheating cables = excess I overheats & melts insulation/ cause fire
3)damp conditions = water to live wire = fire/ path for I from person to earth (electric shock)
4)excess current from overload of plugs = heat melts insulation/ fire
part of mains electricity
-blue - neutral wire; form a complete circuit with live wire
-yellow & green - earth wire; safety wire to stop appliances from being live - prevent electric shocks
-brown - live wire; carry ac from mains supply to circuit - touch without earth = electrocution
-fuse - connected to live wire to open circuit if current is too high
-cable grip - keeps wires in plug secure
-plastic insulating case
-plastic wire outer covering
in socket pins:
earth
neutral live
safety features for appliances
-double insulation = wire is covered with insulating material & a non- metallic case = don’t need earth wire
-earthing = for wires with metal cases = low R path to earth, I melts & breaks fuse, I supply to appliance is cut off
-fuse & trip switches (circuit breaker) = cut flow of electricity when I is too big
-outer casing of an electrical appliance has to be double insulated or earthed
exceptions for mains electricity
-some countries only have live & neutral wire
-in UK mains current = 230V ac - current flow back & forth 50 times per second (50hz)
-some appliances don’t have earth wire because they have double insulation
eg. radio - outer case is plastic, not metal, = extra insulation
how does fuse wok with earth wire in mains electricity
-if there is fault, big current flows through the live wire, case & down the earth wire
-surge in current blows fuse & cuts off live supply to isolate appliance
-prevents electric shock from case & fire from heating current
-fuses rating should be a bit higher than operating current
what happens when wrong fuse is used
-if there is fault, circuit overheats & catches fire without fuse blowing
how does a trip switch (circuit breaker) work
-automatic switches that trip (turn off) when current rises above specific value
-can be reset by pressing a button
formula to find fuse rating
I = P/ V
current = power / voltage
-then increase it a little
-3A, 5A, 13A
how do fuses work
-piece of wire with low melting point in fuse
-wire melts when current is too high = break circuit
-have to replace fuse
advantage of trip switches (circuit breakers) over fuses
-fuses have to be replaced but circuit breakers don’t
-provide protection form I surges/ faults
note: current has a heating effect
note: when a conductor moves across a (changing) magnetic field, emf is induced across conductor
factors affecting the magnitude of an induced e.m.f.
-strength of mag field
-rate of change of mag field
how to find I when resistance is diff in branches of parallel circuit.
-find R in branches
-find V in each branch
-use V = IR to find I
how to solve a parallel circuit Q
-find total resistance
-find total I
use of a fixed resistor
to reduce voltage/ current
