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Phy > P4 - electricity & magnetism > Flashcards

P4 - electricity & magnetism Flashcards

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1
Q

properties of magnets

A

-always strongest at its poles (closer the magnetic lines, stronger the attraction)
-points to North when freely suspended
-attracts magnetic materials by inducing them

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2
Q

note:
temp. temporary
per - permanent
mag - magnet

A
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3
Q

properties of magnetic field

A

-lines never cross
-N to S
-closer the lines, stronger the attraction
-Cross North Close, CNC

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4
Q

law of magnetism

A

-like poles repel
-unlike poles attract

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5
Q

define magnetic field

A

-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

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6
Q

define magnetic material

A

-materials that can be magnetized
eg. iron, cobalt, nickel

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7
Q

define non- magnetic material

A

-materials that cannot be magnetized
eg. aluminum, copper, lead, brass, tin

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8
Q

properties of temp mag

A

-made of soft iron
-dont retain magnetism for a long time
-poles change
-eg. electromag

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9
Q

properties of a per mag

A

-made of ferromagnetic material eg. steel, iron, nickel
-retains magnetism
-poles dont change

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10
Q

define electromag

A

-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

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11
Q

ferrous metals

A

iron, steel, cobalt, nickel

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12
Q

ferrous magnets

A

hard - steel, alloys
soft - iron, mumetal

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13
Q

how does electromag work

A

-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

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14
Q

how to increase magnetic field strength in an electromag

A

-more no. coils
-more current
-use a material with high magnetic permeability

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15
Q

how to demagnetize a mag (temp/ permanent)

A

-heating
-dropping magnet frequently
-hammering
-alternating direction current
(domains are forced out of alignment)

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16
Q

define magnetic induction

A

-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)

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17
Q

what are mag keepers made of & why

A

-soft iron because it’s highly magnetic
-keepers complete mag circuit to preserving per mag strength - avoid self demagnetization

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18
Q

note: needles in compass are inverted
-S on needles shows N pole of earth

A
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19
Q

differences btw electromag & per mag

A

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

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20
Q

ways to magnetize

A

-induced magnetism (temp.) - temp
-using current (temp.) - temp & per depending on material
-stroking - temp & per depending on material

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21
Q

how does stroking work

A

-stroke a per. mag to a magnetic material
-pole at end of magnetic material is opp. pole of per mag

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22
Q

diff btw steel & iron

A

steel:
-hard to magnetize
-retains magnetism for long time
iron:
-easy to magnetize
-retains magnetism for short time

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23
Q

drawing of mag. filed

A

-like poles: 1 neutral middle point, magnetic fields cancel
-unlike pole: no neutral points, magnetic fields combine

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24
Q

how do magnets work

A

-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

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25
how to show magnetic field
-drop iron fillings -use compass & follow the needle
26
uses of permanent magnets
-needles of compasses -fridge door seal -loudspeaker
27
uses of electromagnets
-circuit breaker -relay -electric bell
28
uses of temp magnets
-generators -circuit breaker
29
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
30
define current
-flow of charges -( + ) to ( - ); opposite to flow of electrons -driven by a pd -measures in amperes (A)
31
relationship btw current & charge
more current, more charge
32
relationship btw charges
-like charges repel -unlike charges attract
33
define electric field
region in where an electric charge experiences a force -nearer the charge, stronger the repulsion/ attraction
34
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
35
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
36
note: pd = potential difference
37
difference btw static & current
static - build up of charges in place current - flow of charges
38
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)
39
relationship btw charge & distance
-closer the charges, the greater the distance btw them
40
what moves in a current
-electrons only ( - )
41
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
42
is water an electrical conductor
-depends on salinity (minerals containing)
43
define electric I
-charge passing a point per unit time; flow of charge around a circuit.
44
define electric I in metals
flow of electrons around a circuit.
45
what is needed of electric current to flow
delocalized electrons to move
46
define conductoers
-material that allows charge to pass through -has delocalized electron to carry charge eg. copper, carbon
47
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
48
what is PVC
-Poly Vinyl Chloride -insulator around wires
49
define semi conductors
-electrical conductivity is btw conductor & insulator -poor conductor when cold -good conductor when hot eg. silicon, germanium
50
direction of convectional current flow & flow of electrons
convectional current flow - ( + ) ( - ) flow of electrons - ( - ) ( + )
51
circuit drawing
-always mention direction of current flow -switches open unless stated -the longer line in battery is ( + )
52
how is charged measured
-in coulombs C -1 volt = 1 joule of potential energy for each coulomb of charge -each charge has potential energy
53
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
54
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
55
define series
-components are connected in a loop with no branches
56
define parallel
-components connected in branches
57
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
58
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
59
define pd
-work done by a unit charge passing between two points in a circuit -pd btw 2 points is measured in V
60
define battery
series of joined cells
61
formula for momentum
-velocity x mass
62
how to hinder flow of electrons
-uses resistors
63
Ohm's law
V = IR voltage = current x resistance -current is directly proportional to voltage when R is constant
64
note: R = resistance
65
Resistance
-hinders current flow = less current -measure in Ohms Ω
66
why is copper used in wires
-good conductor -little resistance to electrons = current passes easily -less expensive -bad resistor
67
why is nichrome used in heating devices
-bad conductor -lots resistance to electrons = less current flow -expensive -good resistor
68
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
69
resistivity constant unit
rho ( ρ )
70
how is heat created in wires & uses
-when current flows, R causes heat -atoms vibrate more, more heat eg. bulbs, toaster
71
R relationship with length & cross sectional area of a wire
-resistance is directly proportional to length -resistance is inversely proportional to cross-sectional area
72
formula to get resistance from ρ, length & cross-sectional area
R = ρ x (length/ area)
73
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
74
rate where energy is transferred
-power -unit = Watt -1 W = 1 J of energy transferred per second -diff electrical components need diff power
75
energy transfer in circuit
-chem energy in cell -potential energy in electrons -thermal & electrical energy in bulb
76
formula for power with voltage & current/ energy
-P = IV -p = E/ t (energy/ time)
77
difference btw ammeter & galvanometer
ammeter - how much current flows galvanometer - if current flows or not
78
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
79
use of fuse
-blows & opens circuit when high V/I -like a resistor but opens circuit
80
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
81
V, I, R calculation in series
-R: Rt = R1 + R2 + R3 -I: same in all places -V: depends in resistor/ component (use V= IR)
82
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)
83
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
84
how to remove charge from insulator
-heating: air ionizes & produce charges which neutralizes the charge in insulator
85
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
86
note: direction of a magnetic field at a point = direction of the force on the N of mag that point
87
note: direction of an electric field at a point = direction of force on a + charge at that point
88
define voltage of the source
- is cause of I in circuit
89
I & V graph when R is constant
-I is directly proportional to V
90
diff btw magnetic, magnetized & magnetic, unmagnetized
magnetic, magnetized - temp mag, poles change magnetic, unmagnetized - per mag, poles don't change
91
formula to get I from charge & time
I = Q/ t current = charge/ time amps, C, s
92
define voltage
energy given to charge to move through the circuit
93
resistance effects for V & I -series -parallel
-reduces I & affects voltage distribution -affects I distribution
94
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
95
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)
96
define transuder
electronic device that converts energy into different forms
97
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
98
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
99
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
100
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
101
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
102
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
103
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
104
what happens when wrong fuse is used
-if there is fault, circuit overheats & catches fire without fuse blowing
105
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
106
formula to find fuse rating
I = P/ V current = power / voltage -then increase it a little -3A, 5A, 13A
107
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
108
advantage of trip switches (circuit breakers) over fuses
-fuses have to be replaced but circuit breakers don't -provide protection form I surges/ faults
109
note: current has a heating effect
110
note: when a conductor moves across a (changing) magnetic field, emf is induced across conductor
111
factors affecting the magnitude of an induced e.m.f.
-strength of mag field -rate of change of mag field
112
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
113
how to solve a parallel circuit Q
-find total resistance -find total I
114
use of a fixed resistor
to reduce voltage/ current

Phy (13 decks)

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