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Phy > Chap 6 - magnetism & electricity > Flashcards

Chap 6 - magnetism & electricity Flashcards

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
-loses magnetism unless near a per mag
-dont retain magnetism for a long time
-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
Q

how to show magnetic field

A

-drop iron fillings
-use compass & follow the needle

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26
Q

uses of permanent magnets

A

-needles of compasses
-fridge door seal
-loudspeaker

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27
Q

uses of electromagnets

A

-circuit breaker
-relay
-electric bell

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28
Q

uses of temp magnets

A

-generators
-circuit breaker

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29
Q

define static electricity

A

-build up of charged particles in 1 place which causes a small shock when discharged
-results from transfer of only electrons by friction

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30
Q

define current

A

-flow of charges
-driven by a pd
-measures in amperes (A)

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31
Q

relationship btw current & charge

A

more current, more charge

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32
Q

relationship btw charges

A

-like charges repel
-unlike charges attract

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33
Q

note: pd = potential difference

A
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34
Q

difference btw static & current

A

static - build up of charges in place
current - flow of charges

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35
Q

eg of insulating materials & how to charge them

A

eg. polythene, perspex
-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)

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36
Q

relationship btw charge & distance

A

-closer the charges, the greater the distance btw them

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37
Q

what moves in a current

A

-electrons only ( - )

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38
Q

uses of static electricity

A

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

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39
Q

is water an electrical conductor

A

-depends on salinity (minerals containing)

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40
Q

define electricity

A

flow of electrons around a circuit.

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41
Q

what is needed of electric current to flow

A

delocalized electrons to move

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42
Q

define conductoers

A

-material that allows charge to pass through
-has delocalized electron to carry charge
eg. copper, carbon

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43
Q

define insulator

A

-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

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44
Q

what is PVC

A

-Poly Vinyl Chloride
-insulator around wires

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45
Q

define semi conductors

A

-electrical conductivity is btw conductor & insulator
-poor conductor when cold
-good conductor when hot
eg. silicon, germanium

46
Q

direction of convectional current flow & flow of electrons

A

convectional current flow - ( + ) ( - )
flow of electrons - ( - ) ( + )

47
Q

circuit drawing

A

-always mention direction of current flow
-switches open unless stated
-the longer line in battery is ( + )

48
Q

how is charged measured

A

-in coulombs C
-1 volt = 1 joule of potential energy for each coulomb of charge
-each charge has potential energy

49
Q

define ammeter

A

-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

50
Q

define voltmeter

A

-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

51
Q

define series

A

-components are connected in a loop with no branches

52
Q

define parallel

A

-components connected in branches

53
Q

diff btw series & parallel

A

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

54
Q

define EMF

A

-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

55
Q

define pd

A

-work done by a unit charge passing between two points in a circuit
-pd btw 2 points is measured in V

56
Q

define battery

A

series of joined cells

57
Q

formula for momentum

A

-velocity x mass

58
Q

how to hinder flow of electrons

A

-uses resistors

59
Q

Ohm’s law

A

V = IR
voltage = current x resistance
-current is directly proportional to voltage when R is constant

60
Q

note: R = resistance

A
61
Q

Resistance

A

-hinders current flow = less current
-measure in Ohms Ω

62
Q

why is copper used in wires

A

-good conductor
-little resistance to electrons = current passes easily
-less expensive
-bad resistor

63
Q

why is nichrome used in heating devices

A

-bad conductor
-lots resistance to electrons = less current flow
-expensive
-good resistor

64
Q

factors affecting R

A

-↑ 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

65
Q

resistivity constant unit

A

rho ( ρ )

66
Q

how is heat created in wires & uses

A

-when current flows, R causes heat
-atoms vibrate more, more heat
eg. bulbs, toaster

67
Q

formula to get resistance from ρ, length & cross-sectional area

A

R = ρ x (length/ area)

68
Q

types of resistors
resistor - keep I & V at the levels needed to work properly

A

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

69
Q

rate where energy is transferred

A

-power
-unit = Watt
-1 W = 1 J of energy transferred per second
-diff electrical components need diff power

70
Q

energy transfer in circuit

A

-chem energy in cell
-potential energy in electrons
-thermal & electrical energy in bulb

71
Q

formula for power with voltage & current/ energy

A

-P = IV
-p = E/ t (energy/ time)

72
Q

difference btw ammeter & galvanometer

A

ammeter - how much current flows
galvanometer - if current flows or not

73
Q

types of transformers & their uses

A

-step up - increases V/E/P
-step down - reduces V/E/P
voltage/ energy/ power

74
Q

use of fuse

A

-blows & opens circuit when high V/I
-like a resistor but opens circuit

75
Q

use of relay

A

turns on & off by itself

76
Q

uses of diodes

A

-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

77
Q

V, I, R calculation in series

A

-R:
Rt = R1 + R2 + R3
-I: same in all places
-V: depends in resistor/ component (use V= IR)

78
Q

V, I, R calculation in parallel

A

-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)

79
Q

uses of potential dividers

A

-divide V in circuit so that each component only receive V requires
-2 or more resistors in series across a power supply

80
Q

define electric field

A

-region where particle experience an electrical force
-nearer the charge, stronger the repulsion/ attraction

81
Q

how to remove charge from insulator

A

-heating:
air ionizes & produce charges which neutralizes the charge in insulator

82
Q

define relay

A

-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

83
Q

note: direction of a magnetic field at a point = direction of the force on the N of mag that point

A
84
Q

note: direction of an electric field at a point = direction of force on a + charge at that point

A
85
Q

define voltage of the source

A
  • is cause of I in circuit
86
Q

I & V graph when R is constant

A

-I is directly proportional to V

87
Q

diff btw magnetic, magnetized & magnetic, unmagnetized

A

magnetic, magnetized - temp mag, poles change
magnetic, unmagnetized - per mag, poles don’t change

88
Q

formula to get I from charge & time

A

I = Q/ t
current = charge/ time
amps, C, s

89
Q

define voltage

A

energy given to charge to move through the circuit

90
Q

resistance effects for V & I
-series
-parallel

A

-reduces I & affects voltage distribution
-affects I distribution

91
Q

types of components

A

-cell
-battery
-filament lamp
-switch
-heater
-fixed resistor
-variable resistor
-light dependent resistor (LDR)
-thermistor
-fuse
-ammeter
-voltmeter
-galvanometer
-magnetizing coil
-relay
-bell
-transformer
-dc power supply
-ac power supply
-diode
-light emitting diode

92
Q

uses of rectification

A

-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)

93
Q

define transuder

A

electronic device that converts energy into different forms

94
Q

how does thermistor in fire alarm work

A

-at room temp, high R = low V
-as temp rises, lower R = more V to transistor, alarm if switched on

95
Q

how does light dependent resistor work in security light circuit

A

-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

96
Q

hazards of electricity

A

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

97
Q

part of mains electricity

A

-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

98
Q

safety features for appliances

A

-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

99
Q

exceptions for mains electricity

A

-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

100
Q

how does fuse wok with earth wire in mains electricity

A

-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

101
Q

what happens when wrong fuse is used

A

-if there is fault, circuit overheats & catches fire without fuse blowing

102
Q

how does a trip switch (circuit breaker) work

A

-automatic switches that trip (turn off) when current rises above specific value
-can be reset by pressing a button

103
Q

formula to find fuse rating

A

I = P/ V
current = power / voltage
-then increase it a little
-3A, 5A, 13A

104
Q

how do fuses work

A

-piece of wire with low melting point in fuse
-wire melts when current is too high = break circuit
-have to replace fuse

105
Q

advantage of trip switches (circuit breakers) over fuses

A

-fuses have to be replaced but circuit breakers don’t
-provide protection form I surges/ faults

106
Q

note: current has a heating effect

A
107
Q

note: when a conductor moves across a (changing) magnetic field, emf is induced across conductor

A
108
Q

factors affecting the magnitude of an induced e.m.f.

A

-strength of mag field
-rate of change of mag field

109
Q

how to find I when resistance is diff in branches of parallel circuit.

A

-find R in branches
-find V in each branch
-use V = IR to find I

110
Q

how to solve a parallel circuit Q

A

-find total resistance
-find total I

Phy (11 decks)

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