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Physics > Electricity > Flashcards

Electricity Flashcards

1
Q

Potential Difference (Voltage)

A

between two points in an electric field is the work required to move a charge of 1C from one point to another

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2
Q

Electromotive Force

A
  • the amount of potential energy a battery has when a circuit is switched off (potential difference between terminals)
  • the voltage that is actually applied to a circuit by a battery
  • difference in potential between two terminals which gives rise to current
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3
Q

Ohms Law

A

the current flowing through a conductor between two points is directly proportional to the potential difference between the points if the temperature remains constant. ( v on y axis)

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4
Q

Static Electricity

A

electrons unmoving

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5
Q

current electricity

A

electrons moving

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6
Q

conductor

A

a medium which allows electric charge to flow through it

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7
Q

insulator

A

a medium which does not allow electric charge to flow through it

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8
Q

colombs law

A

the electrostatic force between two points is directly proportional to the product of their charges and inversely proportional to the square distance between them

(each charge experiences a force equal in magnitude but opposite in direction)

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9
Q

electrostatic law

A
  • depends on charge
  • attractive and repulsive
  • strong
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10
Q

Newton’s gravitational law

A
  • depends on mass (too small = none)
  • attractive
  • weak
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11
Q

Dielectric Material

A 
- good insulator 
( allows charge to build up on plates ) 
- paper 
- plastic
- ceramic
- air/free space
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12
Q

Charge on Sphere

A
  • centre

- factor in radius

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13
Q

Demonstrating Separation of Charge in Two Spheres by Induction

A

1 - Two conducting spheres with Even charge distribution touching (with insulated stands)
2 - Bring (-) charged rod near
3 - Electrons repelled by rod (attracted if +) causing polarisation
4 - keep rod in place and separate spheres
5 - now charged by induction
6 - spheres are now oppositely charged by induction

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14
Q

Charging a Single Sphere by induction

A

1) Conducting sphere with even charge distribution and insulated stand
2) Bring (-) rod near - polarised
3) ground sphere so electrons free to move - will be repelled (-)/ attracted (+) and travel out/intro sphere to/from earth
4) Remove Earth
5) Remove rod
6) sphere is now positively charged by induction

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15
Q

To Charge a gold leaf electroscope

A
  • ; - bring + rod close to cap
    • polarised (leaf rises)
    • earth
    • electrons attracted and move in from
      earth (leaf drops as + cancelled)
    • remove earth then rod
    • negative charge spreads out (leaf rises)
\+ ; bring - charged rod close to cap 
    polarised (leaf rises) 
    earth
    electrons repelled
    remove earth then rod

why earth - so electrons free to move

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16
Q

to test Nature of Charge (also size and if charged)

A
  • negative GLE with leaf up
    • rod = leaf goes down
    • rod = leaf stays up

(rod opposite to charge on GLE = leaf goes down)

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17
Q

to test for Conductor or Insulator

A
  • negatively charged GLE with leaf up
  • touch with material
  • stays up = insulator
  • goes down = conductor (takes electrons)
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18
Q

What GLE tests

A
  • if something is charged
  • relative size of charge
  • the nature of the charge - +/-
  • if something is a conductor/insulator
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19
Q

Electric Field Lines

A

Show the strength and direction of a force due to a positive charge in the field

  • go from + to -
  • never cross over
  • never finish mid air
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20
Q

Electric Field

A

A region in space where the force of an electric charge can be felt

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21
Q

Inspection Window

A

Shows Flux density

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22
Q

Electric Field Strength

A
  • the force per unit positive charge at a point in an electric field
  • E = F/Q
  • units - N/C
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23
Q

Electrostatic precipitation in chimneys

A
  • based on point effect
  • waste gas with smoke particles
  • neutral smoke particles (-) ionised by point
    effect
  • attracted to + charged collecting plates
  • knocked to remove particles
  • waste gas without particles emerges
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24
Q

Lightning conductor

A

1) - earthed copper rod - conductor and higher than the building so provides safe passage

2) point effect - reduces likely hood of lightening striking by removing electrons from cloud
- warm and cold clouds rub and create friction
- electrostatic charge builds up
- causes lightning

  • charge accumulates at point
  • cloud repells these electrons and polarises
    rod (+)
  • air particles become ionised by cloud (-)
  • attracted to (+) point
  • release electrons to point (+)
  • return to cloud and remove more e
  • cyclic process
  • reduces electrostatic build up on cloud
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25
Q

Mains electricity ireland

A
  • 50 hz
  • changes direction every 1/100 sec
  • 230 V a.c
  • alternating cycle of electricity
  • produces sinusoidal wave
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26
Q

Capacitor

A

a device capable of storing charge for rapid release

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27
Q

capacitance

A

the ratio of charge stored to voltage

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28
Q

factors that increase capacitance ( to put more charge on an object when it is already at full capacity)

A
  • voltage = work to put on 1 C
  • becomes too high as e repel
  • add a parallel positive plate
  • adds attractive force in opposite direction
  • repulsive force reduced
  • voltage reduced
  • capacitance increased (can hold more charge)

Increase Capacitance -
- shorter distance between two plates

  • increased permittivity of dielectric material (allows charge to build up on plates)
  • larger overlap area
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29
Q

current definition

A
  • flow of electrons
  • how many electrons pass a point every second
I = q/t 
(A = q/t) ( 6.25 x 10^18 e/s)
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30
Q

Joules Law

A

the rate of heat produced in a constant resistance conductor is proportional to the square of the current

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31
Q

1 coulomb

A

the charge that passes when 1 Ampere flows for 1 second

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32
Q

semi conductor

A

a conductor whose resistivity is between then of a good insulator and a good conductor.
as temp increases its resistance goes down

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33
Q

Properties of magnets

A
  • attract ferromagnetic material and cause temporary magnetism in them
  • when freely suspended will point roughly N and S
  • poles - like repels
    - opposite attracts
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34
Q

magnetic field

A
  • a region in space where attractive/repulsive magnetic forces can be can be felt
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35
Q

magnetic field lines

A

lines drawn that show the direction and strength of a magnetic field at any point

  • direction at any point = the same direction as the force that a north pole would experience at that point
  • point north to south
  • closer together = stronger field
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36
Q

who invented the left hand rule?

A
  • Flemming
  • how magnetic field effects a current carrying conductor
  • force = left hand
  • F, B, I
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37
Q

Current in a magnetic field

A
  • current carrying conductors have a magnetic field around them
  • when placed in another magnetic field creates a force
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38
Q

Force

A
  • perpendicular to current and voltage

- will not be produced if current is II to magnetic field

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39
Q

Force on a current carrying coil

A
  • equal opposing forces cause turning effect

- commutator/slip ring keeps forces continually turning

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40
Q

Generator

A
  • movement in (water heated into steam which turns turbine)

- electricity out

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41
Q

turbine

A
  • makes the length of wire longer
  • more current
  • bigger magnetic field
  • more force
  • more electricity
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42
Q

Magnitude of force

A

F=BIL

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43
Q

Magnetic Flux Density

A
  • the magnitude of electric field strength at any point
  • B = 1 Tesla if
  • a 1 m conductor with a 1 A current flowing through it
  • is placed at a right angle to the magnetic field
  • and experiences a 1N force because of it
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44
Q

Force when magnetic field is not perpendicular to current

A

F = BIL x sin( θ
(sin θ to find perpendicular magnetic flux!!)
( Magnetic flux given x sin θ = ⟂B)

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45
Q

Earth wire

A
  • top plug (longer) (safety - N and L can’t be connected without earth)
  • green and yellow for colour blind
  • makes live send its current through the wire instead of you ( we have higher resistance)
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46
Q

Live wire

A
  • on right when looking at back of plug
  • brown
  • attached to fuse
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47
Q

Neutral wire

A
  • On left when looking at back of plug
  • blue
  • carries current but with neutral voltage
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48
Q

Where do safety devices go

A
  • switch and fuse are on live wire in front of device so you can safely work on device
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49
Q

Devices with separate circuit breakers

A
  • electric shower
  • oven
  • a lot of current
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50
Q

Only device not connected to circuit breaker box

A
  • alarm

- because master switch is outside before box so can be pulled out by thieves

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51
Q

MCB

A
  • miniture circuit breaker
  • electromagnetic switch set for a certain amount of current
  • if current exceeds that it turns off
  • REUSABLE
    (usually on lights)
52
Q

RCD

A
  • Residual current devices
  • measures the balance of current in and out of the electric circuit (expects them to =)

if current flowing in differs from current flowing out because of leakage to earth relay switches off circuit

  • if someone is being electrocuted current will go through them and current in will not equal current out
53
Q

Fuse in plug

A
  • will melt if current exceeds fuse expectancy
  • value slightly above current value
  • NOT REUSABLE
54
Q

Safety devices

A
  • large ceramic fuse
  • master switch
  • RCD
  • MCB
  • fuse in socket
  • fuse in plug
  • earth
55
Q

Plugging in

A
  • connects live to neutral wire so current flows
56
Q

ring main circuit

A
  • 3 wires in parallel - live, neutral, earth
  • power travels both ways
    (less current if current has to go through two paths)
  • if one has a fault the other will still work
57
Q

radial ciruit

A
  • 1 arm out and back
  • individual appliances (oven)
  • own neutral, fuse in the live as higher voltage required
  • mcb.
58
Q

lighting

A

connected in parallel so one fault does not cause more

59
Q

KWh

A
  • kilowatt hour
  • 1500 W uses 1.5 kwh
  • kwh x number of hours used = kwh used
60
Q

energy used

A
  • figure out time used in seconds
  • 1500 W heater = 1500 J/s
  • times J/s by number of seconds
61
Q

p-n junction

A

where a p-type semi conductor and an n-type semi conductor meet

62
Q

electric current

A

flow of electric charge, usually carried by moving electrons in a wire

63
Q

semi- conductor diode

A
  • a device which allows current to flow in only one direction
  • p-type and n-type semi conductor in contact
64
Q

effects of electric current

A
  • heating - electric current flowing through a wire generates heat
  • chemical - passing a current through a liquid can cause a chemical reaction (electrolysis)
  • magnetic - electric current passing through a wire generates an electric field around the wire
65
Q

RCD has rating of 30 micro amps, significance?

A

current is cut off if the difference between the live an neutral currents is greater than 30 micro amps

66
Q

galvanometer can be changed into a voltmeter

A

large resistance

in series

67
Q

resistance vs resistivity

A
  • equations and label
68
Q

effect on resistance of a wire if diameter is incresed by 3 §

A

decreases by 9

69
Q

RCD will disconnect

A
  • when current flowing differs to the current flowing out

- leaking of current to earth

70
Q

residual circuit ring

A
  • reduces current as current takes two paths to socket reducing the current in each path
71
Q

EHT used to transport electricity

A
  • high voltage
    = lower current
    = less heat (energy loss)
72
Q

one amp

A
  • one coulomb per second

- 6.25 x 10^18 electrons/ second

73
Q

electricity

A

form of energy involving electrons

74
Q

to demonstrate charge accumulates at a point

A
  • touch pear shaped conductor off of dome
  • hold round end near GLE
  • hold pointy end near GLE
    observe difference in relative charge present
75
Q

to demonstrate charge resides on the outside of a metal conductor

A
  • touch outside of sphere with proof plane
  • hold near GLE
  • earth
  • touch inside
  • hold near
76
Q

xerography

A
  • drum negatively charged
  • light reflected by item being copied cancels charge in a pattern
  • ink particles are positively ionised and stick to charged parts of drum that remain
77
Q

importance of earthing

A

discharging planes

  • charge builds up on outside from friction with clouds
  • tether (earths)
  • copper strips in tyres

electronics

  • microchips - small voltage
    - large electrostatic charge
  • engineers have earthed bracelets

flour mill

  • airbourne flour particles become ionised
  • electronic machinery earthed
78
Q

under what circumstances will point discharge occur

A

when a charge accumilates at a point so there is large potential at that point

79
Q

batteries

A

convert chemical energy to electrical

80
Q

primary cell - not rechargeable

A
  • electrodes = two different metal plates
  • copper electrode - releases electrons
  • zinc electrode - releases positive ions
  • Electrolyte - allows flow of ions
    - sulfuric acid
  • connected above solution - allows flow of electrons
81
Q

dry cell

A
  • paste electrolyte - MnO2
  • carbon and zinc electrodes
  • exposed positive terminal - mixture of powdered carbon and MnO2
  • bottom = exposed negative zinc can
  • insulating cover
82
Q

voltmeter connected in parallel

A
  • high resistance - would hinder current flow
  • also measured potential difference between two points so two points on conductor needed
    ( in parallel V=V=V)
83
Q

ammeter connected in series

A 
low resistance 
( in series I = I = I)
84
Q

Power derivation

A

1) V = W/q
2) Vq = W
divide by t (seconds)
3) Vq/t = W/t
but I = coulombs per second (q/t)
and P = W/t
4) VI = P

85
Q

secondary cell - rechargeable by reversing current

A
  • lead electrodes
  • gas vents
  • electrolyte = sulfuric acid

car / phone batteries

86
Q

joules law easier

A

the rate of heat produced in a conductor is directly proportional to the square of the current provided its resistance is constant

rate of heat produced = J/s = P

87
Q

semi conductor diode (pn diode)

A
    • device which allows current to flow in only one direction
  • p-type and n-type semi conductor in contact - joined to form a single semi conductor
88
Q

Doping

A

the addition of small controlled amounts of certain impurities to a pure semi conductor to increase its conductivity

89
Q

N-type doping (boron)

A
  • when the impurity added provides more positive holes for conduction
90
Q

P-type

A
  • when the impurity added provides more negative electrons for conduction
91
Q

PN junction

A

where a p-type semi conductor and an n-type semi conductor meet

92
Q

Depletion layer

A

the region around a PN junction which contains virtually no free charge carriers so behaves as an insulator

93
Q

Junction voltage

A

the potential difference that exists across a PN junction caused by electrons and holes moving across the junction when it was formed

94
Q

Intrinsic Conduction

A

conduction in a pure semiconductor due to the movement of electrons and holes

  • equal number of electrons and positive holes
95
Q

Extrinsic conduction

A

increased conductivity in a semiconductor due to the controlled addition of impurities

  • number of electrons and holes is
    never equal
  • increases number of free charge carriers
    (p = more holes/ n = more e)
96
Q

Electromagnets

A
  • hans orsted
  • electric current passed through a coil creates a magnetic field
  • creates a switchable elecromagnet capable of exerting a magnetic effect on ferromagnetic materials
97
Q

magnetic field direction test

A

right hand grip

98
Q

magnetic flux density/current/force test

A

left hand rule
(Loser)
(lefts are stronger)

99
Q

solenoid

A
  • length of wire longer than radius of coil it produces
  • magnetic core = stronger
  • shape = magnetic field cancels in some areas
  • leaves same magnetic field as a bar magnet
100
Q

electromagnetic induction

A

when the magnetic field in a coil changes

  • an emf is induced in the coil
  • which induces a current
101
Q

Magnetic flux

A
  • not the same as magnetic flux density
  • weber
  • coil must be ⟂ to magnetic field
  • Φ = BA (x sinθ)

= 1 Wb if magnetic flux density = 1T over 1m² when it is perpendicular to a magnetic field

(if in a 1m² window Φ = B)

102
Q

Faraday’s law of electromagnetic induction

A

the magnitude of induced emf is directly proportional to the rate of change of flux

103
Q

what did faraday discover

A
  • magnetic field
  • electromagnetic induction
  • electrolysis
  • did experiments
104
Q

increasing electricity produced

A
  • faster moving magnet = faster changing flux

- stronger magnet

105
Q

Lenz’s law

A

direction of an induced current is always such as to oppose the charge producing it

(if not law of conservation of energy would be broken and magnet would be accelerated to a velocity of infinity)

106
Q

derivation of E (mf) = - dΦ/dt

A

1) according to Faraday’s law
E = change in Φ/ time

2) E = Φ2 - Φ1/t
3) E = k (Φ2 - Φ1/t)
if k = 1
4) E = Φ2 - Φ1/t
5) E = - dΦ/dt

(actually E = - (number of turns in coil) dΦ/dt

107
Q

Calculating heating effect of alternating current

A
  • constantly rising and falling so can’t calculate using P = I²R
  • need root mean square to change A.C to D.C equivalent (230V) (gets rid of - current)
108
Q

alternating current

A
  • produced by alternating voltage
  • one cycle (back and forth) = 1/50s = 0.02s
    (50 hz)
  • will be less bright than d.c with same peak values as actual voltage is not peak value
  • dc value reduced by a factor of √2 gives same value
109
Q

to find peak voltage/current

converting d.c to a.c

A

ac / √2 = dc (rms)

dc (rms) x √2 = ac

e.g 230v x √2

for current or voltage ^

needed to calculate P in

P = Irms x Vrms
P = I²rms x R
110
Q

mutual induction

A

when a changing magnetic field in one coil induces an emf in a nearby coil (adjacent)

111
Q

self induction

A

when a changing magnetic field in a coil induces a (back) emf in the coil itself that opposes the changing current

112
Q

induced emf depends on

A
  • higher rate of change of magnetic flux
  • soft iron core
  • increasing number of turns in coils
  • coils closer
113
Q

why effective voltage is less than applied

A

1) resistance of components

2) back emf (in ac)

114
Q

emf on dc

A
  • little effect
    only provides resistance (current flows in a constant direction)
  • only delays current flowing doesn’t stop it
115
Q

emf on ac

A
  • constantly opposes ac (current always changing direction)
  • back emf opposes rise and fall of current
  • smooths out fluctuations in current
  • dimmer switches
  • fine tuning radio stations (+ capacitors)
116
Q

inductor

A

coils with the property of self inductance

117
Q

Transformer

A

a device used to change the value of alternating voltage

with no electrical contact

118
Q

why soft iron core

A
  • improves magnetic effect

- increases induced emf

119
Q

How transformers work

A
  • alternating Vi causes alternating current to flow in Primary coil
  • alternating current causes alternating magnetic flux
  • alternating magnetic flux induces an emf in the secondary coil
120
Q

Step up

A

Np < Ns

Vi < Vo

121
Q

Step down

A

Np > Ns

Vi > Vo

122
Q

Transformer equations

A

Vi/Vo = Np/Ns

Vi Ip = Vo Is (not in tables book)

123
Q

Uses of Transformers

A

1) Long distance electricity transport
- joules law = I high = heat (energy) loss
- step up v (EHT) = lower I
- step down to 230v before entering
buildings

2) appliances that require higher/lower voltages

  • mobiles have transformer in plug
  • car spark plugs usually 12v - stepped up to allow spark (EHT)
124
Q

time taken for rectangular coil to enter field

A

length/speed = time
(distance needed to be traveled)/speed

  • if 6 cm side and travelling at 5m/s II to side
    0. 06m/5 = 0.012s
125
Q

ratio of Ip : Is compared to Np : Ns

A

opposite

126
Q

if dc supply replaced with ac supply?

A
  • self induction will occur

- back emf will be produced

Physics (16 decks)

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