electricity Flashcards

1
Q

static electricity

A

occurs when there is a build up of electric charge on the surface of a material

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2
Q

why its called static electricity

A
  • bc the charges dont move

- the electricity we use everyday involves moving charges

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3
Q

cause of static electricity

A
  • static electricity is caused when certain materials are rubbed against each other
  • electrons can be rubbed off one material and onto another
  • the material that has got extra electrons is now negatively charged. The material that has lost electrons is positively charged
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4
Q

dangers of static electricity

A
  • static charges can be dangerous
    eg: planes must be earthed before refuelling to allow the charge to leave, so there is not an explosion
    eg: if dust gets into certain machines and work areas they can do serious damage (car painting, flour mill, food preperation)
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5
Q

unit of electric charge

A

the Coulomb (c)

-it is the amount of charge in 6.25 x 10¹⁸ electrons

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6
Q

induced charge

A
  • in a metal, there are free electrons that can move
  • if a negatively charged rod is brought near the metal, these free electrons will move to the opposite side of the material
  • the protons will move to the side nearest the rod
  • if the rod is taken away, they move back again
  • the charges produced is INDUCED CHARGE
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7
Q

atoms

A

an tom as a whole is electrically neutralo ie. it has no overall electric charge

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8
Q

charges

A

negatively charged -> object has gained electrons

positively charged -> object has lost electrons

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9
Q

what moves when objects become charged

A

it is only the electrons that actually move when objects become charged

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10
Q

charging by induction

A

-if we bring a negatively charged rod near to two metal spheres that are touching each other, the free negative electrons will go into the sphere that is furthest from the rod while positive charge will go to the near one

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11
Q

insulator

A

any substance through which electric charge cannot flow is called an insulator

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12
Q

conductor

A

any substance through which electric charge can flow is called a conductor

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13
Q

charging a single object by induction

A
  • a single insulated conductor can be charged by induction
  • bring a negatively charged rod to a conductor, touch the conductor with your finger, the negative charge will travel through you to earth
  • remove your finger, then the red, the conductor will be positively charged
  • the same can be done to make a negatively charged conductor
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14
Q

gold leaf electroscope

A

-consists of:
•v thin gold leaf attached to one end of metal rod
•other end of rod has metal disc (cap) attached
•leaf + rod are in metal case w/ window

  • case + window stop droughts from casing leaf to move
  • rod insulated from case so charge on rod does not flow away
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15
Q

uses of gold leaf electroscope

A
  • detect charge
  • indicate approximate size of a charge
  • test whether a charge is + or -
  • test if object is an insulator or a conductor
  • indicate the size of a potential difference
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16
Q

detect charge

A
  • if charged object brought near, induced charges appear on electroscope
  • due to force of repulsion between charges, leaf diverges
  • thus, it detects electric charge
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17
Q

indicate approximate size of a charge

A
  • place objects w/ diff charges the same distance from electroscope
  • larger the charge on object, the greater the divergence produced
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18
Q

test whether a char is + or -

A
  • give electroscope a charge of known sign
  • bring object w/ unknown charge near cap
  • if divergence of leaf increases, object + electroscope have charge of same sign
  • if leaf collapses, opposite charges (provided that when charge removed, leaf diverges again)
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19
Q

test if object is an insulator or a conductor

A
  • charge electroscope, then, holding object in hand, touch cap w/ object
  • if leaf collapses, object is a conductor
  • otherwise, insulator
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20
Q

conductors

A
  • all charges resides on outside of a conductor eg. dome of VdG generator
  • static charge tends to accumulate where it is most pointed
  • can be showen using VdG generator
  • these things lead to development of lightning conductor, that safely brings charge to ground to protect buildings etc
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21
Q

point discharge

A
  • if there is a sharp point on an object there is a large charge
  • this leads to v strong force, forming around point
  • ions are attracted + repelled to and from the point
  • leads to other ions being attracted + repelled and eventually the charge on point is cancelled out
  • this loss of charge is known as “point discharge” or “point effect”

know diagram of how it occurs - diagram with concentration of charge at point

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22
Q

experiment: to show that all static charge resides on the outside of a hollow metal conductor

A

1) connect a cylindrical metal can (the hollow conductor) to dome of Van de Graaff generator + turn on generator

2) Touch a proof plane against inside of can + bring proof plane very near cap of an uncharged electroscope
- -leaf will not diverge

3) touch a proof plane against outside of of can + bring proof plane very near cape of an uncharged electroscope
- -leaf will diverge (showing static charge is on outside)

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23
Q

Van de Graaff generator

A

needs to be smooth so it does not lose its charge (any charge on it will stay on the dome and not leak off)

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24
Q

Coulumb’s Law

A

states that the force of attraction or repulsion between two points is directly proportional to the product of the charges and inversely proportional to the square of the distance between them

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25
Q

Coulomb’s Law equation

A

in hback

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26
Q

Coulumb’s Law - Inverse square law

A
  • this law is an example of inverse square law
  • if distance doubled, force is four times smaller
  • if distance is made three times bigger, force is 9 times smaller
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27
Q

Permitivity

A
  • The ability of a substance to store electrical energy in an electric field.
  • Permitivity, ε, is different for different media
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28
Q

Permitivity of free space/a vacuum

A

If forces are in a vacuum, permitivity is known as permitivity of free space or of a vacuum ε0

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29
Q

Unit of permitivity

A

the Farad per metre

F/m

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30
Q

equation

A

ε = εr x ε0

ε = permitivity
εr = relative permitivity
ε0 = in a vacuum
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31
Q

electric field

A

Any region of space where a static electric charge experiences a force other than the force of gravity

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32
Q

electric field line

A
  • A line drawn in an electric field showing the direction of the force on a positive charge placed in the field
  • The stronger the electric field the closer the lines are together
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33
Q

applications

A
  • electrostatic precipitators
  • the photocopier
  • effects on integrated circuit
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34
Q

electrostatic precipitators

A

a charge is transferred to dust by the point effect then attracted to plates used to clean air in chimneys

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35
Q

the photocopier

A

see earlier notes

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36
Q

effects on integrated circuit

A

static off people moving around can ruin computers, people working with ICs often need to be earthed

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37
Q

electric field strength, type, unit

A

-electric field strength E at a point in an electric field in the force per unit charge at that point

formula + notation

  • type: vector quantity
  • unit: newton per coulomb (N/C)
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38
Q

electric field strength equation

A

E = F/Q

Q = charge
F = force
E = electric field strength
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39
Q

to show electric field patterns

A
  • use equipment in diagram
  • connect a high voltage source to the metal plates which are in the oil
  • the semolina lines up in the direction of the field, showing the electric field
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40
Q

potential difference (V), type, unit

A

potential difference between two points in an electric field is the work done in bringing a charge of +1C from one point to the other

  • unit: joule per coulomb (J/C) also known as the volt (V)
  • type: scalar quanitity
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41
Q

volts and joule per coulomb

A

1 Volt = 1 Joule per Coulomb

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42
Q

Work done equation

A

W = QV

work done = charge transferred x voltage

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43
Q

How voltage is measured

A

using a voltmeter, can be estimated w/ a gold leaf electroscope

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44
Q

volt

A

the potential difference between two points is 1 volt if 1 joule of work is done when 1 coulomb is brought from one point to the other

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45
Q

potential at a point

A

potential difference between a point and the Earth is called the potential of that point

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46
Q

Earth potential

A

The Earth is at zero potential

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47
Q

Positive charge on a conductor

A

if a positive charge is added to a conductor it becomes more difficult to bring a charge from Earth to it, its potential increases

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48
Q

capacitance, unit

A

capacitance of a conductor is the ratio of the charge on the conductor to its potential

C = Q/V + notation

-unit: Farad (F)

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49
Q

capacitance equation

A

C = Q/V

C = capacitance
Q = charge
V = voltage/p.d
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50
Q

farads and coulomb per volt

A

1 farad = 1 coulomb per volt (C/V)

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51
Q

parallel plate capacitor

A
  • two parallel plates separated by an insulator (dielectric)
  • a capacitor stores charges
  • when, the plates carry equal but opposite charges
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52
Q

parallel plate capacitor - to show it stores energy:

A
  • set up equipment
  • charge capacitor by connecting the battery
  • remove battery + connect bulb
  • bulb will flash as capacitor discharges - capacitor stores energy
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53
Q

Capacitors and current

A

capacitors conduct alternating current but not direct

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54
Q

uses of capacitors

A
  • tuning radios
  • flash guns on cameras
  • smoothing out variations in direct currents
  • filtering unwanted frequencies in sound systems
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55
Q

diagrams

A

hardback

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56
Q

capacitance of parallel plate capacitor equation

A

C = εA/d

C = capacitance
ε = permitivity of the dielectric
A = overlap of plates
d = distance between plates
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57
Q

energy stored in a charged capacitor equation

A

W = 1/2 CV²

W = energy stored
C = capacitance
V = p.d
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58
Q

capacitance of a parallel plate capacitor depends on:

A
  • the distance between the plates
  • the common area of the plates
  • the nature of the dielectric
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59
Q

to show that the capacitance of a parallel plate capacitor depends on:

  • the distance between the plates
  • the common area of the plates
  • the nature of the dielectric
A

Procedure:

  • use equipment in diagram. Divergence of leaf is measure of the potential difference between plates
  • since C = Q/V and amount of charge Q is fixed, follows that the greater the divergence, the smaller the capacitance and vice versa
  • charge plates by connecting them across a high voltage source (say, 2000V)
  • move plates closer together ie. decrease d. Divergence of leaf decreases => C increases. If d is increased, opposite happens
  • decrease overlap area + divergence increases => C decreases. If overlap area increased, opposite happens.
  • Place diff slabs of insulating material between the plates. Divergence will be seen to be less than what it is for air
  • Diff materials cause capacitance to increase over its value when dielectric in air
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60
Q

current

A

a flow of electric charge

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61
Q

conductor

A

allows charge to flow eg. wire, acid

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62
Q

insulator

A

tries to stop the flow of electrical charge eg. plastic, glass

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63
Q

3 effects of current

A
  • heating
  • magnetic
  • chemical
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64
Q

3 effects of current - heating

A

bulbs, battery + wire heat up as current flows

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65
Q

3 effects of current - magnetic

A

current causes a compass to deflect off N-S

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66
Q

3 effects of current - chemical

A

wires in sulphuric acid + wire will bubble as current flows

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67
Q

charge, unit

A

particles that exert electrostatic forces on each other are said to be charged

-unit: Coulomb (C)

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68
Q

current, unit

A
  • an electric current is charged particles moving
  • in a metal conductor, the electric current is a flow of electrons

-unit: Ampere (A)

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69
Q

ampere and coulomb per second

A

1 ampere = 1 coulomb per second

1 A = 1 C/S

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70
Q

Charge equation

A

C = At

charge = current x time

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71
Q

Electrons charge

A

the charge on an electron is 1.6 x 10⁻¹⁴ coulombs

72
Q

Protons charge

A

same as electron

73
Q

Conventional current

A

a conventional current is said to flow from positive to negative, but in a metal, electrons actually flow in the opposite direction

74
Q

current in a series circuit

A

the current is the same at every point in a series circuit

75
Q

direct current (DC)

A

-current flows in one direction

76
Q

where direct current happens

A

happens in electric cells or batteries

77
Q

measuring size of an electric current

A
  • use an ammeter (must be connected in series)

- for very small currents, a galvonometer can be used

78
Q

alternating current

A

if an electric current is a circuit reverses directions every so often

eg. the electricity in our homes is alternating current

79
Q

sum of currents flowing in a junction

A

the sum of the currents flowing into a junction is the same as the sum of the currents leaving a junction

80
Q

current in series

A

i = i₁ = i₂

the current is the same at every point in a series circuit

81
Q

current in parallel

A

i = i₁ + i₂

82
Q

potential difference

A

the potential difference between two point is V volts and the energy given out between the point is W joules when a charge of Q coulombs passes any point in the circuit

83
Q

potential difference equation

A

V = W/Q

V = energy given out/charge gone past

84
Q

P = VI proof

A

hardback

85
Q

voltage in series

A

sum of the voltages across each part

V = V₁ + V₂ + V₃

86
Q

voltage in parallel

A

V₁ = V₂ = V₃

87
Q

electromotive force, symbol, unit

A

-to keep a current flowing in a circuit, an electric field must be maintained in that circuit. There must be a p.d between the ends of the circuit. This p.d is called an electromotive force (emf)

  • symbol: E
  • unit: volt
88
Q

what drives electrons to keep moving around a circuit?

A

electromotive force

89
Q

sources of emf

A
  • electric cells (2 plates in an electrolyte)
  • simple cell
  • primary and secondary cells
  • thermocouple
  • mains electricity
90
Q

a simple cell

A
  • a typical simple cell consists of a copper plte and a zinc plate in a beaker of dilute sulphuric acid
  • the plates react chemically with the acid, causing the zinc plate to become negatively charged + the copper plate positively charged
  • as current is drawn from the cell, the chemicals are used up
  • when they are fully used, no more current can be got from the cell. Such a cell cannot be recharged. This cell is not very practical and its emf is about -1V
91
Q

resistance (R), unit

A

the ratio of the p.d. across a conductor to the current flowing through it

-unit: ohm Ω

92
Q

resistance equation

A

R = V/I

93
Q

ohm’s law

A

states that for certain conductors (mainly metals) the current flowing through them is direction proportional to the p.d. across them at a constant temperature

V α I or V = IR

94
Q

resistors in series

A

in series, the total resistance is:

R = R₁ + R₂ + R₃

95
Q

resistors in parallel

A

1/R = 1/R₁ + 1/R₂ + 1/R₃

96
Q

factors affecting resistance of a conductor

A
  • temperature
  • material of conductor
  • length
  • cross-sectional area
97
Q

temperature

A
  • the resistance of a metallic conductor increases as the temperature increases eg. copper
  • the resistance of a semiconductor/insulator decreases as the temperature increases eg. thermistors
98
Q

length

A

resistance of a uniform conductor is directly proportional to its length

ie. R α L

99
Q

cross-sectional area

A

resistance of a uniform conductor is inversely proportional to its cross-sectional area

ie. R α 1/A

100
Q

material

A

material also affects the resistance of a conductor by a fixed amount for different materials
this is known as resistivity (ρ)

101
Q

Resistance equation

A

R = pL/A

p = constant of proportionality
unit: ohm meter (Ωm)

102
Q

resistivity equation

A

ρ = Rπ²/4L

103
Q

wheatstone bridge

A

hardback, know diagram and formula and what each symbol stands for in it

104
Q

uses of wheatstone bridge

A
  • temperature control
  • fail-safe device (switch circuit off)
  • measure an unknown resistance
  • galvonmeter detects small current
105
Q

potential divider circuits

A
  • a variable potential divider

- the moving constant moves from A to B

106
Q

effects of an electric current

A

heat
chemical
magnetic

107
Q

joule’s law

A

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

ie. P = I²R

108
Q

power lines

A

in order to prevent power lines from overheating, electricity is transmitted at a very high voltage

EHT = Extra High Tension

109
Q

joule’s law - current and heat

A

from Joule’s Law, the larger the current, the more heat produced. Hence, a transformer is use to increase voltage and lower current

ie. P = VI

110
Q

effects of an electric current

A
  • electrolysis
  • voltameter
  • inactive electrodes
  • active electrodes
  • ion
  • charge carries
111
Q

electrolysis

A

the chemical effect of an electric current

112
Q

voltameter

A

consists of electrodes, an electrolyte and a container

113
Q

inactive electrodes

A

inactive electrodes are electrodes that don’t take part in the chemical reaction eg. platinum in H₂SO₄

114
Q

active electrodes

A

active electrodes are electrodes that take part in the chemical reaction eg. copper in CuSO₄

115
Q

ion

A

ion is an atom or molecule that has lost or gained 1 or more electrons

116
Q

charge carriers

A

charge carriers in an electrolyte are + and - carries

117
Q

uses of electric currents

A
  • electroplating to make metal look better, prevent corrosion
  • purifying metals
  • making electrolyte capacitors
118
Q

relationship between V and I for conductors

A
metallic conductor
filament bulb
semiconductor
active electrodes
inactive (inert) electrodes
gas
vacuum
119
Q

metallic conductor (metals)

A

negative electrons are the charge carriers

120
Q

filament filament bulb

A

negative electrons are the charge carriers

121
Q

semiconductor

A

negative electrons and positive holes are the charge carriers

122
Q

active electrodes

A

positive and negative ions are the charge carriers

123
Q

inactive (intert) electrodes

A

positive and negative ions and electrons are the charge carriers

124
Q

gas

A

positive and negative ions and electrons are the charge carriers

125
Q

vacuum

A

electrons are the charge carriers

126
Q

domestic electric circuits

A
  • electricity entering the home is supplied at 230are the charge carriers a.c.
  • 2 wires enter the house from the mains: live + neutral and pass through the meter box
  • these 2 wires pass into a distribution box with fuses
127
Q

radial circuit

A

used for appliances that take a large current

  • each closed circuit has its own live wire, neutral wire, and fuse
    eg. cooker, electric shower
128
Q

ring circuit

A

used for connections to sockets

live terminals are connected together as are the neutral terminal

129
Q

lights

A

connected in parallel and a number of them are connected to the same fuse

130
Q

safety in house circuits

A
switch
fuse
MCBs
RCDs
bonding
earthing
131
Q

switch

A

should always be connected in the live wire

132
Q

fuse

A

piece of wire that will melt when a current of a certain size passes through it. Connected to the live wire

133
Q

MCBs

A

miniature circuit breakers are found in the distribution box. They are bimetallic (for small currents) and electromagnets (for large currents). Can be reset when the switch trips, faster than fuse

134
Q

RCDs

A

residual current devices protect sockets and people against electrocution by detecting a difference between current in live and neutral wire (30 mA)

135
Q

bonding

A

all metal taps, pipes, water tank etc are connected to the earth

136
Q

earthing

A

earth wire prevents electrocution from touching metal parts of appliances by providing a path of least resistance when faults occur

137
Q

E.S.B - Kilowatt-hour kWh:

A
  • the amount of energy used by a 1000W appliance in one hour

- the ESB charge bills based on the no. of units, kWh, used in the home

138
Q

charge carriers when an electric current passes through an electrolyte

A

ions

139
Q

I, Q, t, fomula

A

I = Q/t

140
Q

ratio of current through resistors in parallel

A

know how ratio goes (notes on booklet)

141
Q

wheatstone bridge diagram

A

in notes

142
Q

how you would know a wheatstone bridge is balanced

A

zero reading on galvanometer

143
Q

radius of a wire is doubled .effect on resistance?

A

resistance decreases by a factor of four

144
Q

What is the net charge of a capacitor?

A

0 C

145
Q

diagram of electric field between charged plates of a parallel plate capacitor

A

know diagram

146
Q

Farad definition

A

coulomb per volt

147
Q

why is it more economical to transmit electrical energy at high voltage?

A

low current

less heat loss

148
Q

write an expression for the charge stored on one plate of a parallel plate capacitor in terms of the pd between the plates, their common area, distance between them, and the permitivity of the dielectric

A

use C = εA/d and C = Q/V

and rearrange

149
Q

electric field definition

A

an electric field is a region of space where electrostatic forces are experienced

150
Q

define the volt

A

potential difference between two points if 1 J of work is needed to move 1 C from one point to the other

151
Q

factors that affect the capacitance of a parallel plate capacitor

A
  • common area of plates
  • distance apart
  • permitivity of dielectric
152
Q

how does a fuse operate?

A
  • fuse in live part of circuit gets hot if current exceeds a certain rated value
  • melts/breaks
  • circuit is broken
153
Q

what test would you use to determine that a wheatstone bridge is balanced?

A

connect galvanometer (across points AC)

no deflection in galvanometer when balanced

154
Q

when an unknown resistor is covered by a piece of black paper, the wheatstone bridge goes out of balance. What type of resistor is it?

A

light dependent resistor / l.d.r / photoresistor

155
Q

use of a light dependent resistor

A
  • used in light meters
  • to control street lights
  • security alarms
  • control traffic lights
  • used in re-charging circuits
156
Q

resistivity + unit

A

resistance of cube of material of side 1m
formula + notation

unit: ohm metre Ωm

157
Q

the toaster has exposed metal parts. How is the risk of electrocution minimised ?

A

metal parts are earthed

158
Q

why is light emitted when a metal is heated

A
  • electrons excited/gain energy
  • jump to higher energy state
  • return to lower state
  • emit energy/light
159
Q

what happens to resistance of wire as temp falls bellow 0

A

R decreases

160
Q

what happens to resistance of wire as length increases

A

R increases

161
Q

what happens to resistance of wire as diameter is increased

A

R decreases

162
Q

devices used to measure resistance

A

-ohmmeter

-wheatstone bridge
-multimeter
ammeter + voltmeter

163
Q

Advantage of using an ohmmeter/wheatstone bridge

A

ohmmeter: -compact, portable, faster method

wheatstone bridge: compact, portable, more accurate

164
Q

Disadvantage of using ohmmeter/wheatstone bridge

A

ohmmeter: less accurate, fragile, difficult to calibrate

wheatstone bridge: ‘black box’ difficult to comprehend, expensive

165
Q

explain why resistance of thermistor decreases as temp increases

A
  • more energy added to thermistor
  • more electrons produced/released
  • resistance is reduced bc more electrons/charge carriers are available for conduction
166
Q

why potential increases as temp increases (thermistor)

2005 q9 qs

A
  • resistance of thermistor decreases
  • pd across thermistor and x decreases
  • potential at A increasess
167
Q

explain how point discharge occurs

A
  • charge accumulates at a point
  • air is ionised around the point by large electric field
  • opposite charges neutralise the charge at the point
168
Q

diagram of gold leaf elecroscope

A

in notes

169
Q

describe how an electroscope can be given a negative charge by induction

A
  • positively charged rod brought close to cap
  • earth cap
  • remove earth and then remove rod
170
Q

when asked to find electric field strength

A

find the value, then write direction

eg. E = 3.0x10^-2 away (from centre of the dome)

171
Q

input vs output voltage diagrams

A

in notes

172
Q

electric field strength diagram between two like charges

A

know how to draw, has curved area between them

173
Q

electric field strength diagram between two unlike charges

A

know how to draw, also has curved area between them going into each other

174
Q

where do u connect the galvanometer in a wheatstone bridge

A

across the points that aren’t connected to the power supply? (look at diagram)

175
Q

current, charge, time formula (not in log table)

A

I = q/t