electricity Flashcards
(175 cards)
1
Q
static electricity
A
occurs when there is a build up of electric charge on the surface of a material
2
Q
why its called static electricity
A
- bc the charges dont move
- the electricity we use everyday involves moving charges
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
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)
5
Q
unit of electric charge
A
the Coulomb (c)
-it is the amount of charge in 6.25 x 10¹⁸ electrons
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
7
Q
atoms
A
an tom as a whole is electrically neutralo ie. it has no overall electric charge
8
Q
charges
A
negatively charged -> object has gained electrons
positively charged -> object has lost electrons
9
Q
what moves when objects become charged
A
it is only the electrons that actually move when objects become charged
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
11
Q
insulator
A
any substance through which electric charge cannot flow is called an insulator
12
Q
conductor
A
any substance through which electric charge can flow is called a conductor
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
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
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
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
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
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)
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
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
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
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)
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)
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
25
Coulomb's Law equation
in hback
26
Coulumb's Law - Inverse square law
- 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
27
Permitivity
- The ability of a substance to store electrical energy in an electric field.
- Permitivity, ε, is different for different media
28
Permitivity of free space/a vacuum
If forces are in a vacuum, permitivity is known as permitivity of free space or of a vacuum ε0
29
Unit of permitivity
the Farad per metre
| F/m
30
equation
ε = εr x ε0
```
ε = permitivity
εr = relative permitivity
ε0 = in a vacuum
```
31
electric field
Any region of space where a static electric charge experiences a force other than the force of gravity
32
electric field line
- 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
33
applications
- electrostatic precipitators
- the photocopier
- effects on integrated circuit
34
electrostatic precipitators
a charge is transferred to dust by the point effect then attracted to plates used to clean air in chimneys
35
the photocopier
see earlier notes
36
effects on integrated circuit
static off people moving around can ruin computers, people working with ICs often need to be earthed
37
electric field strength, type, unit
-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)
38
electric field strength equation
E = F/Q
```
Q = charge
F = force
E = electric field strength
```
39
to show electric field patterns
- 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
40
potential difference (V), type, unit
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
41
volts and joule per coulomb
1 Volt = 1 Joule per Coulomb
42
Work done equation
W = QV
work done = charge transferred x voltage
43
How voltage is measured
using a voltmeter, can be estimated w/ a gold leaf electroscope
44
volt
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
45
potential at a point
potential difference between a point and the Earth is called the potential of that point
46
Earth potential
The Earth is at zero potential
47
Positive charge on a conductor
if a positive charge is added to a conductor it becomes more difficult to bring a charge from Earth to it, its potential increases
48
capacitance, unit
capacitance of a conductor is the ratio of the charge on the conductor to its potential
C = Q/V + notation
-unit: Farad (F)
49
capacitance equation
C = Q/V
```
C = capacitance
Q = charge
V = voltage/p.d
```
50
farads and coulomb per volt
1 farad = 1 coulomb per volt (C/V)
51
parallel plate capacitor
- two parallel plates separated by an insulator (dielectric)
- a capacitor stores charges
- when, the plates carry equal but opposite charges
52
parallel plate capacitor - to show it stores energy:
- set up equipment
- charge capacitor by connecting the battery
- remove battery + connect bulb
- bulb will flash as capacitor discharges - capacitor stores energy
53
Capacitors and current
capacitors conduct alternating current but not direct
54
uses of capacitors
- tuning radios
- flash guns on cameras
- smoothing out variations in direct currents
- filtering unwanted frequencies in sound systems
55
diagrams
hardback
56
capacitance of parallel plate capacitor equation
C = εA/d
```
C = capacitance
ε = permitivity of the dielectric
A = overlap of plates
d = distance between plates
```
57
energy stored in a charged capacitor equation
W = 1/2 CV²
```
W = energy stored
C = capacitance
V = p.d
```
58
capacitance of a parallel plate capacitor depends on:
- the distance between the plates
- the common area of the plates
- the nature of the dielectric
59
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
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
60
current
a flow of electric charge
61
conductor
allows charge to flow eg. wire, acid
62
insulator
tries to stop the flow of electrical charge eg. plastic, glass
63
3 effects of current
- heating
- magnetic
- chemical
64
3 effects of current - heating
bulbs, battery + wire heat up as current flows
65
3 effects of current - magnetic
current causes a compass to deflect off N-S
66
3 effects of current - chemical
wires in sulphuric acid + wire will bubble as current flows
67
charge, unit
particles that exert electrostatic forces on each other are said to be charged
-unit: Coulomb (C)
68
current, unit
- an electric current is charged particles moving
- in a metal conductor, the electric current is a flow of electrons
-unit: Ampere (A)
69
ampere and coulomb per second
1 ampere = 1 coulomb per second
1 A = 1 C/S
70
Charge equation
C = At
charge = current x time
71
Electrons charge
the charge on an electron is 1.6 x 10⁻¹⁴ coulombs
72
Protons charge
same as electron
73
Conventional current
a conventional current is said to flow from positive to negative, but in a metal, electrons actually flow in the opposite direction
74
current in a series circuit
the current is the same at every point in a series circuit
75
direct current (DC)
-current flows in one direction
76
where direct current happens
happens in electric cells or batteries
77
measuring size of an electric current
- use an ammeter (must be connected in series)
| - for very small currents, a galvonometer can be used
78
alternating current
if an electric current is a circuit reverses directions every so often
eg. the electricity in our homes is alternating current
79
sum of currents flowing in a junction
the sum of the currents flowing into a junction is the same as the sum of the currents leaving a junction
80
current in series
i = i₁ = i₂
the current is the same at every point in a series circuit
81
current in parallel
i = i₁ + i₂
82
potential difference
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
potential difference equation
V = W/Q
V = energy given out/charge gone past
84
P = VI proof
hardback
85
voltage in series
sum of the voltages across each part
V = V₁ + V₂ + V₃
86
voltage in parallel
V₁ = V₂ = V₃
87
electromotive force, symbol, unit
-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
what drives electrons to keep moving around a circuit?
electromotive force
89
sources of emf
- electric cells (2 plates in an electrolyte)
- simple cell
- primary and secondary cells
- thermocouple
- mains electricity
90
a simple cell
- 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
resistance (R), unit
the ratio of the p.d. across a conductor to the current flowing through it
-unit: ohm Ω
92
resistance equation
R = V/I
93
ohm's law
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
resistors in series
in series, the total resistance is:
| R = R₁ + R₂ + R₃
95
resistors in parallel
1/R = 1/R₁ + 1/R₂ + 1/R₃
96
factors affecting resistance of a conductor
- temperature
- material of conductor
- length
- cross-sectional area
97
temperature
- 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
length
resistance of a uniform conductor is directly proportional to its length
ie. R α L
99
cross-sectional area
resistance of a uniform conductor is inversely proportional to its cross-sectional area
ie. R α 1/A
100
material
material also affects the resistance of a conductor by a fixed amount for different materials
this is known as resistivity (ρ)
101
Resistance equation
R = pL/A
p = constant of proportionality
unit: ohm meter (Ωm)
102
resistivity equation
ρ = Rπ²/4L
103
wheatstone bridge
hardback, know diagram and formula and what each symbol stands for in it
104
uses of wheatstone bridge
- temperature control
- fail-safe device (switch circuit off)
- measure an unknown resistance
- galvonmeter detects small current
105
potential divider circuits
- a variable potential divider
| - the moving constant moves from A to B
106
effects of an electric current
heat
chemical
magnetic
107
joule's law
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
power lines
in order to prevent power lines from overheating, electricity is transmitted at a very high voltage
EHT = Extra High Tension
109
joule's law - current and heat
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
effects of an electric current
- electrolysis
- voltameter
- inactive electrodes
- active electrodes
- ion
- charge carries
111
electrolysis
the chemical effect of an electric current
112
voltameter
consists of electrodes, an electrolyte and a container
113
inactive electrodes
inactive electrodes are electrodes that don't take part in the chemical reaction eg. platinum in H₂SO₄
114
active electrodes
active electrodes are electrodes that take part in the chemical reaction eg. copper in CuSO₄
115
ion
ion is an atom or molecule that has lost or gained 1 or more electrons
116
charge carriers
charge carriers in an electrolyte are + and - carries
117
uses of electric currents
- electroplating to make metal look better, prevent corrosion
- purifying metals
- making electrolyte capacitors
118
relationship between V and I for conductors
```
metallic conductor
filament bulb
semiconductor
active electrodes
inactive (inert) electrodes
gas
vacuum
```
119
metallic conductor (metals)
negative electrons are the charge carriers
120
filament filament bulb
negative electrons are the charge carriers
121
semiconductor
negative electrons and positive holes are the charge carriers
122
active electrodes
positive and negative ions are the charge carriers
123
inactive (intert) electrodes
positive and negative ions and electrons are the charge carriers
124
gas
positive and negative ions and electrons are the charge carriers
125
vacuum
electrons are the charge carriers
126
domestic electric circuits
- 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
radial circuit
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
ring circuit
used for connections to sockets
live terminals are connected together as are the neutral terminal
129
lights
connected in parallel and a number of them are connected to the same fuse
130
safety in house circuits
```
switch
fuse
MCBs
RCDs
bonding
earthing
```
131
switch
should always be connected in the live wire
132
fuse
piece of wire that will melt when a current of a certain size passes through it. Connected to the live wire
133
MCBs
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
RCDs
residual current devices protect sockets and people against electrocution by detecting a difference between current in live and neutral wire (30 mA)
135
bonding
all metal taps, pipes, water tank etc are connected to the earth
136
earthing
earth wire prevents electrocution from touching metal parts of appliances by providing a path of least resistance when faults occur
137
E.S.B - Kilowatt-hour kWh:
- 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
charge carriers when an electric current passes through an electrolyte
ions
139
I, Q, t, fomula
I = Q/t
140
ratio of current through resistors in parallel
know how ratio goes (notes on booklet)
141
wheatstone bridge diagram
in notes
142
how you would know a wheatstone bridge is balanced
zero reading on galvanometer
143
radius of a wire is doubled .effect on resistance?
resistance decreases by a factor of four
144
What is the net charge of a capacitor?
0 C
145
diagram of electric field between charged plates of a parallel plate capacitor
know diagram
146
Farad definition
coulomb per volt
147
why is it more economical to transmit electrical energy at high voltage?
low current
| less heat loss
148
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
use C = εA/d and C = Q/V
| and rearrange
149
electric field definition
an electric field is a region of space where electrostatic forces are experienced
150
define the volt
potential difference between two points if 1 J of work is needed to move 1 C from one point to the other
151
factors that affect the capacitance of a parallel plate capacitor
- common area of plates
- distance apart
- permitivity of dielectric
152
how does a fuse operate?
- fuse in live part of circuit gets hot if current exceeds a certain rated value
- melts/breaks
- circuit is broken
153
what test would you use to determine that a wheatstone bridge is balanced?
connect galvanometer (across points AC)
no deflection in galvanometer when balanced
154
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?
light dependent resistor / l.d.r / photoresistor
155
use of a light dependent resistor
- used in light meters
- to control street lights
- security alarms
- control traffic lights
- used in re-charging circuits
156
resistivity + unit
resistance of cube of material of side 1m
formula + notation
unit: ohm metre Ωm
157
the toaster has exposed metal parts. How is the risk of electrocution minimised ?
metal parts are earthed
158
why is light emitted when a metal is heated
- electrons excited/gain energy
- jump to higher energy state
- return to lower state
- emit energy/light
159
what happens to resistance of wire as temp falls bellow 0
R decreases
160
what happens to resistance of wire as length increases
R increases
161
what happens to resistance of wire as diameter is increased
R decreases
162
devices used to measure resistance
-ohmmeter
-wheatstone bridge
-multimeter
ammeter + voltmeter
163
Advantage of using an ohmmeter/wheatstone bridge
ohmmeter: -compact, portable, faster method
wheatstone bridge: compact, portable, more accurate
164
Disadvantage of using ohmmeter/wheatstone bridge
ohmmeter: less accurate, fragile, difficult to calibrate
wheatstone bridge: 'black box' difficult to comprehend, expensive
165
explain why resistance of thermistor decreases as temp increases
- more energy added to thermistor
- more electrons produced/released
- resistance is reduced bc more electrons/charge carriers are available for conduction
166
why potential increases as temp increases (thermistor)
2005 q9 qs
- resistance of thermistor decreases
- pd across thermistor and x decreases
- potential at A increasess
167
explain how point discharge occurs
- charge accumulates at a point
- air is ionised around the point by large electric field
- opposite charges neutralise the charge at the point
168
diagram of gold leaf elecroscope
in notes
169
describe how an electroscope can be given a negative charge by induction
- positively charged rod brought close to cap
- earth cap
- remove earth and then remove rod
170
when asked to find electric field strength
find the value, then write direction
| eg. E = 3.0x10^-2 away (from centre of the dome)
171
input vs output voltage diagrams
in notes
172
electric field strength diagram between two like charges
know how to draw, has curved area between them
173
electric field strength diagram between two unlike charges
know how to draw, also has curved area between them going into each other
174
where do u connect the galvanometer in a wheatstone bridge
across the points that aren't connected to the power supply? (look at diagram)
175
current, charge, time formula (not in log table)
I = q/t
