Electricity Flashcards
(126 cards)
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
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
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)
4
Q
Static Electricity
A
electrons unmoving
5
Q
current electricity
A
electrons moving
6
Q
conductor
A
a medium which allows electric charge to flow through it
7
Q
insulator
A
a medium which does not allow electric charge to flow through it
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)
9
Q
electrostatic law
A
- depends on charge
- attractive and repulsive
- strong
10
Q
Newton’s gravitational law
A
- depends on mass (too small = none)
- attractive
- weak
11
Q
Dielectric Material
A
- good insulator ( allows charge to build up on plates ) - paper - plastic - ceramic - air/free space
12
Q
Charge on Sphere
A
- centre
- factor in radius
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
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
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
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)
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)
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
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
20
Q
Electric Field
A
A region in space where the force of an electric charge can be felt
21
Q
Inspection Window
A
Shows Flux density
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
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
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
25
Mains electricity ireland
- 50 hz
- changes direction every 1/100 sec
- 230 V a.c
- alternating cycle of electricity
- produces sinusoidal wave
26
Capacitor
a device capable of storing charge for rapid release
27
capacitance
the ratio of charge stored to voltage
28
factors that increase capacitance ( to put more charge on an object when it is already at full capacity)
- 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
29
current definition
- flow of electrons
- how many electrons pass a point every second
```
I = q/t
(A = q/t) ( 6.25 x 10^18 e/s)
```
30
Joules Law
the rate of heat produced in a constant resistance conductor is proportional to the square of the current
31
1 coulomb
the charge that passes when 1 Ampere flows for 1 second
32
semi conductor
a conductor whose resistivity is between then of a good insulator and a good conductor.
as temp increases its resistance goes down
33
Properties of magnets
- attract ferromagnetic material and cause temporary magnetism in them
- when freely suspended will point roughly N and S
- poles - like repels
- opposite attracts
34
magnetic field
- a region in space where attractive/repulsive magnetic forces can be can be felt
35
magnetic field lines
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
36
who invented the left hand rule?
- Flemming
- how magnetic field effects a current carrying conductor
- force = left hand
- F, B, I
37
Current in a magnetic field
- current carrying conductors have a magnetic field around them
- when placed in another magnetic field creates a force
38
Force
- perpendicular to current and voltage
| - will not be produced if current is II to magnetic field
39
Force on a current carrying coil
- equal opposing forces cause turning effect
| - commutator/slip ring keeps forces continually turning
40
Generator
- movement in (water heated into steam which turns turbine)
| - electricity out
41
turbine
- makes the length of wire longer
- more current
- bigger magnetic field
- more force
- more electricity
42
Magnitude of force
F=BIL
43
Magnetic Flux Density
- 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
44
Force when magnetic field is not perpendicular to current
F = BIL x sin( θ
(sin θ to find perpendicular magnetic flux!!)
( Magnetic flux given x sin θ = ⟂B)
45
Earth wire
- 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)
46
Live wire
- on right when looking at back of plug
- brown
- attached to fuse
47
Neutral wire
- On left when looking at back of plug
- blue
- carries current but with neutral voltage
48
Where do safety devices go
- switch and fuse are on live wire in front of device so you can safely work on device
49
Devices with separate circuit breakers
- electric shower
- oven
- a lot of current
50
Only device not connected to circuit breaker box
- alarm
| - because master switch is outside before box so can be pulled out by thieves
51
MCB
- miniture circuit breaker
- electromagnetic switch set for a certain amount of current
- if current exceeds that it turns off
- REUSABLE
(usually on lights)
52
RCD
- 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
Fuse in plug
- will melt if current exceeds fuse expectancy
- value slightly above current value
- NOT REUSABLE
54
Safety devices
- large ceramic fuse
- master switch
- RCD
- MCB
- fuse in socket
- fuse in plug
- earth
55
Plugging in
- connects live to neutral wire so current flows
56
ring main circuit
- 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
radial ciruit
- 1 arm out and back
- individual appliances (oven)
- own neutral, fuse in the live as higher voltage required
- mcb.
58
lighting
connected in parallel so one fault does not cause more
59
KWh
- kilowatt hour
- 1500 W uses 1.5 kwh
- kwh x number of hours used = kwh used
60
energy used
- figure out time used in seconds
- 1500 W heater = 1500 J/s
- times J/s by number of seconds
61
p-n junction
where a p-type semi conductor and an n-type semi conductor meet
62
electric current
flow of electric charge, usually carried by moving electrons in a wire
63
semi- conductor diode
- a device which allows current to flow in only one direction
- p-type and n-type semi conductor in contact
64
effects of electric current
- 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
RCD has rating of 30 micro amps, significance?
current is cut off if the difference between the live an neutral currents is greater than 30 micro amps
66
galvanometer can be changed into a voltmeter
large resistance
| in series
67
resistance vs resistivity
- equations and label
68
effect on resistance of a wire if diameter is incresed by 3 §
decreases by 9
69
RCD will disconnect
- when current flowing differs to the current flowing out
| - leaking of current to earth
70
residual circuit ring
- reduces current as current takes two paths to socket reducing the current in each path
71
EHT used to transport electricity
- high voltage
= lower current
= less heat (energy loss)
72
one amp
- one coulomb per second
| - 6.25 x 10^18 electrons/ second
73
electricity
form of energy involving electrons
74
to demonstrate charge accumulates at a point
- touch pear shaped conductor off of dome
- hold round end near GLE
- hold pointy end near GLE
observe difference in relative charge present
75
to demonstrate charge resides on the outside of a metal conductor
- touch outside of sphere with proof plane
- hold near GLE
- earth
- touch inside
- hold near
76
xerography
- 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
importance of earthing
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
under what circumstances will point discharge occur
when a charge accumilates at a point so there is large potential at that point
79
batteries
convert chemical energy to electrical
80
primary cell - not rechargeable
- 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
dry cell
- paste electrolyte - MnO2
- carbon and zinc electrodes
- exposed positive terminal - mixture of powdered carbon and MnO2
- bottom = exposed negative zinc can
- insulating cover
82
voltmeter connected in parallel
- 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
ammeter connected in series
```
low resistance
( in series I = I = I)
```
84
Power derivation
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
secondary cell - rechargeable by reversing current
- lead electrodes
- gas vents
- electrolyte = sulfuric acid
car / phone batteries
86
joules law easier
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
semi conductor diode (pn diode)
- - 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
Doping
the addition of small controlled amounts of certain impurities to a pure semi conductor to increase its conductivity
89
N-type doping (boron)
- when the impurity added provides more positive holes for conduction
90
P-type
- when the impurity added provides more negative electrons for conduction
91
PN junction
where a p-type semi conductor and an n-type semi conductor meet
92
Depletion layer
the region around a PN junction which contains virtually no free charge carriers so behaves as an insulator
93
Junction voltage
the potential difference that exists across a PN junction caused by electrons and holes moving across the junction when it was formed
94
Intrinsic Conduction
conduction in a pure semiconductor due to the movement of electrons and holes
- equal number of electrons and positive holes
95
Extrinsic conduction
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
Electromagnets
- 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
magnetic field direction test
right hand grip
98
magnetic flux density/current/force test
left hand rule
(Loser)
(lefts are stronger)
99
solenoid
- 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
electromagnetic induction
when the magnetic field in a coil changes
- an emf is induced in the coil
- which induces a current
101
Magnetic flux
- 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
Faraday's law of electromagnetic induction
the magnitude of induced emf is directly proportional to the rate of change of flux
103
what did faraday discover
- magnetic field
- electromagnetic induction
- electrolysis
- did experiments
104
increasing electricity produced
- faster moving magnet = faster changing flux
| - stronger magnet
105
Lenz's law
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
derivation of E (mf) = - dΦ/dt
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
Calculating heating effect of alternating current
- 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
alternating current
- 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
to find peak voltage/current
| converting d.c to a.c
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
mutual induction
when a changing magnetic field in one coil induces an emf in a nearby coil (adjacent)
111
self induction
when a changing magnetic field in a coil induces a (back) emf in the coil itself that opposes the changing current
112
induced emf depends on
- higher rate of change of magnetic flux
- soft iron core
- increasing number of turns in coils
- coils closer
113
why effective voltage is less than applied
1) resistance of components
| 2) back emf (in ac)
114
emf on dc
- little effect
only provides resistance (current flows in a constant direction)
- only delays current flowing doesn't stop it
115
emf on ac
- 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
inductor
coils with the property of self inductance
117
Transformer
a device used to change the value of alternating voltage
| with no electrical contact
118
why soft iron core
- improves magnetic effect
| - increases induced emf
119
How transformers work
- 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
Step up
Np < Ns
| Vi < Vo
121
Step down
Np > Ns
| Vi > Vo
122
Transformer equations
Vi/Vo = Np/Ns
Vi Ip = Vo Is (not in tables book)
123
Uses of Transformers
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
time taken for rectangular coil to enter field
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
ratio of Ip : Is compared to Np : Ns
opposite
126
if dc supply replaced with ac supply?
- self induction will occur
| - back emf will be produced
