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Physics > Topic 5 - Electricity > Flashcards

Topic 5 - Electricity Flashcards

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1
Q

what is a voltmeter?

A

measures p.d and connected in parallel to the component

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2
Q

what is a ammeter?

A

measures current and connected in series to the component

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3
Q

what is current?

A

the rate of flow of electric charge ( A )
which flows from positive to negative terminal

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4
Q

charge equation

A

Q = I t
charge = current x time

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5
Q

potential difference

A

a source of potential difference allows current to flow through a circuit ( driving force ). Meausred in V

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6
Q

How does changing the resistance affect the size of the current?

A

the greater the resistance of the component the smaller the current ( for a given p.d )

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7
Q

equation linking current, p.d , resistance

A

V = IR
p.d = current x resistance

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8
Q

I-V characteristic of ohmic conductor

A

the current through ohmic conductor is directly proportional to the p.d across it.
- it has a constant resistance ( for given temp )

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9
Q

I-V characteristic of filament lamp

A
  • as the current increases the resistance also increases
  • less current can flow per unit of p.d and so the graph gets shallower
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10
Q

why does the resistance of a filament lamp increase with current

A
  • when an electrical charge flows through it transfers energy to thermal energy store of filament
  • this heats up and glows
  • resistance increases with temperature
  • as more current flows the lamp heats up more, increasing resistance
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11
Q

I-V characteristic of diode

A
  • only lets current pass through one direction
  • in the opposite direction R initially high but drops drastically as p.d increases letting current flow
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12
Q

safety precaution with diode

A
  • use a protective resistor to stop the current becoming too high and damaging the diode
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13
Q

p.d in series circuits

A

total V = V₁ +V₂ + …..

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14
Q

current in series circuits

A

total I = I₁ = I₂ = …etc

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15
Q

resistance in series circuits

A

total R = R₁ +R₂ + …..

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16
Q

p.d in parallel circuits

A

total V = V₁ = V₂ = …etc

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17
Q

current in parallel circuits

A

total I = I₁ +I₂ + …..

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18
Q

resistance in parallel circuits

A

adding more resistors in parallel decreases the overall resistance of the circuit
total resistance less than smallest individual resistance

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19
Q

resistance of LDRs

A
  • in bright light the resistance falls
  • in darkness the resistance is highest
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20
Q

resistance of thermistors

A
  • in hot conditions the resistance falls
  • in cold conditions resistance is highest
21
Q

uses of LDRs

A

automatic night lights, burglar detectors

22
Q

uses of thermistors

A

thermostats

23
Q

sensor circuits

A
  • if it gets hot, the resistance of the LDR decreases so takes a lesser share of the p.d
  • this means the p.d across the output ( e.g : fan ). will increase, meaning it will get switched on ( faster )
24
Q

what is a static charge?

A
  • an electric charge that can’t move
  • found on electrical insulators where charge can’t flow freely or isolated conductors
25
Q

what happens when two insulators are rubbed together?

A
  • creates friction which gives the electrons energy
  • negatively charged electrons transferred from one materila to the other
  • gain electron = N , lose electron = P
26
Q

how do sparks form?

A
  • when charge builds up on an object the p.d between it and the earth ( 0V ) increases.
  • if the p.d is large enough, electrons can jump across the gap between charged object and earth/ earthed conductor
27
Q

what is an electric field ?

A
  • a region around an electrically charged object in which forces can act on an object, causing it to attract or repel
28
Q

what happens when two electrically charged object are brought close?

A

they exert a force on each other called electrostatic attraction / repulsion

29
Q

electric field strength

A
  • further away the weaker the force of attraction/repulsion
  • field lines further apart
30
Q

how do sparks work in terms of electric fields

A
  • the object has a large static charge and so there is a large p.d between it and earth
  • causes a strong electric field
  • electric field ionises air particles so the air can conduct electricity
  • spark jumps from object to earthed conductor
31
Q

alternating current ( A.C )

A

current is constantly changing direction as the positive and negative terminals alternate

32
Q

direct current ( D.C )

A

current always flows in the same direction ( uni - directional )

33
Q

why is A.C. used for mains electricity

A
  • easier to generate
  • easy to increase/decrease p.d of transformer
34
Q

voltage of mains electricIty in UK

A

230V

35
Q

frequency of mains electricIty in UK

A

50 Hz

36
Q

why is each wire in a three core cable colour coded?

A

so that it is easy to tell different wires apart

37
Q

live wire colour

A

brown

38
Q

neutral wire colour

A

blue

39
Q

earth wire colour

A

green and yellow

40
Q

purpose of live wire

A

carries the A.C from supply to appliance at 230V

41
Q

purpose of neutral wire

A

carries the A.C away from appliance and completes the circuit

42
Q

purpose of earth wire

A

for protection and safety
- provides an alternate low-resistance path for the high current to flow in case of a fault
- this prevents the current from going to the person

43
Q

purpose of fuse

A

fuse connected to the live wire
if a fault occurs high current will cause fuse to heat up and melt
- this will break the circuit and stop dangerously current from flowing

44
Q

power equation

A

P = VI

45
Q

energy equation

A

E = Pt or VIt

46
Q

step up transformer

A
  • increases p.d and
  • decrease current
47
Q

step down transformer

A
  • decreases p.d to safe domestic levels
  • and increases current
48
Q

why is the p.d increased first

A
  • for a given power, if you increase p.d you decrease current
  • less current means wires heat up less and less thermal energy is dissipated
  • this makes National grid more efficient
49
Q
A

Physics (9 decks)

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