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BMAT > Physics - Electricity > Flashcards

Physics - Electricity Flashcards

1
Q

How to create charge between two objects?

A

Two insulators moving relative to each other - friction makes them both either positive/negative,t charged
Plastic with duster/hair being combed

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2
Q

Can conductors become charged?

A

Only retain the charge if they are insulated from their surroundings ; if not insulated then charge will leak away

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3
Q

Induction

A

A neutral object becomes magnetised when placed near a magnet
Electrostatic induction ; charged object induces charge on a neutral object (charge is polarised to opposite sides)

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4
Q

How to keep the charge permanent?

A

If one end of an object charged by induction is momentarily earthed then that charge will leak away and become permanently charged (with the other charge)

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5
Q

Total charge?

A

Charge is always conserved - for every object that becomes positively charged, another one must become negatively charged

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6
Q

Which way the electrons move?

A

Determined by whichever object has nuclei that attract the electrons less strongly (loses electrons)

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7
Q

If an object is made out of insulating material (balloon)

A

Charge will not flow away

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8
Q

Two factors which affect the electrostatic force

A

Larger the charges = larger the force
Larger distance = smaller the force

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9
Q

Sparking

A

Air between two objected becomes ionised by a large voltage and starts conducting
Two charged objects that have air between them can discharge by a spark between them (when charge is large enough/distance is small enough)

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10
Q

How can risk of sparking be eliminated

A

By earthing ; or if they are connected together by a wire then electrostatic charging cannot take place

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11
Q

Photocopying and printing

A

Charge being placed on the paper ; exposed to toner powder which sticks to the paper at those locations as a result of electrostatic induction

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12
Q

Aircraft refuelling

A

Large volumes of fuel flow through the pipe - large amounts of friction - pipe is electrostatically charged - thus pipe is always earthed to prevent build up of charge

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13
Q

Battery

A

Group of cells (two cells)

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14
Q

Series connection

A

Connected in a line

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15
Q

Parallel connection

A

Branches

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16
Q

Direct current

A

Always in the same direction

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17
Q

Alternating current

A

Repeatedly changes direction ; usually very rapidly

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18
Q

Sources of dc

A

Cells or batteries

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19
Q

Power supply from mains electricity

A

Converted from ac to dc using diodes as a rectifier ; diode only allows current in one direction (direction of the arrow)

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20
Q

Source of ac

A

Generators in power stations produce ac

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21
Q

Ac current?

A

Change is repeated regularly to produce a waveform

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22
Q

Frequency of mains supply in U.K.

A

50Hz (current changes 100 times each second to produce 50 complete to and fro cycles )

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23
Q

What is ac wave in the form of?

A

Sin wave
Positive in one direction
Negative in the other

24
Q

Voltage on ac supply

A

Switches from positive to negative and back

25
Q

Good conductors

A

Metals
Carbon (graphite)
Ionic solutions

26
Q

Good insulators

A

Non-metals (plastic/rubber etc)
Water is a conductor (not good insulator)

27
Q

Insulators

A

Large resistance to flow of charge

28
Q

Current formula

A

Current = charge/time
Charge is in coulombs
I = rate of flow of electric charge
Current = coulombs per second = ampere

29
Q

Milliamperes

A

1*10^-3 ampere

30
Q

Microamperes

A

1*10^-6 Ampere

31
Q

If voltage is connected across a metal

A

Positive end of the metal attracts electrons and negative end repels electrons ; creates flow of charge

32
Q

Direction of current

A

Positive end of conductor to negative end

33
Q

Direction of electron flow

A

From negative end to positive

34
Q

Voltmeter

A

Connected in parallel to a component and measures voltage

35
Q

Ammeter

A

Connected in series with component and measures current

36
Q

Voltmeter resistance

A

Very high as if it was not then it would short circuit the component because there would be a lot of current in the voltmeter instead of the component

37
Q

Ammeter

A

Very low resistance - otherwise it would tend to reduce amount of current that it was being used to measure

38
Q

Resistance formula

A

Voltage/current

39
Q

In metallic conductors at a constant temperature

A

Current is direct,y proportional to voltage ; ohm’s law

40
Q

Fixed resistor

A

Has constant resistance ; ohmic conductor so resistance = voltage/current

41
Q

Graph of V against I for fixed resistor

A

Straight line passing through the origin

42
Q

Resistance of fixed resistor graph

A

Gradient of the graph

43
Q

Filament lamp

A

Becomes very hot so as temperature changes - current changes and resistance of filament is not constant
Temperature increases = resistance also increases = ratio of V:I increases and graph curves

44
Q

To calculate resistance from a graph of filament lamp?

A

Divide voltage at that point by current at that point

45
Q

Thermistor

A

Resistance depends on temperature ; as temperature increases it’s resistance decreases

46
Q

LDR

A

Light intensity increases, resistance of LDR decreases

47
Q

Components in series

A

Current in each component is the same

48
Q

Components in series

A

Total voltage across components = sum of voltages across each individual component

49
Q

Components in parallel

A

Voltage is the same
Current moving in branch = current moving out of it

50
Q

What is voltage

A

Difference in energy carried by each unit of charge either side of a circuit component ; energy lost/gained per unit charge

51
Q

Combined resistance of resistors in series

A

Sum of individual resistances ; combined resistance of two or more resistors connected in series is always greater than that of individual resistors

52
Q

Resistors in parallel

A

Combined resistance of two or more resistors connected in parallel is less than resistance of any of the individual resistors

53
Q

Voltage formula

A

Energy/charge
Energy = joules

54
Q

Power =

A

Current * voltage

55
Q

Thus power also =

A

Current ^2 * resistance

56
Q

Power also =

A

V^2 / R

BMAT (7 decks)

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