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Physics > Electricity > Flashcards

Electricity Flashcards

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1
Q

Current

A

Rate of flow of charge (I = Q/t)

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2
Q

Voltage

A

Work done per unit charge (V = W/Q)

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3
Q

Resistance

A

Opposition to the flow of charge (R = V/I)

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4
Q

Ohm’s Law

A
  • current in a conductor is proportional to the potential difference applied to it
  • provided physical conditions such as temperature remain constant
  • constant of proportionality given by R, resistance
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5
Q

Describe I-V graph for ohmic conductor

A
  • current is directly proportional to potential difference
  • since resistance remains the same
  • passes through origin
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6
Q

Describe I-V graph for filament lamp (non-ohmic conductor)

A
  • s-shaped curve passing through origin
  • current directly proportional to potential difference initially
  • then resistance increases with current due to increasing temperature
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7
Q

Describe I-V graph for semiconductor diode

A
  • n shaped curve passing through origin
  • forward biased diode allows current to pass through with low resistance (steep gradient) when threshold voltage is reached
  • reverse biased diode does not allow current to pass through but there is some leaking current
  • if potential difference is high enough in reverse then diode can break at break down voltage allowing infinite current to pass
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8
Q

How to draw V-I graph based on I-V graph

A

inverse graphs so reflection in y=x

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9
Q

State resistance of ammeter

A

zero

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10
Q

State resistance of voltmeter

A

infinite resistance

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11
Q

Suggest why voltmeters are not connected in series when measuring voltage through a component

A
  • voltmeter has infinite resistance
  • impedes the flow of charge
  • smaller current would flow through the component so measured voltage would be lower than true value
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12
Q

Suggest why an ammeter connected in parallel would stop a circuit from working

A
  • ammeter has zero resistance
  • if connected in parallel a large current would flow
  • since current moves through the path of least resistance
  • wire in the ammeter would burn out resulting in short circuit
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13
Q

Resistivity

A
  • quantifies how strongly a given material opposes the flow of electric current
  • ρ = RA/L where ρ= resistivity (Ωm), R=resistance (Ω), A=cross-sectional area (m2), L=length (m)
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14
Q

Metallic Conductors

A

A substance through which current flows due to movement of delocalised electrons

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15
Q

Insulator

A

A substance through which electrons cannot flow since all electrons are fixed in outer shells of atoms

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16
Q

Semiconductor

A

A substance through which more current flows at higher temperatures as electrons break free from atoms and are free to move

17
Q

Explain why resistance of metallic conductors increases with temperature

A
  • positive temperature coefficient
  • positive metal ions vibrate more at higher temperatures
  • higher frequency of collisions between them and charge carriers
  • so charge cannot pass through easily
18
Q

Explain why resistance of ntc thermistor decreases as temperature increases

A
  • negative temperature coefficient
  • semiconductor
  • number of charge carriers increases since more electrons break free at higher temperatures
19
Q

Give application of thermistors

A

temperature sensors

20
Q

Superconductor

A
  • zero resistance
  • when critical temperature reached
  • resistivity decreases with temperature
21
Q

Explain why superconductors do not heat up over time

A
  • current passes through with no resistance

- no voltage across it thus no heating effect

22
Q

Suggest uses of superconductors

A
  • high power electromagnets in MRI scanners or maglev trains

- supercomputers due to reduction in energy loss in transmission of electrical power so high efficiency

23
Q

Kirchhoff Current Laws

A
  1. Current passing through components in a series circuit is the same
  2. At a junction total current in = total current out
24
Q

Kirchhoff Voltage Laws

A
  1. Total potential difference across components is equal to the sum of potential difference across each component in series
  2. Potential difference is constant across parallel branches
  3. For any complete loop sum of emfs equals sum of potential difference drops
25
Q

How to calculate sum of resistance for resisters in series

A

RT=R1+R2+R3+ …

26
Q

How to calculate sum of resistance for resisters in parallel

A

1/RT=1/R1+1/R2+1/R3+ …

27
Q

How to calculate terminal pd of cells in series when internal resistance is not negligible

A
  • add potential differences of cells arranged in same direction
  • minus potential differences of cells arranged in opposite directions
  • total internal resistance is sum of individual internal resistances
  • V = e - Ir
28
Q

How to calculate terminal pd of identical cells in parallel when internal resistance is not negligible

A
  • total emf of cells = emf of each individual cell

- V = e - Ir/n where n is number of identical cells

29
Q

Diode

A

Component which only allows current to flow in one direction

30
Q

Potential divider

A
  • two or more resistors in series
  • connected to a source of emf
  • supplies constant or variable potential difference that is a fraction of voltage between its ends
31
Q

State formula for output voltage of potential divider

A

Vout = Vin [R2 / (R1 + R2)]

32
Q

Suggest an advantage of using potential dividers (potentiometers) to variable resistors in a sensor circuit

A
  • potential difference can be reduced to zero with potential dividers
  • only be reduced to a minimum with a variable resistor
33
Q

Electromotive Force (EMF)

A

Electrical energy per unit charge supplied by a power source
e = E/Q
e = V + Ir

34
Q

Internal Resistance

A

Resistance due to materials within a source of electrical energy

35
Q

Suggest why parallel circuits have a lower resistance relative to series

A
  • more paths for current to flow through

- less resistance to the flow of charge

36
Q

State and explain whether two cells connected in a series or in a parallel combination would run out first

A
  • series cells run out first

- double current through them compared to parallel cells

37
Q

Cons of internal resistance

A
  • lost volts
  • energy wasted
  • limits current of circuit

Physics (5 decks)

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