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wow woah insane its the best science flash cards in the observable universe confirmed > Electricity > Flashcards

Electricity Flashcards

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1
Q

Find current (using charge)

A

Charge passing a point/time

Q/t

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2
Q

Find voltage (using work done)

A

Work done/charge

W/Q

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3
Q

Charge of an electron in coulombs (or e)

A

1.6 x10^-19

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4
Q

Kirchhoff current law

A

Sum of current into a point = sum of current out
(ΣIin = ΣIout)
Because conservation of charge

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5
Q

Number density

A

Number of free electrons per cubic metre of material

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6
Q

Find current (using mean drift velocity)

A 
Current = cross sectional area*number density*e*mean drift velocity
(I = Anev)
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7
Q

Find voltage (using current)

A 
Voltage = current*resistance
(V = IR)
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8
Q

Find resistance (using resistivity)

A 
Resistance = resistivity*length/cross sectional area
(R = ρL/A)
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9
Q

Find resistivity

A 
Resistivity = resistance*cross sectional area/length
(ρ = RA/L)
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10
Q

Resistivity

A

How much a material opposes electron flow

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11
Q

Resistance

A

How much a component opposes electron flow

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12
Q

Current

A

Rate of flow of charge

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13
Q

Potential difference (p.d.)

A

Energy lost (work done) by each unit of charge flowing through a component

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14
Q

Electromotive force (e.m.f.)

A

Energy gained by each unit of charge passing through a cell

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15
Q

How to measure p.d. of a component?

A

Connect a voltmeter in parallel

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16
Q

How to measure current through a component?

A

Connect an ammeter in series

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17
Q

Power

A

Rate of work done by a component

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18
Q

Find power (of a component)

A 
Power = voltage*current
(P = VI)
19
Q

Find power (using only current and resistance)

A 
Power = resistance*current^2
(P = I²R)
20
Q

Find power (using only voltage and resistance)

A 
Power = voltage^2/resistance
(P = V²/R)
21
Q

Find kinetic energy of an electron from an electron gun

A

Kinetic energy = e*accelerating p.d.

0.5mv² = eV

22
Q

Diode

A
  • only allows current in one direction
  • very high resistance until threshold p.d. is reached
  • non ohmic component (V not proportional to I)
23
Q

Thermistor

A
  • can have positive or negative temperature coefficient
  • NTC decreases resistance with temp rise
  • PTC increases resistance with temp rise
  • non ohmic
24
Q

Photoresistor (or LDR)

A
  • decreases resistance as light intensity increases

- non ohmic

25
Q

Find total resistance in series

A

Rt = R1 + R2 + …

26
Q

Find total resistance in parallel

A

1/Rt = 1/R1 + 1/R2 + …

27
Q

Kirchhoff voltage law

A

In any loop,
Sum of e.m.f. = sum of p.d.
(Σε = ΣV)
Because conservation of energy

28
Q

Find p.d. across R1 of a potential divider

A

V1 = Vt*R1/Rt

29
Q

Find e.m.f. of a cell (using internal resistance r)

A

e.m.f. = terminal p.d. + p.d. of internal resistance
(ε = V + Ir
or
ε = I(R + r))

30
Q

On a terminal p.d. vs current graph, how do you find:

  • e.m.f.
  • internal resistance
A 
ε = V + Ir (because of voltage law)
V = -rI + ε
so:
- e.m.f. = y intercept
- internal resistance = negative of gradient
31
Q

Terminal p.d.

A

p.d. across a cell (same as p.d. across rest of circuit)

32
Q

Direction of current flow

A

Positive to negative

33
Q

Direction of electron flow

A

Negative to positive

34
Q

n values for different materials

A

Conductors - high
Semiconductors - lower
Insulators - very low

35
Q

I-V graph of a resistor

A
  • resistance stays the same
  • straight line through the origin
  • ohmic conductor
36
Q

I-V graph of a filament lamp

A
  • resistance increases with V (because of heating)

- curve with gradient that reduces to 0

37
Q

I-V graph of a thermistor

A
  • resistance decreases with V (because of heating)

- current curves upwards like a tan graph

38
Q

I-V graph of a diode

A
  • resistance goes from high to low at threshold voltage

- curves sharply upwards at V = 0.7 (different for LEDs)

39
Q

Explain the effect of temperature on resistance / resistivity

A

High temperature causes ions in the metal to vibrate, making them more likely to block electron flow

40
Q

Find work done by a component

A

Work done = voltage*charge
(W = VQ
or
W = VIt)

41
Q

Kilowatt hour

A

Unit of energy used to calculate energy costs

42
Q

Ratio of voltages in a potential divider

A

Ratio of voltages = ratio of resistances

V1/V2 = R1/R2

43
Q

Potentiometer

A

Resistor with three terminals and a sliding contact - can be used as a variable resistor or a potential divider

44
Q

Ohm’s law

A

Voltage is proportional to current at constant temperature

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