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AS-Level Physics > Electricity > Flashcards

Electricity Flashcards

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1
Q

What does EMF stand for?

A

Electromotive force

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2
Q

What is a ideal cell?

A

A source of EMF

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3
Q

What is a real cell?

A

A source of EMF with an internal resistance.

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4
Q

Explain why a real cell has internal resistance.

A
  • The current produced has to flow through the cell itself.
  • The chemicals in the cell have an electrical resistance.
  • The resistance inside the cell forms part of the circuit.
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5
Q

Calculate the current that would flow from a 1.5V AA cell when short circuited and describe the heating effect. (R=0, r=1))

A
  • 1.5A

- The cell will get warm.

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6
Q

Calculate the current that would flow from a 1.2V AA NiMH cell when short circuited and describe the heating effect. (R=0, r=0.1))

A
  • 12A

- Substantial heating effect.

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7
Q

Calculate the current that would flow from a 12V car battery when short circuited and describe the heating effect. (R=0, r=0.01))

A
  • 1200A

- Explosive heating effect.

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8
Q

Which is higher v(emf) or V (terminal pd)?

A

v

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9
Q

Define emf

A

The potential difference across the cell terminals when no current is drawn from the cell.

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10
Q

What property must a voltmeter have to measure emf?

A

Infinitely high resistance

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11
Q

What two instruments can be used to measure emf?

A
  • Voltmeter

- Oscilloscope

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12
Q

What does the term rated mean?

A

The max p.d. that can be applied to a component without damaging it.

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13
Q

Give five equations that relate to emf.

A
  • ε = V + v
  • V = IR
  • v = Ir
  • ε = I( R+r )
  • ε = V + Ir
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14
Q

What is a conductor?

A

A component with a very small resistance and allow current to pass through with ease.

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15
Q

What is an insulator?

A

A component with a high resistance making it hard for current to pass through.

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16
Q

What is a semiconductor?

A

Materials that can change resistance by electrical means or by external factors.

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17
Q

Define resistance

A

R = V/I
where: R is the components RESISTANCE measured in OHMS
V is the p.d. across the component measured in VOLTS.
I is the current through the component measured in AMPS.

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18
Q

What is Ohm’s Law?

A

For a metallic conductor, the current flowing through the conductor is directly proportional to the potential difference across the conductor provided the temperature remains constant.

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19
Q

What is a diode?

A

A component that only allows current to flow in one direction.

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20
Q

What is forward bias?

A

When a diode allows current through with little or no resistance.

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21
Q

What is reverse bias?

A

When a diode doesn’t allow current though due to almost infinite resistance.

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22
Q

What is an LED?

A
  • Light emitting diode.

- Emits light in forward bias.

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23
Q

What does LDR stand for?

A

Light dependent resistor

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24
Q

Describe how the resistance of an LDR changes.

A

The resistance of the LDR falls as the light intensity increases.

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25
Q

Explain how an LDR works.

A
  • Made from CdS.
  • CdS forms a covalent structure with almost all available electrons in covalent bonds.
  • When a photon hits, the energy is transferred to an electron giving it enough energy to break the bond.
  • The electron is then free for conduction.
  • More electrons means a lower resistance.
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26
Q

Describe how the resistance of a thermistor changes.

A

The resistance falls as the temperature increases. (ntc)

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27
Q

What does ntc stand for?

A

Negative temperature coefficient

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28
Q

Explain how an ntc thermistor works.

A
  • Made from silicon.
  • Silicon forms a giant covalent structure.
  • Heat energy causes bonds to break releasing electrons.
  • More electrons means a lower resistance.
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29
Q

Explain how the reading of the voltmeter will change as the temperature of the thermistor falls.

A
  • As the temperature increases, the resistance of the thermistor falls.
  • As the resistance of the thermistor falls the total resistance in the circuit falls.
  • The current in the circuit will increase.
  • The pd across the fixed resistor increase so the pd across the thermistor decreases.
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30
Q

What is the relationship between pd and current of an Ohmic conductor?

A

They are directly proportional as long as the temperature remains constant.

31
Q

How is the resistance calculated from the graph of a non-Ohmic conductor?

A

V/I

Not the gradient

32
Q

How is charge flow calculated?

A

ΔQ = IΔt

33
Q

How are electrons arranged in an insulator?

A

An electron is attached to an atom and cannot move away from the atom.

34
Q

How are electrons arranged in a metallic conductor?

A

Most electrons are attached to electrons but the ones that are not can carry charge.

35
Q

How are electrons arranged in a semiconductor?

A

The number of charge carrying electrons increases with temperature.

36
Q

Define potential difference.

A

Work done per unit charge

37
Q

How is resistivity calculated?

A

ρ = RA/L

38
Q

What is the unit of resistivity?

A

Ohm metre

39
Q

What is a superconductor?

A

A wire or device with zero resistivity at or below a critical temperature.

40
Q

What can be used to measure the variation of current with pd for a component?

A
  • Potential divider

- Variable resistor

41
Q

Give an advantage of using a potential divider.

A

Pd and current can be reduced to zero.

42
Q

What is the current passing through a diode in reverse bias?

A

Extremely small

43
Q

What is the current passing through a diode in forward bias?

A

The current increases up to about 0.6V when the increases exponentially.

44
Q

How does the resistance of a metal change with temperature?

A

As the temperature increases, the resistance of the metal increases.

45
Q

How does the resistance of a thermistor change with temperature?

A

A thermistor has a negative temperature coefficient.

46
Q

What are the three main rules for current?

A
  • At any junction, the total current leaving the junction is equal to the total current entering the junction.
  • The current entering a component is equal to the current leaving the component.
  • The current passing through two or more components in series is equal in each component.
47
Q

What are the three main rules for potential difference?

A
  • The total pd across components is series is equal to the sum of the pd of each component.
  • The pd across components in parallel is equal.
  • In a loop, the sum of the emfs is equal to the sum of the potential drops round the loop.
48
Q

What is the total resistance of a set of resistors in series?

A

R1 + R2 + R3…

49
Q

What is the total resistance of a set of resistors in parallel?

A

1/Rtotal = 1/R1 + 1/R2…

50
Q

How is the rate of heat transfer calculated?

A

P=I^2R

51
Q

How is emf calculated?

A

ε = E/Q

52
Q

When is maximum power delivered to the load?

A

When the load resistance is equal to the internal resistance of the source.

53
Q

How is cell current calculated?

A

cell emf / total circuit resistance

54
Q

How is the pd across each resistor in a circuit calculated?

A

current x resistance of each resistor

55
Q

How is the current through each resistor in parallel in a circuit calculated?

A

pd across the parallel combination / resistors resistance

56
Q

What are the three ways a potential divider can deliver pd?

A
  • A fixed pd between zero and the source pd.
  • A variable pd.
  • A pd that varies with a physical condition.
57
Q

What is the ratio of the pds across each resistor equal to?

A

The resistance ratio of the two resistors.

58
Q

Give two uses of potential dividers.

A
  • A volume control

- Vary the brightness of a bulb.

59
Q

What is a temperature sensor circuit made from?

A

A thermistor and variable resistor

60
Q

What is a light sensor circuit made from?

A

An LDR and variable resistor

61
Q

What is the frequency of mains electricity?

A

50Hz

62
Q

How is time period calculated?

A

1/freq

63
Q

What is the peak value of an alternating current?

A

The maximum current or pd which is the same in either direction.

64
Q

What is the peak pd in a mains circuit?

A

325V

65
Q

How does increasing the output pd affect an oscilloscope screen?

A

The trace gets taller.

66
Q

How does increasing the frequency affect an oscilloscope screen?

A

The number of cycles on the screen increases.

67
Q

What does I0 represent?

A

Peak current

68
Q

What is the root mean square value of AC?

A

The value of direct current that would give the same heating effect as the AC in the same resistor.

69
Q

How is Irms calculated?

A

I0/√2

70
Q

How is Vrms calculated?

A

V0/√2

71
Q

Which value is used to calculate mean power of an AC circuit?

A

rms

72
Q

If a pd is applied across the X-plates of an oscilloscope, how will the spot deflect?

A

Horizontally

73
Q

If a pd is applied across the Y-plates of an oscilloscope, how will the spot deflect?

A

Vertically

AS-Level Physics (4 decks)

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