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Electricity 2 Flashcards

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1
Q

Define current. Include formula.

A

Current is the flow of charge. I=Q/t

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2
Q

Define the Coulomb.

A

1 Coulomb = 1 Ampere x 1 second

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3
Q

Distinguish between actual current and convention current.

A

Flow from negative (short end of battery) is actual current.
Flow from positive (long end of battery) is conventuel current.

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4
Q

State Ohm’s law.

A

Voltage is directly proportional to the current if the temperature of the conducting material is constant.

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5
Q

Define the unit ohm.

A

1 ohm = 1 volt / 1 ampere

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6
Q

Define resistance.

A

Is the voltage across a conductor divided by the current flowing through it.

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7
Q

State the relationship of each of the factors that affect resistance.

A

Resistance proportional to length of the conductor.
Resistance proportional to material of conductor (resistivity)
Resistance inversely proportional to cross-sectional area

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8
Q

Define resistivity. What is the unit?

A

The resistivity of a conductor is the resistance of the conductor of length 1m and cross sectional area 1m2.

Unit is Ohm metre.

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9
Q

ME: To measure resistivity of a wire: How to read micrometer?

A

Read main scale first than vernier scale.

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10
Q

ME: To measure resistivity of a wire: Procedure to measure length.

A

Make sure the wire is straight and taut. Measure with metre stick between points where resistance is being measured.

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11
Q

ME: To measure resistivity of a wire: Procedure for average diameter. Why was it done.

A

Check micrometer for zero error.
Place wire between jaws of micrometer.
Tighten grip on wire with slip control and read the main scale and vernier scale.
Repeat along several points of wire to calculate average value.

Diameter could be non-uniform. Several values allows for more accurate average.

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12
Q

ME: To measure resistivity of a wire: Affect of warm day?

A

Higher Temperature increase in resistance
Higher Temperature increase in diameter

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13
Q

ME: To measure resistivity of a wire: Two accuracy points.

A

Check the micrometer for zero error and adjust diameter values if necessary.
Avoid error of parallax when measuring length of wire with the metre stick.

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14
Q

What is the formula for a balanced Wheatstone bridge? Draw locations of each or explain in words.

A

R1 / R2 = R3 / R4

R1 and R2 connected
R3 and R4 connected

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15
Q

Give three advantages and one disadvantage of Wheatstone bridge.

A

+: compact, portable, accurate value of resistance
-: expensive

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16
Q

Give the formula of a metre bridge. Draw diagram or explain locations in words.

A

R1 x L2 = R2 x L1

Length furthest away from each resistor.

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17
Q

Give one advantage and two disadvantages of a metre bridge.

A

+: Gives accurate value of resistance.
-: large physical size, expensive

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18
Q

Give three advantages and three disadvantages of an ohmmeter.

A

+: compact, portable, fast
-: can be fragile, must check for zero error, less accurate

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19
Q

Give two practical uses of a Wheatstone bridge.

A

Temperature control
Fail safe device

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20
Q

Give the three laws for series calculations.

A

I(t) = I(1) = I(2) = I(3)
V(t) = V(1) + V(2) + V(3)
Therefore R(t) = R(1) + R(2) + R(3)

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21
Q

Give the three laws for parallel calculations.

A

V(t) = V(1) = V(2) = V(3)
I(t) = I(1) + I(2) + I(3)
Therefore 1/R(t) = 1/R(1) + 1/R(2) + 1/R(3)

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22
Q

Explain why when temperature increases the (i) resistance of thermistor decreases and (ii) the potential increases at a point BEFORE it.

A
  1. As the temperature increases the number of electrons and holes increases. More charge carriers means conduction and less resistance.
  2. A smaller resistance means smaller potential difference across the thermistor and greater potential BEFORE it.
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23
Q

Define emf.

A

The electromotive force is equal to the potential difference across the terminals of a battery or generator when no current is being drawn.

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24
Q

Give two sources of emf.

A

E.S.B. Mains
A thermocouple

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25
Q

What is an l.d.r.? Give a use for this type of resistor.

A

Light dependent resistor. Control traffic lights.

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26
Q

ME: Variation of resistance of metallic conductor and thermistor with temperature: 3 Accuracy points and differences between graphs.

A

Heat water bath slowly allowing time for metallic conductor and oil to have the same temperature.
Check and correct the ohmmeter for any zero error.
Ensure good electrical contacts as poor contacts affect the resistance.

Metallic Conductor: Straight line cutting y-axis a little above the origin.
Thermistor: Resistance starts high and decreases in a non linear fashion with increased temperature.

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27
Q

ME: To investigate variation of current with voltage of metallic conductor, filament bulb, copper electrodes: 2 Accuracy points

A

Check the voltmeter and the milliammeter for zero error and correct if necessary.
Ensure good electrical contacts as poor contacts cause variations in resistance.

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28
Q

ME: To investigate variation of current with voltage of metallic conductor, filament bulb, copper electrodes both active and inert: Compare graph’s does it obey Ohm’s law.

A

Metallic conductor: Straight Line passing through origin. Obeys.
Filament Bulb: non-linear increase from origin which approaches horizontal line. Does not obey.
Active Copper Electrodes: Straight line passing through origin. Obeys.
Inert Copper Electrodes: Straight line passing through a point on + sense of x-axis. Does not obey.

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29
Q

ME: Investigate variation of current with voltage for a semiconductor dioide in forward and reverse bias: Give differences between data collection.

A

Forward Bias: uses milliammeter
Voltage measurements go up in 0.1 V

Reverse Bias: uses microammeter
Voltage measurements go up in 1V

30
Q

ME: Investigate variation of current with voltage for a semiconductor dioxide in forward and reverse bias: Common accuracy points.

A

Ensure good electrical contacts as poor contacts cause variation in resistance.
Check the voltmeter and milliammeter/micrometer for a zero error and correct if necessary.

31
Q

ME: Investigate variation of current with voltage for a semiconductor dioxide in forward and reverse bias: Differences between graphs. Do they obey Ohm’s law.

A

Forward Bias- starts at origin is almost horizontal than rises steeply from junction voltage. Does not obey.

Reverse Bias-starts at origin and goes backward in negative sense of y and x, rises a little than goes horizontal. Does not obey.

32
Q

ME: Investigate variation of current with voltage for a semiconductor dioxide in forward and reverse bias: Differences between circuit setup.

A

Forward Bias: Resistor limits the current through the diode preventing overheating.
Reverse Bias: Voltmeter is connected across the diode and microammeter. Ensures only current through the diode is measured. Polarity of battery reversed.

33
Q

What is the junction voltage of silicon and germanium?

A

Silicon - 0.6 V
Germanium - 0.2 V

34
Q

For current voltage graphs of metallic conductor, filament bulb, active and inactive copper electrodes what are the charge carriers? What is it for gases, a vacuum and a semiconductor and do these three obey Ohm’s law?

A

Metallic Conductor-Electrons
Filament Bulb-Electrons
Active copper electrodes-Positive and negative ions
Inactive copper electrodes-Positive and negative ions

Gases-Positive ions, negative ions and electrons. Do not obey.
Vacuum-Electrons. Do not obey.
Semiconductor-Electrons and Positive Holes. Do not obey.

35
Q

Define heat energy formula in electricity. Define electrical power formula.

A

Heat = VIt (V = Voltage, I = Current, t = time)

P = VI (P = Power, V = Voltage, I = Current)

36
Q

Define a unit of electricity. Give its other name.

A

1 unit = 1000 Watts x 3600s = 3600000 Joules
Called Kilowatt-hour.

37
Q

Give an example of the heating effect of electricity.

A

Electrical kettle

38
Q

Give an example of the chemical effect of electricity.

A

Electroplating

39
Q

Give an example of the magnetic effect of electricity.

A

Electrical motor

40
Q

State Joules Law.

A

The rate of conversion of electrical energy to heat is proportional to current squared.

P is proportional to I^2

41
Q

ME: Experiment to verify Joule’s Law: What is on the axis of graph? 2 accuracy points?

A

Temperature rise on y-axis
Current squared on x-axis

Make sure the time the current flows is fixed and the mass of the water is constant.
Check and correct the ammeter for any zero error.

42
Q

Give the formula for calculating temperature rise of water given current, resistance, time and mass.

A

I^2Rt = mc(Delta T)

43
Q

Why is transmission at high voltage (E.H.T.) desirable?

A

Voltages increased by factor of 1000
Current reduced by factor of 1000 (Ohm’s Law)
Therefore heat generated reduced by a factor of 1000000 (Joule’s Law)
Huge reduction in energy losses and huge financial savings.

44
Q

Give the three types of wires and corresponding colours in a domestic circuit.

A

Live wire-brown insulation
Neutral wire-blue insulation
Earth wire-green/yellow insulation

45
Q

What is the advantage of a ring circuit as opposed to a radial circuit.

A

In a ring circuit the current takes two paths from fuse to socket.
Current along each path is half the current appliance needs.
Heat generated is reduced by factor of four (Delta T proportional to I^2)
Reduced risk if overheating is safer.

46
Q

Explain how a fuse works.

A

Is a protective device that limits the flow of current in a circuit by cutting it off before it exceeds a certain value. Prevents a wire from overheating and reduces risk of fire. Does not protect against electrocution.

47
Q

Explain the function of an earth wire.

A

Only for appliances with metal on their outer surface. If live wire comes in contact with metal on outer surface, earth wire provides good conducting path from outer metal surface to earth. So current flows to earth rather than through a person in contact with the appliance. Protects a person from electric shocks.

48
Q

Explain the functionality of a M.C.B.

A

A miniature circuit breaker does the same job as a fuse limiting the current flowing in the circuit. When a normal fuse blows it must be removed and replaced but M.C.B. trips so can be reset by the simple flick of a switch.

49
Q

Explain the functionality of a R.C.D.

A

If the live current and the neutral current are not the same the Residual current device will be activated and stop the current immediately. Response of R.C.D. very fast giving great protection to a person against dangers of electrocution.

50
Q

Define semiconductor.

A

Is a material with a conductivity or resistivity between that of a conductor and an insulator.

51
Q

Define intrinsic conduction.

A

Intrinsic conduction is when a semiconductor starts to conduct due to heating the material.

52
Q

Explain how intrinsic conduction works.

A

At low temperatures the outer electrons in silicon are involved in bonding and don’t act as conductors. Almost no conduction essentially an insulator.

At room temperatures the atoms vibrate vigorously and some of the silicon bonds are broken. There are now free electrons and positive holes to act as conductors. The higher the temperature the more charge carriers and greater conductance.

53
Q

What is the ratio of electrons to positive holes in intrinsic conduction?

A

Equal

54
Q

Define extrinsic conduction.

A

Extrinsic conduction (doping) is when the addition of a controlled quantity of an impurity increases conduction.

55
Q

Explain how a n-type semiconductor is produced through doping.

A

A controlled quantity of Arsenic is added to the silicon. It has five electrons in its outer shell four of which bond with the silicon and the fifth is available as a conductor. Adding Arsenic to silicon makes it a conductor. Electron-hole pairs from intrinsic conduction also available. Fifth electron due to Arsenic is the majority charge carrier which is a Negative charge. Hence N-type doping.

56
Q

Explain how a p-type semiconductor is produced through doping.

A

A controlled quantity of Boron is added to the silicon. It has three electrons in its outer shell which bond with the silicon with the missing electron resulting in a positive hole which acts as a conductor. Adding Boron to silicon makes it a conductor. Electron-hole pairs from intrinsic conduction also available. Positive Hole due to Boron is the majority charge carrier which is a Positive charge. Hence P-type doping.

57
Q

Explain how a P-N junction is formed.

A

Silicon is p-type doped on the left and n-type doped on the right.
Free electrons and positive holes diffuse across the junction.
N-type region loses electrons and becomes positively charged.
P-type region gains electrons and becomes negatively charged.
When the charged region are sufficient to prevent further diffusion a potential barrier is set up. This occurs at junction voltage.
There are no charge carriers on either side of the junction and this area is called the depletion region.

58
Q

Which way does the arrow in the diode point in forward and reverse bias?

A

In forward bias diode points away from positive junction.
In reverse bias diode points toward positive junction.

59
Q

Explain how a reverse bias P-N junction functions.

A

Voltage applied across P-N junction with negative terminal to p-type and positive terminal to n-type.
External voltage reinforces potential barrier increasing width of depletion region.
Resistance of P-N junction is very high, does not conduct.

60
Q

Explain how a forward bias P-N junction functions.

A

Voltage applied across P-N junction with positive terminal to p-type and negative terminal to n-type.
Width of depletion region greatly reduced
Resistance of P-N junction is very low, is a conductor with a significant current flowing when above junction voltage.

61
Q

Give a use of a semiconductor dioide?

A

Rectifier

62
Q

What is meant by doping?

A

Adding of impurities to increase conductivity.

63
Q

Define conductor.

A

Conductors are good at allowing current to flow.

64
Q

Define insulator.

A

Insulators are poor at allowing current to flow.

65
Q

Give two advantages of LED over a filament bulb.

A

Safe, cheap.

66
Q

Explain why LED will not light if connected the wrong way around.

A

Only allows current to flow one way.

67
Q

Explain why it is necessary to use R.M.S. values when comparing a.c. and d.c. voltage.

A

So as to make the power output equivalent between a.c and d.c.

68
Q

Give two differences between electrostatic Force and gravitational force.

A

Gravitational force is weaker
Gravitational force is attractive, electrostatic force is repulsive.

69
Q

How would an observer no that a wheatstone bridge was balanced?

A

0 reading on galvanometer

70
Q

Explain why the resistance of a filament bulb is different when it is not connected to the mains.

A

Mains heats up filament, hot filament has higher resistance.

71
Q

Explain the operation of a fuse in a circuit (i.e. how it stops current)

A

Fuse is in live part of circuit, it gets hot if current exceeds a certain value, it melts under this heat and breaks the circuit, cutting it off before it reaches dangerous temperatures.

Physics (16 decks)

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