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X AS Level Physics > Electricity > Flashcards

Electricity Flashcards

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0
Q

Define current

A

The rate of flow of charge.

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1
Q

Which way do electrons flow in a circuit?

A

From the - to the + terminal.

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2
Q

Which way does charges flow in a circuit?

A

From the + to the - terminal.

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3
Q

Define a coulomb.

A

The amount of charge that passes 1 second when the current is 1 ampere.

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4
Q

How is current calculated?

A

Change in charge

Change in time.

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5
Q

Define PD

A

The energy converted per unit charge moved.

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6
Q

How is voltage calculated ?

A

Work divided by charge.

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7
Q

Define a volt.

A

The potential difference across a circuit is 1 volt when 1 joule of energy is converted moving 1 coulomb of charge through the component.

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8
Q

What happens when PD is put through an electrical component?

A

A current will flow.

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9
Q

How is resistance calculated?

A

Voltage divided by current.

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10
Q

What is a resistance of 1 ohm?

A

A component has a resistance of 1 when a PD of 1v makes a current of 1A flow through it.

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11
Q

What is Ohm’s law?

A

Provided the temperature is constant, the current through an ohmic conductor is directly proportional to the potential difference across it.

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12
Q

What is an ohmic conductor ?

A

A component with constant resistance.

The voltage is directly proportional to the current.

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13
Q

What does the gradient of an I/V graph show?

A

The resistance of an electrical component. The flatter the gradient, the greater the resistance.

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14
Q

What is the circuit you might use to calculate the resistance of a component?

A

Have a battery connected to a variable resistor. Connect this to the component, this to an ammeter and this to the battery. A voltmeter should be in parallel to the component.

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15
Q

What is the I/V graph of a filament lamp?

A

It is a curve. At the origin it is steep but as voltage increases in both directions, it gets flatter.

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16
Q

Explain the shape of the I/V graph of the filament lamp.

A

The filament lamp is a coiled up piece of wire. Thus current flowing though it causes the temperature to increase and the resistance increases.

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17
Q

How does a semiconductor work?

A

They have less charge carriers available in them so they aren’t as good at conducting as metal. If energy is supplied, then there are more charge carriers so the resistance drops.

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18
Q

Name 3 types of semiconductors

A

Thermistor, LDR and diode.

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19
Q

How do diodes work?

A

They let current flow in one direction only as they have a very high resistance in one direction.

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20
Q

What do most diodes have a threshold voltage of ?

A

0.6 v. They need this before they can conduct.

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21
Q

What 3 things can affect the resistance of a piece of wire?

A

Length(longer it is, the more stuff the electrons have to get past)
Area(the wider, the more ways the electrons can get past)
Resistivity.(this depends on the material. It can be changed by the structure or by environmental factors like temperature and light intensity.

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22
Q

What is the definition of restistivity?

A

State the equation from the formula sheet and state which each letter represents.

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23
Q

What is the unit for restistivity ?

A

Ohm-meters. Don’t write Ω/m. Write Ωm

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24
Q

What is the formula for restistivity ?

A

P = RA

I

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25
Q

How big are the typical restistivity values for conductors?

A

Very small. For copper is is 0.00000000172

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26
Q

What is the disadvantage if resistance in a circuit?

A

Energy gets waste as heat.

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27
Q

How can the restistivity of a material be reduced?

A

Cool it down.

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28
Q

What is the transition temperature?

A

If a material is cooled to below this point, then restistivity goes away.

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29
Q

What would be uses of superconductors ?

A

Power cables that didn’t lose any power could be made.
Really strong electromagnets that don’t need a constant power source could be made for uses in trains and hospital equipment.
Really fast electronic circuits could be made.

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30
Q

Define power and a watt.

A

Power is defined as the rate of transfer of energy.

1 watt is equivalent to 1 joule per second.

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31
Q

How is power calculated ?

A

Voltage X current.

Energy/time.

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32
Q

What is internal resistance?

A

The resistance within a battery or cell. It causes them to heat up.
You can think of it as a perfect cell with a resistor next to it.

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33
Q

What is the electromotive force?

A

The amount of electrical energy a battery produces for each coulomb of charge. (It is measured in volts)
The PD with an open circuit.

34
Q

What is the PD across the load resistor?

A

The energy transfered when one coulomb of charge flows through the load resistance. This is the terminal PD.

35
Q

What would the terminal PD be if there was no internal resistance?

A

The same as the EMF.

36
Q

What are lost volts?

A

The energy wasted per coulomb overcoming the internal resistance.

37
Q

When is a low total internal resistance needed?

A

Things that must produce a really high current like car battery’s and torches.

38
Q

When is a larger internal resistance wanted?

A

When high voltages are used. High tension and extremely high tension supplies have a high resistance that means if there accidentally short circuited only a very small current will flow.

39
Q

What is Kirchoff’s law?

A

The total current entering a junction = the total current leaving it.

40
Q

What does R mean?

A

Load resistance. The resistance the circuit has.

41
Q

What does r mean?

A

Internal resistance.

44
Q

In a series circuit what must the voltages of the components add up to?

A

The voltage provided by the battery.

45
Q

In different branches of a parallel circuit, what is the voltage?

A

The voltage across each branch will be the same though the current will be different.

46
Q

What happens to current in a series curcuit?

A

Nothing. It is the same throughout the whole circuit.

47
Q

When is the only time when current will change?

A

When it comes to a split in a parallel circuit it will also split.

48
Q

What circuit would you use to calculate the internal resistance of a battery?

A

Connect a battery up to an ammeter, than a voltmeter and then back to the battery again. A second resistor should be around the voltmeter in parallel.

49
Q

What is the symbol for terminal pd?

A

V

50
Q

What is the symbol for lost volts?

A

v

51
Q

In a closed loop, what must always be equal?

A

The total emf around a series circuit= the sum of the pd across each component.

52
Q

How is frequency found from an oscilloscope ?

A

Frequency = 1

Time period

53
Q

What is the root mean squared voltage ?

A

The square root of the mean of the squares of all the values of the voltage in one cycle.
To get this you divide the peak voltage by √2 . This also works with current.

54
Q

What are the 3 equations you have to be able to produce from the emf equation?

A

ℰ=V+v
V=ℰ-v
V=ℰ-Ir

55
Q

If you had four cells and when in series they produced 6v what would happen if you put them in parallel, 2 in each branch?

A

the voltage would halve and the current would double.

56
Q

If you had a circuit with four identical cells, 2 in parallel and 2 in series, which ones would run out quicker and why?

A

The ones in parallel last twice as long as they provide half the current per second than the ones in series. This means they provide half the energy per second and so last longer.

57
Q

How is energy related to time and charge?

A

E=T X Q

58
Q

Define the root mean square ?

A

the square root of the mean of the squares of all the values of the voltage in one cycle

59
Q

What is the relation ship between emf, charge and energy?

A

Energy ÷ charge= emf

60
Q

Compare the resistance of an ideal ammeter with that of an ideal voltmeter. Which has the larger resistance? Why?

A

An ammeter needs to measure current without changing the amount of current that would normally be coming through a particular brand of a circuit. As a result, it needs to have very low resistance. On the other hand, a voltmeter measures the voltage difference between two different points (say, on different sides of a resistor), but it should not change the amount of current going through the element between those two points. So, it should have very high resistance so that it doesn’t “draw” current through it.

61
Q

When finding the area of the wire, how can inaccuracies be reduced?

A

Take the radius from several points on the wire.

62
Q

What is the name of the temperature you have to cool something down to, to get no resistance?

A

The critical temp.

63
Q

When is superconductivity useful?

A

When the energy saved is less than the energy required to maintain the superconductor at or below the critical temperature

64
Q

When is the power of a battery at its maximum?

A

When the internal resistance is equal to that of the circuit resistance.

65
Q

What is the emf if the cells are connected in series but one of them is in the wrong way round? (positive to positive)

A

the emf will be that of one minus that of the other.

66
Q

What is meant by the internal resistance of a battery?

A

The electrical resistance of the materials in the battery.

67
Q

How is the PD across a battery found?

A

If there was no internal resistance it would be the same as the emf.
Use R= V÷I to find the voltage with the internal resistance and the current going through it. This is lost volts and subtract this from the emf.

68
Q

What will the I/V graph of an ohmic conductor look like?

A

A straight line that goes through the origin with constant gradient.

69
Q

What will the I/V graph of a diode look like?

A

There will be nothing until you get to 0.7 v at which point a straight line will go up.

70
Q

What is the PD of a diode when conducting?

A

0.7 v. It is always 0.7 v

71
Q

What happens when a diode is put in the wrong way?

A

It means it will not conduct and have infinite resistance. iit would have all the voltage of the cell around it.

72
Q

What happens when 2 cells of identical emf are put in parallel?

A

The emf of the total will be the same as that of a single cell. However the internal resistance will be less meaning they will produce a higher current. This is why in car battery’s they have many cells in parallel to produce a very high current.

73
Q

What is the type of electricity we get in the UK?

A

AC, 50 hertz, 325 voltage peak.

74
Q

In essence, what is a potential divider?

A

A voltage source and a couple of resistors in series. The potential of the voltage is divided in the ratio of the resistances.

75
Q

If you had a potential divider with a 2 and 3 ohm resistor, how would the pd be divided?

A

2/5 of the pd would be around the 2 ohm one and 3/5 of the pd would be around the 3 ohm resistor. By changing the resistances of one of the resistors you can change the pd you get out of it.

76
Q

What are the uses of potential dividers?

A

Put a thermistor in it and you have a temperature sensor. Use a LDR and you have something that will turn a light on in the dark.

77
Q

What is a potentiometer ?

A

A potential divider but with a variable resistor replacing the 2 resistors that you would normally have. By changing the resistance you can have a voltage from 0 to the source voltage.

78
Q

What is a use of a potentiometer ?

A

They can change the volume of a stereo by changing the voltage the speaker gets.

79
Q

When measuring I/V values by changing the resistance, what should you do between readings?

A

Disconnect the circuit

80
Q

Why is a filament bulb more likely to break when first turned on?

A

To start the resistance is lower.
This means the current is greater.
This means a greater temperature change.

81
Q

What is emf?

A

energy changed to electrical energy per unit charge/coulomb passing through
[or electrical energy produced per coulomb or unit charge] [or pd when no current passes through/or open circuit

82
Q

How is the emf equation re-arranged to give a graph?

A

ℰ=I(r+R) multiply by I
ℰ=Ir+V
V=ℰ-Ir. This means that if you draw a graph of terminal pd against current, the y intercept is the emf and the negative of the gradient is the internal resistance.

83
Q

If you wanted to measure the internal resistance of a battery, what experiment would you do?

A

Connect the battery up to an ammeter, to a variable resistor and back to the cell again. Have a voltmeter around the cell. Change the current in the circuit by changing the variable resistance. Record V and I values and draw a graph of V against I.

84
Q

Why when measuring the pf around a resistor connected to a battery, is the voltage less than the emf?

A

Battery has internal resistance
current passes through (this resistance)
work done/voltage lost, which reduces the value of the emf

85
Q

On an oscioscope what are the two controles?

A

Voltage sensitivity and time base.

X AS Level Physics (10 decks)

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