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OCR A-level Physics A > Electricity > Flashcards

Electricity Flashcards

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1
Q

What is current?

A

The rate of flow of charge.

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2
Q

What is the unit of charge?

A

The coulomb (C)

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3
Q

What is the elementary charge, e?

A

1.6 x 10^-19

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4
Q

In terms of the elementary charge, e, what is the charge of an electron and a proton?

A

Electron -e
Proton +e

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5
Q

An object’s charge is quantised. Explain what this means.

A

The charge of the object has to be a multiple of e. This is because an object cannot gain or lose part of an electron.

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6
Q

What are the charge carriers in metals and electrolytes?

A

Metals - electrons
Electrolytes - ions

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7
Q

What is the direction of conventional current?

A

Positive to negative

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8
Q

Which direction do electrons flow in?

A

From negative to positive, the opposite of conventional current.

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9
Q

What is Kirchoff’s first law?

A

The total current flowing into a junction is equal to the total current flowing out of a junction.

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10
Q

Current is conserved according to which of Kirchoff’s laws?

A

Kirchoff’s first law

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11
Q

Describe the motion of electrons in a wire that does not have a potential difference across it.

A

They move randomly and at high speeds in zig-zag direction

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12
Q

What is the name given to the low average speed of charge carriers along a conductor when there is a potential difference across the conductor?

A

Mean drift velocity

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13
Q

State the formula relating mean drift velocity to current.

A

I = Anev, where I = electrical current (A) ; A = cross sectional area (m^2), n = number density of electrons (m^-3), e = the elementary charge, v = mean drift velocity (ms^-1)

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14
Q

What is number density, n, and what is its unit?

A

The number of free charge carriers per unit volume of a material; m^-3

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15
Q

Explain the difference between conductors, semi-conductors, and insulators, in terms of n.

A

Conductors have high n, semi-conductors have medium n, and insulators have very low n.

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16
Q

Why are metals more conductive than plastics?

A

Metals have lots of free electrons free to carry charge around the circuit.

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17
Q

Give the value of n for a perfect insulator

A

Zero

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18
Q

What is potential difference?

A

The energy transferred per coulomb of charge (between two points in a circuit)

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19
Q

What is emf?

A

The energy transferred by a power supply from chemical energy to electrical energy per unit charge

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20
Q

What is resistance?

A

The ratio of p.d. to current; ultimately, how hard it is for current to flow through a component

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21
Q

State Ohm’s law.

A

V = IR. Current is directly proportional to potential difference in a resistor, assuming a constant temperature.

22
Q

What is the name for a conductor that obey’s Ohm’s law.

A

Ohmic conductor.

23
Q

A metal has a positive temperature coefficient for resistance. What does this mean?

A

Its resistance increases with temperature

24
Q

What causes a metal to have a positive temperature coefficient?

A

At higher temperatures, the atoms and ions in a metal vibrate more, increasing the chance of collisions with the free electrons, which slows the free electrons down, decreasing the current per volt of p.d. across the metal, and increasing the resistance.

25
Q

In a negative temperature coefficient thermistor, why does the resistance decrease as temperature increases?

A

An increase in temperature leads to an increase in the number density of free electrons.

26
Q

Describe and explain the I-V graph for an ohmic conductor, such as a fixed resistor.

A

Current is directly proportional to p.d., so the I-V graph is s straight line through the origin.

27
Q

How can the resistance be found from I-V graph for an ohmic conductor?

A

The resistance is constant and can be found from 1 / gradient of the IV graph.

28
Q

Describe and explain the I-V graph for a filament lamp.

A

The gradient decreases, showing that the resistance of the filament lamp increases.

This is because the temperature of the filament lamp increases as the current increases. The atoms and ions in the wire vibrate more, and collide more often with electrons flowing through it, so there is more resistance, so the gradient of the graph decreases.

29
Q

Describe and explain the I-V graph for a diode or LED.

A

The I-V graph shows that a diode only lets current flow in one direction, known as the forward direction. The graph also shows that the p.d. must be above a certain threshold before any current can flow.

30
Q

Describe and explain the I-V graph for an NTC thermistor.

A

The gradient increases, showing that the resistance of the thermistor decreases. This is because the number density of the semiconductor increases as the temperature of the thermistor increases. This increases the current for a given p.d. and so decreases the resistance.

31
Q

How can the resistance be found from I-V graph for a filament lamp, diode or LED, or NTC thermistor?

A

The resistance is different throughout the graph, and can be calculated at any specific point by using R = V / I

32
Q

Define the resistivity of a metal.

A

The resistance of a 1m length of the material that has a cross sectional area of 1m^2

33
Q

State the unit of resistivity

A

Ohm metre (Ωm)

34
Q

When light intensity increases on an light dependant resistor (LDR), how does the resistance change?

A

The greater the intensity of light, the lower its resistance.

35
Q

How does the resistivity of a metal change with temperature?

A

The resistivity of a metal increases as temperature increases, because the electrons in the metal vibrate more when it is heated.

36
Q

How does the resistivity of a semiconductor change with temperature?

A

If energy is supplied to some types of semiconductors then their resistivity decreases because more charge carriers are released.

37
Q

What is power?

A

The rate of doing work.

38
Q

What is Kirchoff’s second law?

A

The sum of the e.m.f.s in a closed circuit loop is equal to the potential differences in that loop.

39
Q

Energy is conserved according to which of Kirchoff’s laws?

A

Kirchoff’s second law

40
Q

How do you find the total resistance in a series circuit?

A

Add together the individual resistances of each resistor:
Total resistance = R1 + R2 + R3

41
Q

How do you find the total resistance in a parallel circuit?

A

1 / Total resistance = (1/ R1) + (1/ R2) + (1/ R3)

42
Q

True or false: Adding a resistor to a circuit in parallel will always reduce the total resistance.

A

True. Total resistance drops the more paths there are for the current to take.

43
Q

In series circuits, is current split or the same across components?

A

It is the same across all components.

44
Q

In series circuits, is potential difference split or the same across components?

A

It is split across components.

45
Q

How can the p.d. across any individual component in a series circuit be found?

A

Using V = IR.

46
Q

In parallel circuits, is current split or the same across components?

A

It is split across components.

47
Q

In parallel circuits, is potential difference split or the same across components?

A

It is the same across all components.

48
Q

What is a power supply’s internal resistance?

A

The resistance of the power supply itself

49
Q

What are “lost volts”?

A

The difference in voltage between that supplied by the source and the amount available to the circuit. They are ‘lost’ due to the internal resistance of the source.

The work done per unit charge to overcome the battery’s internal resistance.

50
Q

What is a of a potential divider?

A

A simple circuit containing resistors in series, across which the source voltage is divided. It allows only a fraction of the total voltage to be used a the output voltage.

51
Q

When more bulbs are connected to a curcuit in parallel, they get dimmer and the temperature of the battery increases. Explain this.

A

The resistance of the circuit decreases as more bulbs are added, so current in the battery increases. P.d. across internal resistance increases so terminal p.d. of battery decreases and bulbs get dimmer. More energy/power is transferred in the battery, so thermal energy increases in battery

OCR A-level Physics A (5 decks)

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