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Physics A Level > Electricity > Flashcards

Electricity Flashcards

1
Q

How can you increase the electrostatic force between two charged particles?

A
  • increase the size of the charges
  • bring the particles closer together
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2
Q

What happens when you pull a charged object acros the disc on a gold-leaf electrscope?

A
  • Electrons flow to try and neutralise the charged object across the disc.
  • The disc then has too many or too few electrons and becomes charged.
  • The charge spreads through the metal rod to the leaves and since like charges repel, the gold leaves repel each other and rise.
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3
Q

What happens when a charged object comes close (but does not make contact with) the disk on a gold-leaf electroscope)?

A
  • The charged object repels/attracts electrons on the disk (depending on whether the object is positively or negatively charged).
  • The imbalance of electrons causes the gold leaves to become charged and repel each other and rise.
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4
Q

What is charge abbreviated to and measured in?

A

Charge is known as Q, measured in coulombs (C).

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5
Q

What direction do electrons flow in a circuit?

A

Negatively charged electrons flow from the negative end of the battery to the positive end.

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6
Q

What can current be defined as?

A

The rate of flow of charged particles.

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7
Q

What is the unknown current?

A
  • The sum of the currents going in = the sum of the currents going out (Kirchhoff’s first rule).
  • 1 = 0.5 + 0.3 + ?
  • 1 - 0.5 - 0.3 = ?
  • ? = 0.2A
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8
Q

What is the voltage across the power supply?

A

The amount of energy the power supply gives to one coloumb of charge.

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9
Q

What is the voltage across a component in a circuit?

A

The amount of energy one coulomb of charge gives to the component.

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10
Q

What is another way of describing voltage?

A

potential difference

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11
Q

What component is this a voltage-current graph for?

A

resistor - current and voltage are directly proportional

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12
Q

What component is this a voltage-current graph for?

A

bulb - resistance increases as the temperature of the filament (and the voltage) increases

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13
Q

What component is this a voltage-current graph for?

A

diode - the diode has a high resistance in one direction, so current can only flow in one direction

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14
Q

What equation links power, time and energy?

A

power (W) = energy converted (J) ÷ time taken (s)

p = E ÷ t

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15
Q

What can power be described as?

A

The amount of energy converted by a component every second.

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16
Q

What will happen to the reading on a voltmeter in a circuit with a thermistor as the thermistor increases in temperature?

A

As the temperature of the thermistor increases, the current increases. The resistance decreases therefore, as more charge is flowing through the thermistor, and so the reading on the voltmeter increases.

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17
Q

What is the effect on a voltmeter in a circuit where there is internal resistance of a battery (that isn’t negligable)?

A

If resistance increases, the voltmeter’s reading will decrease and the current will also decrease, as less charge is flowing through.

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18
Q

In what direction does conventional current flow?

A

Positive to negative.

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19
Q

In what direction do electrons flow?

A

Negative to positive.

20
Q

What is electromotive force?

A

Electromotive force (ℰ) is the voltage of a component which is putting electrical energy into a circuit, such as a cell.

ℰ (V) = energy transferred (J) ÷ charge passing (C)

emf = E ÷ Q

21
Q

What is potential difference?

A

Potential difference (pd) is the term for the voltage of a component which is using electrical energy in a circuit and transferring it into a different form of energy, such as a bulb.

pd (V) = energy transferred (J) ÷ charge passing (C)

pd = W ÷ Q

22
Q

What is an electron volt?

A

How much energy an electron gains by passing through a voltage of 1V.

23
Q

What is Ohm’s law?

A
  • Assuming the temperature of a component stays constant, voltage will be directly proportional to current (I ∝ V).
  • R = V ÷ I, so resistance will remain constant also.
24
Q

What is threshold voltage (on a diode)?

A

The minimum driving voltage necessary for a diode to conduct (have current) in a forwards direction.

typically around 0.6V

25
Q

What is the structure inside a metal?

A
  • There are free electrons (moving in continuous and random motion) which surround fixed positive ions).
  • The electrons are constatntly colliding with the positive ions, so much so that the net flow is 0.
26
Q

What is the drift velocity?

A

The net rate of the movement of the electrons as they cause a current flow.

27
Q

What is the transport equation?

A
  • current = density of electrons x cross-sectional area x drift velocity x charge on each electron
  • I = nAve
28
Q

If the temperature of a metal increases, what happens to the atoms?

A
  1. The atoms vibrate more as temperature increases.
  2. More collisions between the electrons and positive ions.
  3. Drift velocity is slowed down more.
  4. Resistance therefore increases.
29
Q

If the current in a metal increases, what happens to the atoms?

A
  1. Higher current causes the electrons to flow at a greater speed.
  2. A higher speed means that there are more collisions between the electrons and the positive ions.
  3. The increases in collisions causes the temperature to increase.
30
Q

If the gap between energy bands is higher…

A

…the more insulating the material will be (more energy is required for electrons to be elevated to the conduction band).

31
Q

What happens when a semiconductor’s temperature increases?

A
  • More electrons are elevated into the conduction band, as they have more energy.
  • More conduction electrons are therefore available, so the current increases as the semiconductor is able to carry a greater current.
32
Q

What are electrical insulators?

A

Materials in which there is such a large gap between the valence and conduction band that an extremely high amount of energy would be required for electrons to move into the conduction band. As a result of this, the material is a very good insulator.

33
Q

What is the critical temperature?

A

The temperature below which the resistivity of a material instantly drops to zero (often around -243˚C).

34
Q

What is the rule for current in series circuits?

A

Provided there are no junctions in the circuit, any group of components will have the same current throughout them all.

35
Q

What is the rule for current in parallel circuits?

A

The sum of the currents in each branch will add up to the total original current.

the current will not always split equally between each branch, the higher the resistance of the path, the smaller the current will be down that path

36
Q

What is the rule for voltage in series circuits?

A
  • The total emf, provided there are no junctions in the circuit, will be the sum of the individual emfs of each cell.
  • The total pd will be the sum of each pd across each component.
37
Q

What is the rule for voltage in parallel circuits?

A

The total voltage across any branch of a parallel circuit is the same as the total voltage across any of the branches.

38
Q

In a series circuit, why does Rtotal = R1 + R2 + R3?

A
  • In series, Vtotal = V1 + V2 + V3

​V = IR

  • IRtotal = IR1 + IR2 + IR3

Itotal = I1 = I2 = I3

  • I can be taken out of the equation therefore*
  • Rtotal = R1 + R2 + R3
39
Q

In a parallel circuit, why does 1/Rtotal = 1/R1 + 1/R2 +1/R3?

A
  • In parallel, Itotal = I1 + I2 + I3

I = V/R

  • V/Rtotal = V/R1 + V/R2 +V/R3

Vtotal = V1 = V2 = V3

  • V can be taken out of the equation therefore*
  • 1/Rtotal = 1/R1 + 1/R2 +1/R3
40
Q

What is the rule for resistance in a series circuit?

A

Rtotal = R1 + R2 + R3

41
Q

What is the rule for resistance in a parallel circuit?

A

1/Rtotal = 1/R1 + 1/R2 + 1/R3

42
Q

What is Vinternal in a cell?

this is the pd over the internal resistance

A
  • current x internal resistance
  • Ir
43
Q

What is Vterminal in a cell?

this is the pd across the terminals of the cell, i.e. the actual pd of the cell

A
  • electromotive force - (current x internal resistance)
  • ℰ - Ir
44
Q

What equation for a circuit incorporates the internal resistance (include working)?

A
  • Vterminal = Vload = ℰ - Ir

this is because all parallel branches in a circuit have the same voltage across them

  • due to energy conservation:

Σℰ = ΣV

ℰ = Vload + Vinternal

ℰ = Vload + Ir = IR + Ir

ℰ = V + Ir

45
Q

What is a potential divider?

A

A set up in a circuit where there are two (or more) resistors connected in series across a power source. They enable you to alter the voltage (pd) across components in a circuit.

46
Q

How would having an LDR in a potential divider be useful?

A
  • If the lamp is parallel with the LDR, then it will have the same voltage as that across the LDR.
  • If the ambient light level drops, the LDR’s resistance will increase, so the proportion of voltage supplied to it will also increase.
  • Increased voltage will mean the lamp becomes brighter (and vice versa).

Physics A Level (7 decks)

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