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AQA A-Level Physics > Electricity (AS) > Flashcards

Electricity (AS) Flashcards

1
Q

What is current?

A

The rate of flow of charge.

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2
Q

What is potential difference?

A

Potential difference is the energy transferred per unit charge between two points in a circuit.

W = V x Q

V = W/Q

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3
Q

What is resistance?

A

How difficult it is for current to flow through an appliance.

A component has a resistance of 1Ω if a potential difference of 1V makes a current of 1A flow through it.

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4
Q

Define a coulomb (C).

A

A coloumb is the amount of charge that passes in 1 second if the current is 1A.

Q = I x T

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5
Q

Define a volt (V).

A

The potential difference across a component is 1V when you convert 1 joule of energy moving 1 couloumb of charge through the component.

V = W/Q

1V = 1JC^-1

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6
Q

Which direction does electron flow act in?

A

Negative to positive.

Direction that electrons actually flow in a circuit.

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7
Q

Which direction does coventional current flow act in?

A

Positive to negative.

Opposite to actual electron flow.

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8
Q

How do you measure the potential difference across a component?

A

Voltmeter should be used connected in parallel to the component.

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9
Q

What should an ideal voltmeter have?

A

Infinite resistance, so no current is drawn from the circuit.

V = I/R

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10
Q

What is an ohmic conductor?

A

A conductor that obeys Ohm’s Law:

The current through an ohmic conductor is directly proportional to the potential difference across it when held at a constant temperature.

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11
Q

Ohm’s Law:

The ______ through an ohmic conductor is ______ ______ to the ______ ______ across it, when held at a constant ______.

A

Ohm’s Law:

The (current) through an ohmic conductor is (directly proportional) to the (potential difference) across it, when held at a constant (temperature).

I ∝ V

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12
Q

What does resistance depend on?

A

Resistivity
Cross sectional area
Length

ρ = RA/L

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13
Q

What is resistivity?

A

Resistivity is a quantity that is proportional to an object’s resistance and cross-sectional area, and inversely proportional to the object’s length.

ρ = RA/l

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14
Q

Superconductors are materials that have zero ______ at and below a ______ ______.

A

Superconductors are materials that have zero (resistivity) at and below a (critical temperature).

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15
Q

What are superconductors?

A

Superconductors are materials that have zero resistivity at and below a critical temperature.

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16
Q

Why are superconductors called superconductors?

A

Since there is no resistance, no electrical energy is turned into heat and lost, so no energy is wasted.

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17
Q

What are critical temperatures in terms of superconductors?

A

A temperature at which, if below, a material will have zero resistivity.

They differ for each material, but are currently too low to be useful.

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18
Q

Why would superconductors with high critical temperatures be useful?

A

It wouldn’t be as difficult or expensive to keep them at their critical temperature. (Not too cold)

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19
Q

Give three uses of superconductors.

A
  1. Power cables without losing too much energy as heat.
  2. Really strong electromagnets that don’t need a constant power source.
  3. Really fast electronic circuits, because there’s no resistance to slow them down.
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20
Q

What are semiconductors?

A

Components for which the resistance changes depending on external conditions.

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21
Q

Give two examples of semiconductors.

A
  1. Light Dependant Resistors (LDR) - When light intensity increases, their resistance decreases.
  2. Thermistors - When the temperature increases, their resistance decreases.
22
Q

What does an I/V graph look like for an ohmic conductor?

A

A straight line graph through the origin.

23
Q

What is power?

A

Power is defined as the rate of energy transfer.

P = W/T
Peas walk on tables
P = VI
(Painful vagina illness)

24
Q

What is a Watt?

A

Unit of power.

1 Watt is the power dissipated when 1 joule of energy is transferred each second.

P = W/T

25
Q

What does an I/V graph look like for a filament lamp?

A

A curve that starts steep, but gets shallower as voltage rises.

Current flowing through the lamp increases its temperature.

The resistance of a metal increases as the temperature increases.

26
Q

Why does resistance increase as temperature increases?

A

Higher temperature causes metal ions to vibrate with more kinetic energy, making it harder for charge carriers to pass through.

27
Q

What is a NTC thermistor?

A

NTC stands for ‘Negative Temperature Coefficient’, which means that resistance decreases as temperature increases.

28
Q

What are diodes?

A

Components that only let current flow in one direction.

29
Q

What is the threshold voltage for diodes?

A

0.6V. They require this voltage before they will conduct.

30
Q

What is the current and resistance like for diodes in the reverse direction?

A

In the reverse direction, the resistance of the diode is very high and the current that flows is very tiny.

31
Q

What kind of component is an LED?

A

Diode.

32
Q

What are the voltage and current like in series circuits?

A

The current is the same in all positions in a series circuit.

The potential difference over the loop is split between all the components, in the ratio of their resistances.

33
Q

What are the voltage and current like in parallel circuits?

A

The current is split between each branch, and so is different in different parts of the circuit.

The potential difference over each loop is the same.

34
Q

What is Kirchhoff’s First Law?

A

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

35
Q

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

Which Law is this?

A

Kirchhoff’s First Law.

36
Q

What is Kirchhoff’s Second Law?

A

The total e.m.f. around a series circuit = the sum of the p.d’s of each component.

ε = ΣIR

37
Q

“The total e.m.f. around a series circuit = the sum of the p.d’s of each component.”

Which Law is this?

A

Kirchhoff’s Second Law.

38
Q

What is e.m.f., or electromotive force?

A

The e.m.f. is the amount of energy supplied by the source per unit charge.

ε = E/Q

39
Q

What is internal resistance?

A

Internal resistance is resistance that occurs in batteries/cells due to the movement of electrons colliding with atoms.

40
Q

What makes cells and batteries warm up whilst they’re being used?

A

Internal resistance.

41
Q

Why is the terminal p.d. (voltage provided by battery) never equal to the e.m.f.?

A

Because internal resistance makes the e.m.f. smaller.

42
Q

What is the terminal p.d. equal to?

A 
V = ε - v 
V = ε - Ir
Where:
V = terminal p.d.
ε = e.m.f.
v = lost volts
I = current
r = internal resistance

Need to learn this

43
Q

What is e.m.f. equal to?

A 
ε = I ( R + r )                      (given in exam)
ε = V + v                           (not given in exam)
Where 
ε = e.m.f.
I = current
R = load resistance
r = internal resistance
V = terminal p.d.
v = lost volts
44
Q

What is load resistance?

A

Load resistance is the total resistance of all the components in the external circuit.

Also called ‘external resistance’.

45
Q

How do you find the total e.m.f. of the cells in a series circuit?

A

ε(total) = ε(1) + ε(2) + ε(3) + …

Adding their individual e.m.f.s.

46
Q

How do you find the total e.m.f. of the cells in a parallel circuit?

A

ε(total) = ε(1) = ε(2) = ε(3) = …

Same size as the e.m.f. of each of the individual cells.

47
Q

What is a potential divider?

A

A potential divider is a circuit with a voltage source and two resistors in series.

This results in the potential difference in the circuit being split into a specific ratio.

So, if you had a 2Ω resistor and a 3Ω resistor, you’d get 2/5 of the p.d. across the 2Ω resistor and 3/5 across the 3Ω.

48
Q

Why are variable resistors used in potential dividers?

A

Variable resistors are used in potential dividers to change the voltage supplied to something to a more suitable voltage.

However, this method wastes power.

49
Q

What is a detecting circuit?

A

Add a semiconductor, such as an LDR to a potential divider as the second resistor. This will change the V(out) across the first resistor dependant on the light intensity.

50
Q

What is a potentiometer?

A

A potentiometer has a single variable resistor replacing both resistors of the potential divider.

AQA A-Level Physics (14 decks)

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