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AQA A-Level Physics > DC Electricity > Flashcards

DC Electricity Flashcards

1
Q

Current

A

The rate of flow of charge at a point in a circuit

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2
Q

What is used instead of an ammeter for very small currents?

A

A galvanometer / galvo

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3
Q

What is the resistance of an ideal ammeter?

A

0 ohms

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4
Q

Direction of flow of current

A

Positive to negative

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5
Q

Direction of flow of electrons

A

Negative to positive

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6
Q

Potential difference between two points

A

The energy converted when a unit charge flows between the points

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7
Q

What is the resistance of an ideal voltmeter?

A

Infinite resistance

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8
Q

Resistance of a devise

A

The ratio of the potential difference across its ends to the current flowing through it

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9
Q

What is resistance a measure of?

A

How difficult it is for current to flow

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10
Q

Ohm’s law

A

Provided the temperature and other physical factors remain constant, the current through a conductor is directly proportional to the p.d across it.

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11
Q

Power

A

The rate at which energy is converted

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12
Q

A kilowatt-hour

A

The energy supplied by 1 kilowatt of power in 1 hour

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13
Q

Kirchoff’s 1st law

A

At a junction, the total current entering the junction is equal to the total current leaving the junction

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14
Q

Explanation for the shape of a V-I graph

A
  1. Obays Ohm’s law at low PD and I
  2. As PD increases, I increases
  3. As I increases, more power is dissipated
  4. As more power is dissipated, the temp increases
  5. As temp increases, resistance increases
  6. As resistance increases, there is a smaller increase in I for the same increase in PD
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15
Q

Why does resistance increase at high temperatures?

A

The metal ions vibrate more vigorously, making it more difficult for electrons to flow through a wire

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16
Q

Kirchhoff’s seccond law

A

The sum of the emfs around a circuit loop is equal to the sum of the p.ds around the loop

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17
Q

Breakdown voltage of a semiconductor diode?

A

Around -50V

18
Q

Minimum voltage required for current to flow in a diode

A

Around 0.7V

19
Q

What is the maximum voltage of a diode?

A

Around 0.7V regardless of current

20
Q

What is the resistance of an ideal diode when reverse biased?

A

Infinite resistance

21
Q

What is a protective resistor used for?

A

Preventing voltage from rising too high and destroying a diode in the circuit

22
Q

What determines resistivity?

A

The material

23
Q

Units of resistivity

A

Ωm

24
Q

A low resistivity means that a material is…

A

A good conductor

25
Q

Effect of increasing temperature on resistivity

A

Resistivity increases

26
Q

Why does resistivity increase with temperature?

A
  • Metal ions gain KE
  • More vibrations
  • More collisions with electrons
  • Rate of flow of electrons decreases
  • Current is lower, so resistance increases
27
Q

Superconductors

A

Materials where the resistivity drops to zero at very low temperatures

28
Q

Critical temperature of supercoductors

A

The temperature at or below which the resistivity of a material is zero

29
Q

Uses of superconductors

A
  • Mag-lev trains
  • MRI
  • Computing
  • More efficient power cables
30
Q

Electromotive force (emf)

A

The number of joules of electrical energy transferred to each coulomb that passes through it

31
Q

What causes internal resistance?

A

The materials inside the power supply

32
Q

Why is the terminal voltage lower than the emf?

A

Some of the emf goes across the internal resistance

33
Q

Effect of increasing current on terminal voltage

A
  • Current increases internal resistance
  • A greater proportion of the pd goes across the internal resistance
  • Terminal pd is lower
34
Q

Why would a low internal resistance be used?

A

This will cause the current produced by the supply to be high

35
Q

Why would a high internal resistance be used?

A

In high voltage supplies, this is used to keep current low (prevents electrocution)

36
Q

Short circuit

A

When something connects the terminals of the supply, causing load resistance to be 0

37
Q

Total emf in series

A

Sum of the emfs

38
Q

Total internal resistance in series

A

Sum of the individual resistances

39
Q

Total emf in parallel (identical cells)

A

Emf of a single cell

40
Q

Total internal resistance in parallel (identical cells)

A

Combined resistance of all resistances in parrallel

41
Q

Why would cells be used in parrallel?

A
  • Batteries last longer (produce 1/n of the current)
  • Lower internal resistance reduces heating
  • Total emf is not effected if a cell brakes

AQA A-Level Physics (6 decks)

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