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AS Physics > Electricity > Flashcards

Electricity Flashcards

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1
Q

Define Energy Efficiency

A

A measure of how usefully energy is used by a device

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2
Q

Percentage efficiency = ?

A

Efficiency x 100

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3
Q

What does the voltmeter measure in a closed circuit?

A

Potential difference

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4
Q

A potential difference of 1 volt transfers what?

A

1 Joule per coulomb of charge

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5
Q

Describe the symbol for an electric motor

A

A circle with an M in it

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6
Q

What does the voltmeter measure in an open circuit?

A

Emf

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7
Q

Describe the symbol for a cell

A

A big line and a small line with a small gap between them, with the wire passing perpendicularly through their centres

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8
Q

Describe the symbol for a resistor

A

A rectangle with its long edges parallel to the wires, which are entering through the centres of the short sides

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9
Q

What happens when a potential difference is applied across a metal?

A

The electrons are attracted towards the positive terminal and repelled by the negative terminal

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10
Q

What is the maximum efficiency possible for an object and why?

A

1.0 because the useful energy can never be greater than the energy supplier

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11
Q

Which way does conventional current flow?

A

From positive to negative

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12
Q

Which way is electron flow?

A

Negative to positive

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13
Q

What is electric current?

A

The flow of electric charge

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14
Q

What are the charge carriers in metals?

A

Negatively charged conduction electrons

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15
Q

What happens when the temperature of a semi-conductor is increased?

A

More charge carriers are produced and the semi conductor turns from an insulator to a conductor

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16
Q

Describe the symbol for a light dependent resistor (LDR)

A

A circle with a rectangle in it, with two arrows pointing from the top left of the circle to the rectangle.

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17
Q

What do charge carriers do inside a metal?

A

They move about, repeatedly colliding with each other and the fixed positive ions of the metal

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18
Q

What do charge carriers consist of in other conducting substances such as acids, low pressure gases and molten salt?

A

Both positive and negative ions

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19
Q

Good conductors have many what?

A

Free to move charge carriers

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20
Q

Insulators have few what?

A

Free to move charge carriers

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21
Q

Describe the symbol for a diode

A

A circle with an isosceles triangle and a straight line. The triangle’s longest edge vertical and parallel to the straight line. The straight line is at the point where the two short sides of the triangle meet.

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22
Q

Describe the symbol for a thermistor

A

A rectangle (with its longest sides horizontal) with a diagonal line going through it -and out either side- from bottom left to top right. There is a small horizontal line going right from the top end of the diagonal line.

23
Q

Define resistance

A

Opposition to the flow of charge in a circuit

24
Q

Define emf

A

The electrical energy produced per unit charge from the power supply

25
Q

What are the two equations for efficiency?

A

Useful power output/power input

Useful work output/energy supplied

26
Q

What is charge?

A

The product of the current flowing through a component and the time taken

27
Q

What are charge carriers?

A

Charged particles that move through a substance when a pd is applied across it.

28
Q

What is Kirchoff’s First Law?

A
  1. The current passing through two or more components in series is the same through each component.
  2. At a junction, the total current in = total current out
29
Q

What is potential difference?

A

The work done per unit charge across a component.

30
Q

What is Ohm’s Law?

A

V = I R

31
Q

What is the equation for resistors in series?

A

R(total) = R1 + R2 + R3+…

32
Q

Define current?

A

The rate of flow of charge around a circuit due to the movement of electrons.

33
Q

What is one of Kirchoff’s Laws relating emf and p.d?

A

The total emf = the total potential difference in a closed loop.

34
Q

What is the equation for resistors in parallel?

A

1/R(total) = 1/R1 + 1/R2 + 1/R3 +…

35
Q

What is the potential divider equation?

A

V2= V0 x (R2/(R1 + R2))

36
Q

What does the I/V graph look like for a diode in forward bias?

A

Current starts at 0 or just above 0 until a threshold voltage is reached.
Then there is a rapid increase of I for a small increase in V.

37
Q

What does the I/V graph look like for a diode in reverse bias?

A

The current starts at zero or just below 0 until voltage reaches a particular value (between -50 and -500V).
Then there is a sharp downward curve.

38
Q

What is the threshold voltage for a silicon diode?

A

About 0.7V

39
Q

Explain the I/V curve of a filament lamp.

A

At low voltages, current increases proportionally- Ohm’s Law is obeyed.
As voltage increases, greater currents heat the filament wire.
This causes resistance to increase.
The rate of increase of I with V therefore decreases and the graph becomes shallower. (The gradient is I/R so it decreases).
The same shape happens but in the negative quadrant.

40
Q

What is an equation for potential difference involving energy or work done?

A

V = W / Q
or
V= E / Q

41
Q

When solving equations to do with readings on voltmeters, what is usually the assumption made?

A

That there is no current through the voltmeter and that it has a very high/infinite resistance.

42
Q

What graph should be plotted when determining the resistivity of a wire (if A is known and L and R are measured)?

A

R on the y axis and l on the x axis.

43
Q

What is the unit for resistivity?

A

Ohm meter

44
Q

When a battery is connected to a component, such as a bulb, and the potential difference is measured across the component, why is this measurement lower than the emf of the battery?

A

Because the battery has internal resistance and part of the emf is the voltage across the internal resistance.

45
Q

When components are in a series circuit what is the relationship between emf of the battery and potential difference across each component? (Ignoring internal resistance)

A

Emf = sum of potential differences across each component

46
Q

When components are in parallel what is the relationship between the potential differences across them?

A

Potential difference is the same for all of them

47
Q

What is a potential divider?

A

A combination of resistors in series connected across a voltage source to produce a required pd.

48
Q

What happens when a cell is placed in reverse when in series with other cells?

A

It’s voltage has to be taken away rather than added.

49
Q

What happens when current enters a junction where there are two resistors in parallel?

A

The current is split in the ratio of the resistors, but with the highest resistor getting the least amount of current.

50
Q

State what is meant by superconductivity.

A

A material has zero resistivity

51
Q

What is required for a material to become a superconductor?

A

It must be cooled down enough to get below the transition temperature.

52
Q

Define internal resistance of a battery.

A

The resistance of the materials within the battery

53
Q

How does increasing the temperature of a thermistor affect the resistance?

A

Decreases resistance.

AS Physics (7 decks)

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