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a level physics > electricity > Flashcards

electricity Flashcards

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1
Q

switch

A
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2
Q

cell / battery

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3
Q

resistor

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4
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diode

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5
Q

LED - light emmitting diode

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6
Q

lamp

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7
Q

fuse

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8
Q

voltmeter

A

Potential difference can be measured using a voltmeter
Voltmeters must be set up in parallel with the component being measured

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9
Q

ammeter

A

Current can be measured using an ammeter
Ammeters must be connected in series with the component being measured

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10
Q

thermistor

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11
Q

LDR - Light dependent resistor

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12
Q

power supply

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13
Q

Electric Current

A

The rate of flow of electric charge

Current is measured in units of amperes or amps (A)
1 amp is equivalent to a charge of 1 coulomb flowing in 1 second, or 1 A = 1 C s^−1

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14
Q

current flow vs electron flow

A

In electric circuits, the current is a flow of electrons
Conventional current is defined as the flow of positive charge
This is from the positive terminal of a cell to the negative terminal
This is the opposite of the direction of electron flow
Electrons are negatively charged so they flow from the negative terminal of a cell to the positive terminal

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15
Q

Potential Difference

A

The electrical work done per unit charge flowing between two points
Potential difference is measured in units of volts (V)
1 volt is equivalent to the transfer of 1 joule of electrical energy by 1 coulomb of charge, or 1 V = 1 J C^−1

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16
Q

how a cell works

A
  • A simple cell creates a potential difference through the separation of charge
  • One end (terminal) of the cell has an excess of positive charge and the other an excess of negative charge
  • Negatively charged electrons are repelled by the negative terminal and attracted to the positive terminal
  • Therefore, when a wire is connected between the two terminals, the potential difference causes the flow of electrons (current)
  • As electrons flow through a cell, they gain energy
  • For example, in a 12 V cell, every coulomb of charge passing through gains 12 J of energy
  • As electrons flow through a circuit, they lose energy
  • For example, after leaving the 12 V cell, each coulomb of charge will transfer 12 J of energy to the wires and components in the circuit
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17
Q

Resistance

A
  • The resistance R of a conductor is defined as the ratio of the potential difference V across to the current I in it.
  • Resistance is measured in units of ohms (Ω)
  • A resistance of 1 Ω is equivalent to a potential difference across a component of 1 V which produces a current of 1 A through it
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18
Q

Resistance relationship with current

A
  • The resistance of a component controls the size of the current in a circuit
  • For a given potential difference across a component:
  • The higher the resistance, the lower the current that can flow
  • The lower the resistance, the higher the current that can flow
  • All electrical components, including wires, possess some value of resistance
  • Wires are often made from copper because of its low electrical resistance
  • This is why it is known as a good conductor
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19
Q

Ohm’s Law

A

For a conductor at a constant temperature, the current through it is proportional to the potential difference across it
Constant temperature implies constant resistance

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20
Q

The relation between potential difference across an electrical component and the current can be investigated through a circuit such as…

A

By adjusting the resistance on the variable resistor, the current and potential difference will vary in the circuit

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21
Q

Measuring the variation of current with potential difference through the fixed resistor

A

An ohmic conductor maintains a constant resistance, which means that the current is directly proportional to the potential difference

  • Since the gradient is constant, the resistance R of the resistor can be calculated by using 1 ÷ gradient of the graph
  • An electrical component obeys Ohm’s law if its graph of current against potential difference is a straight line through the origin
  • A resistor does obey Ohm’s law
  • A filament lamp does not obey Ohm’s law
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22
Q

I–V Characteristics - semiconductor diode

A
  • For a semiconductor diode, the I–V graph is a horizontal line that goes sharply upwards
  • A diode is used in a circuit to allow current to flow only in a specific direction

As the potential difference increases (in forward bias) from zero the resistance starts very high and becomes much lower at threshold p.d.
In reverse bias the resistance remains high.

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23
Q

I–V Characteristics - filament lamp

A

For a filament lamp, the I–V graph has an ‘S’ shaped curve - The I–V graph for a filament lamp shows the current increasing at a proportionally slower rate than the potential difference
This is because:

  • As the pd increases the current increases
  • as the current increases the filament increases in temperature
  • therefore the resistance of the filament increases
  • Resistance opposes the current
  • therefore the ratio of V/I increases

Where the graph is a straight line, the resistance is constant
* The resistance increases as the graph curves
* The filament lamp obeys Ohm’s Law for small voltages

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24
Q

Semiconductor Diode - forward bias

A

When the current is in the direction of the arrowhead symbol, this is forward bias. This is shown by the sharp increase in potential difference and current on the right side of the graph

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25
Q

Semiconductor Diode - reverse bias

A

When the diode is switched around, it does not conduct and is called reverse bias. This is shown by a zero reading of current or potential difference on the left side of the graph

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26
Q

Resistivity

A

This is a measure of how much a particular material resists current flow.
It depends on the structure of the material as well as on environmental factors such as temperature and light intensity. It is a property of the material.

The resistivity of a material is defined as the resistance of a 1 m length with a 1 m^2 cross-sectional area. It is measured in ohm-metres.

27
Q

what does resistance depends on:

A

Length (L)
Area (A)
Resistivity (p)

28
Q

trends in length and cross sectional area in wire shown by resistivity equation

A

The resistivity equation shows that:
* The longer the wire, the greater its resistance
* The thicker the wire, the smaller its resistance

29
Q

Semiconductors

A
  • Semiconductors are a group of materials that aren’t as good at conducting electricity as metals, because they have far fewer charge carriers (i.e. electrons) available.
  • if energy is supplied to a semiconductor, e.g. by an increase in temperature, more charge carriers can be released and the resistivity of the material decreases.
  • This means that they can make excellent sensors for detecting changes in their environment. Three common semiconductor components are thermistors, diodes and light dependent resistors (LDRs).
30
Q

Temperature & Resistance

A
  • The electrons collide with the vibrating metal atoms which impede their flow, hence the current decreases.
  • So, if the current decreases, then the resistance will increase (from V = IR)

Therefore, for a metallic conductor which obeys Ohm’s law:
* An increase in temperature causes an increase in resistance
* A decrease in temperature causes a decrease in resistance

31
Q

Thermistors

A

A thermistor is a component with a resistance that depends on its temperature.

Warming the thermistor gives more electrons enough energy to
escape from their atoms. This means that there are more charge carriers available, so the resistance is lower. This sensitivity to temperature makes them really good temperature sensors.

32
Q

NTC thermistors

A

NTC stands for ‘Negative Temperature Coefficient’. This means that the resistance decreases as the temperature goes up,

33
Q

Superconductivity

A
  • Resistance means that when electricity flows through a material, it heats up and the electrical energy is wasted as thermal energy
  • The resistivity of a material can be lowered by lowering its temperature
  • If a material is cooled below a temperature called the critical temperature, its resistivity disappears entirely. It is now a superconductor
34
Q

superconductor

A

A material with no resistance below a critical temperature

35
Q

superconductor

The critical temperature

A

The temperature at which a material becomes superconducting

36
Q

Uses of superconductors

A

Creating strong magnetic fields (e.g. MRI scanner/Maglev/Particle accelerators)
Reductions in energy loss (e.g. power lines / tranformers)

37
Q

electrical Power equation list

A

P = E/t
P=IV
V=IR
P = V^2 / R
P = I^2 / R
E = IVt
E = tV^2 / R
E = I^2 Rt

38
Q

Resistors in Series

A

When two or more components are connected in series, the combined resistance of the components is equal to the sum of individual resistances.

39
Q

Resistors in Parallel

A

When two or component are connected in parallel:
The reciprocal of the combined resistance is the sum of the reciprocals of the individual resistances.

This means as more resistors are added, their combined resistance decreases and is, therefore, less than the resistance of the individual components
For example, If two resistors of equal resistance are connected in parallel, then the combined resistance will half

40
Q
A
41
Q

Current - series circuit

A

In a series circuit, the current is the same for all components

42
Q

Current - parallel circuit

A

In a parallel circuit, the current is split across the different branches (or junction). The total current into a junction must equal the total current out of a junction.

43
Q

Potential Difference - series circuit

A

In a series circuit, the e.m.f of the power supply is shared amongst all the components in different amounts, depending on their resistance

44
Q

Potential Difference - parallel circuit

A

In a parallel circuit, the voltage of all the components in each branch is equal to the e.m.f of the power supply

45
Q

Kirchhoff’s First Law

A

The sum of the currents entering a junction always equal the sum of the currents out of the junction

46
Q

Kirchhoff’s Second Law

A

The total e.m.f. in a closed circuit equals the sum of the potential differences across each component

47
Q

Charge and Energy in a circuit

A

Charge is never used up or lost in a circuit - this is known as conservation of charge

Energy is never used up or lost in a circuit - this is known as conservation of energy

48
Q

electromotive force (e.m.f)

A

The work done by the cell per unit charge (The chemical energy transferred to electrical energy per unit charge)

E.m.f is equal to the potential difference across the cell when no current is flowing

E.m.f can be represented by the symbol ε (greek letter epsilon)
It is not actually a force, and is measured in volts (V)

49
Q

terminal potential difference (p.d)

A
  • The terminal potential difference (p.d) is the potential difference across the terminals of a cell
  • If there was no internal resistance, the terminal p.d would be equal to the e.m.f
  • Since a cell has internal resistance, the terminal p.d is always lower than the e.m.f
50
Q

Lost volts

A

In a closed circuit, current flows through a cell and a potential difference develops across the internal resistance

Since resistance opposes current, this reduces the energy per unit charge (voltage) available to the rest of the external circuit

This difference is called the ‘lost volts’

The work done per unit charge / coulomb to overcome the internal resistance / resistance inside the battery (when current flows)

Lost volts is usually represented by little v
So, from conservation of energy: v = e.m.f − terminal p.d

51
Q

e.m.f eqn

A

v = ε – V = Ir (Ohm’s law)

Where:
v = lost volts (V)
I = current (A)
r = internal resistance of the battery (Ω)
ε = e.m.f (V)
V = terminal p.d (V)

52
Q

Internal Resistance

A

The resistance of the materials within the battery as caused by chemical energy being used to make electrons move which collide with atoms inside the battery

It is internal resistance that causes the charge circulating to dissipate some electrical energy from the power supply itself
This is why causing the cell to become warm

Therefore, over time the internal resistance causes loss of voltage or energy loss in a power supply
A cell can be thought of as a source of e.m.f with an internal resistance connected in series.

53
Q

Potential Divider Circuit

A

At its simplest, a potential divider is a circuit with a voltage source and a couple of resistors in series. The potential difference across the voltage source (e.g. a battery) is split across the resistors in the ratio of the resistances

54
Q

Draw the circuit diagram that would enable you to determine the IV charcteristic of semiconducting diode

A

1) Ballast resistor is included to limit the current through the diode
2) Ammeter connected to diode in series with no junctions between them (except for voltmeter)
3) Voltmeter in parallel to diode only

55
Q

The resistance of an ideal ammeter is…

A

zero

56
Q

The resistance of an ideal voltmeter is…

A

infinite

57
Q

How does increasing the temperature of a metal conductor change its resistivity/resistance?

A

Resistivity and resistance increase with temperature

58
Q

Explain in terms of the current in the circuit why increasing the temperature of the thermistor will cause the reading on the voltmeter to decrease (4)

A

Increasing the temperature of the thermistor decreases the resistance of the thermistor and the total resistance of the circuit
This causes the current in the circuit to increase
Which increases the p.d. across the fixed resistor (as V=IR)
Which causes the p.d. across the thermistor to decrease (as the sum of the pd across R and T must equal the emf of the cell)

59
Q

Use the idea of potential dividers to explain why increasing the temperature of the thermisor will cause the reading on the voltmeter to decrease

A

Increasing the temperature of the thermistor causes its resistance to decrease
The ratio of the resistance of the thermistor to the resistance of the fixed resistor decreases
The ratio of potential differences across components in series is proportional to the ratio of their resistances
Therefore the pd across the thermistor decreases

60
Q

Why is it best to avoid using P=IV to explain changes in power that occur when the current or p.d. changes?

A

Because if the potential difference changes, so does the current, so all three variables change!

61
Q

The cell below has internal resistance. Explain what will happen to the reading on the voltmeter when the switch is closed

A

Closing the switch decreases the external resistance (from infinite to 2.0 ohms)
This causes the current flowing through the cell to increase (from zero to something)
This increases the potential difference across the internal resistance (the “lost volts”) to increase (V= Ir)
Causing the potential difference across the terminals of the cell to decrease

62
Q

When cells are connected in series how do their EMFs and internal resistance combine?

A

The total emf = sum of their individual emfs
The total internal resistance = sum of their total internal resistances

63
Q

When identical cells are connected in parallel how do their EMFs and internal resistance combine?

A

The total emf = the emf of one individual cell
The total internal resistance= the internal resistanc of one cell / number of cells in parallel

64
Q

What do the gradient and y-intercept of this graph show? How can you prove this?

A

grad = -r y-intercept = emf

a level physics (7 decks)

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