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AQA A level Physics > Electricity > Flashcards

Electricity Flashcards

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1
Q

What is meant by superconductivity and what are the conditions for a material to
become superconducting

A

Zero resistivity
As resistivity decreases with temperature a material becomes superconducting when you reach the critical transition temperature

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2
Q

What is current

A

Rate of flow of charge

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3
Q

What is the charge of a single electron

A

1.6x10-19C

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4
Q

What is the unit of charge

A

Coulombs

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5
Q

What is potential difference

A

The work done per unit of charge

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6
Q

How do you measure the potential difference

A

Using a voltmeter, this has to be placed in parallel with the component being measured

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7
Q

What is resistance measured in

A

Ohms

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8
Q

What does ohms law state

A

the current through an ohmic conductor is directly proportional to the potential difference across it as long as the physical conditions such as temperature around it stay constant

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9
Q

What does an IV graph show

A

how current flowing through a component changes as the potential difference is increased, the graph follows ohms law.

The shallower the gradient – the greater the resistance of the component

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10
Q

What assumption can you make about voltmeters and ammeters

A
  • voltmeters are assumed to have infinite resistance so no current can flow through them and ammeters are assumed to have no resistance so potential difference will not flow through them
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11
Q

What are filament lamps and explain the shape of the graph

A
  • filament is a thin coil of metal wire
  • when a current flows through the filament in a filament lamp some of the electrical energy is converted into heat energy and causes the metal to heat up. The extra energy causes particles in the metal filament to vibrate more so it is harder for the charge carrying electrons to pass through the resistor
  • the current cant flow as easily and resistance increases

Graph appears like two opposite curves

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12
Q

What is a diode

A
  • designed to let the current flow only in one direction
  • most diodes require a voltage of 0.6 volts before they will conduct known as the threshold voltage
  • in reverse bias the resistance of the diode is very high and current that flows is very small
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13
Q

What is resistivity

A

The resistance of a 1m length with a 1m ^2 cross-sectional area.

measured in ohm-metres

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14
Q

What three factors effect the amount of current that passes through a wire

A
  • Length – the longer the wire the more difficult it Is to make a current flow through it
  • Area – The wider the wire the easier it will be for electrons to pass along it
  • Resistivity – measure of how much a particular material resists a current flow
    – for example the type of the material and environmental factors
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15
Q

What are semi-conductors

A

– group of materials that aren’t as good at conducting electricity as metals because they have far fewer electrons available. However when energy is supplied more electrons are released and the resistivity of the material decreases . Examples of this is a thermistor and a diode

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16
Q

What is a thermistor

A

component with resistance that depends on its temperature

17
Q

What is the NTC

A

negative temperature coefficient – resistance decreases as temperature increases
- Warming a thermistor means more electrons are available, so the resistance is lower. This makes them really good temperature detectors

18
Q

How could you do an experiment to investigate the resistance of a thermistor

A
  • Place thermistor in a beaker and pour enough boiling water into the beaker to cover the thermistor
  • Measure and record the temperature of the water using a digital thermometer and current through a circuit – the potential difference across the circuit must be kept constant
  • Continue to record the current and temperature for every 5 degree drops in temperature
  • Use recorded values for current and potential difference to calculate the resistance of the thermistor at each temperature
  • As temperature decreases you should find resistance increases
19
Q

What is power

A

Defined as the rate of energy transfer

20
Q

What is the unit of measurement for power

A

watts

21
Q

What is e.m.f

A
  • The amount of electrical energy the battery produces and transfers to each coulomb of charge is called its electromotive force – measured in volts
22
Q

explain internal resistance

A

resistance comes from electrons colliding with atom’s and losing electrons

  • in a battery, chemical energy Is used to make electrons move and as they move they collide with atoms inside the battery – so batteries must have resistance.
  • This is called internal resistance and is what makes batteries heat up when they are used
  • load resistance of all the components in the external circuit.
23
Q

What is Kirchoffs first law

A

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

24
Q

What is Kirchoffs second law

A

The total sum of e.m.f around a series circuit=the sum of p.d.s across each component- fundamentally a statement of the conservation of energy

25
Q

What is a potential divider

A

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

26
Q

Explain the graph for a diode

A

A diode only lets the current flow in one direction.
When the diode is facing the direction of the current flow it is said to be in forward bias. Even in forward bias it will not conduct until a voltage of around 0.6 volts has passed across it.

Called the switch on voltage

After this switch on voltage is reached, resistance decreases as current rises.

27
Q

What is a voltage drop

A

A voltage drop means the charge has lost energy

28
Q

What is fundamental about the potential difference in a series circuit

A

V= V1 + V2 ETC.

29
Q

How can we measure internal resistance

A
  • Using an oscilloscope, as they have effectively and
    infinite resistance
    -graphical method
30
Q

How can we measure internal resistance using the graphical method

A
  • If we replace R with a variable resistor we can get a
    range of results for V and I
  • This gives us a V as our Y variable and I as our X
    variable
  • compare this with Y=mx+c we get a straight line plot of V vs I with a gradient of -r (the internal resistance) and a Y intercept of E
31
Q

How does resistance relate in series circuits

A

R=R1+R2+R3

32
Q

define resistance

A

Resistance is defined as the ratio of the voltage across a component to the current through it.

33
Q

define internal resistance

A

The internal resistance is the opposition to current flow charge experiences inside a cell

34
Q

Under what conditions are the emf and terminal p.d equal

A

When no current is drawn from the cell

35
Q

What fundamental laws are Kirchoffs laws based upon

A

conservation of charge and energy

36
Q

What is meant by a non ohmic conductor

A

no resistance

37
Q

define e.m.f

A

work (done)/energy (supplied) per unit charge (by battery) !
1 (or pd across terminals when no current passing through cell or open
circuit)

38
Q

State and explain why it is important for car batteries to have a very low internal resistance

A

need large current/power to start the car ! (or current too low)
2 internal resistance limits the current/wastes power(or energy)/reduces
terminal pd/increases lost volts !

AQA A level Physics (17 decks)

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