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A-Level Physics > Electricity > Flashcards

Electricity Flashcards

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1
Q

What does NTC mean in regards to thermistors

A

Negative temperature coefficient

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2
Q

what do NTCs mean for resistance

A

resistance decreases as the temperature goes up

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3
Q

What effect does an increase in current have on temperature in a thermistor?

A

The temperature increases with current

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4
Q

On an I-V graph, what does an increasing gradient show?

A

decreasing resistance

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5
Q

Why does warming a thermistor result in a lower resistance?

A

Warming a thermistor gives more electrons enough energy to escape from their atoms, meaning there are more carge carriers available so the resistance is lower

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6
Q

What does LDR stand for

A

Light dependant resistor

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7
Q

In an LDR, how does a greater intensity of light affect resistance?

A

more light = less resistance

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8
Q

Why does a greater instensity of light on an LDR result in decresed resistance?

A

same as in a thermistor, only it is light that provides the energy that allows electrons to escape and result in more charge carriers which decreases the overall resistance

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9
Q

Describe a thermistor circuit symbol

A

resistor (resctangle) with line across it diagonally but has little flat line at bottom of diagonal one

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10
Q

Describe LDR circuit symbol

A

resistor (rectangle) with cirle arround it and two diagonal arrows gointing towards circle from top left corner

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11
Q

What is forward bias in regard to diodes?

A

the direction in which the current is allowed to flow

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12
Q

What is the direction the triangle points in a diode called?

A

forward bias

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13
Q

What is the threshold voltage of most diodes?

A

6V

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14
Q

What is the threshold voltage?

A

The minimum voltage required in the forward direction for a diode to conduct

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15
Q

In reverse bias, what is the nature of the current and what is the nature of the resistance?

A

The current is very tiny
The resistance is very high

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16
Q

What is power?

A

The rate of transfer of energy or the rate of doing work

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17
Q

What is power measured in

A

watts

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18
Q

What is 1 watt equivalent to

A

1 joule of work done per second

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19
Q

Power =
Power =

A

work done over time (W/t) or P=VI

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20
Q

why does P=VI make sense?

A

V is the work done per coulomb, I is the number of coulonbs transferred per second. So VI is the work done per second, i.e power

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21
Q

Using V=IR, which two equations can also be made for power

A

P = V^2 / R and P = I^2 R

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22
Q

What is easy to calculate if you know the power?

A

energy

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23
Q

Why is energy easy to calculate if you know power?

A

It’s just power times time

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24
Q

What do electrical companies use instead of joules and watts?

A

units - kilowatt-hours (KWh)

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25
What do you need in order to work out the work done in kWh?
The power of an appliance and the length of time its used for
26
Why isn't the joule used for household energy?
its too tiny a unit of measurement and therefore impractical
27
If a 1500 W hairdryer is on for 10 mins, how would you calculate the energy used in 1: Joules 2: KWh ?
1) 1500 x (10 x 60) For (1) Watts is already in the correct unit, but 10 mins must be converted to seconds 2) (1500 / 1000) x (10/60) For (2), W must be converted to KW and mins must be converted to hours
28
What is the cost of electricity?
The Price per unit times the Number of units needed
29
What is a unit in electricity?
A KWh
30
What are the effects of burning fossil fuels?
they realease carbon dioxide into the atmosphere, adding to the greenhouse effect and contributing to global warming
31
What can cause acid rain?
Sulfur dioxide released by burning oil and coal
32
What do nuclear power stations produce?
radioacive waste, this is dangerous to environment and human health if not disposed of safely
33
How does renewable energy like biofuel have a negative impact?
lots of land needed to produce them
34
What are two benefits to energy saving deivces?
Save money, reduce environmental impacts
35
Two positives of energy saving flourescent light bulbs?
four times more efficient than traditional ones and last 10 years instead of 1 despite being more expensive. This reduces environmental impacts of landfill and making new bulbs
36
LED light bulbs
more efficient thn energy saving bulbs, last even longer, but cost more
37
What do domestic appliances in the UK have? How does this help?
an energy rating, this helps to choose an appliance that's most effient to run and reduce environmental impact
38
efficiency equation?
(useful output energy / total input energy) x 100
39
Where does resistance come from?
electrons colliding with atoms and lsing energy
40
What is used to make electrons move in a battery?
chemical energy
41
Why must batteries have resistance?
As electrons move, they collide with atoms inside the battey and resistance comes from electrons colliding with atoms and losing energy
42
What causes batteries and cells to warm up when they're used?
internal resistance
43
What is internal resistance?
The resistance inside of a cell or battery
44
in general, the internal resistance of a battery is less than - ohms
1
45
Why is the internal resistance for a battery typically less than 1 ohm?
to allow a high current to flow
46
What is load resistance?
The total resistance of all the components in the external circuit (so excluding batteries and cells)
47
What is another name for load resistance?
external resistance
48
What does e.m.f stand for?
electromotive force
49
What is the emf?
the total amount of work the battery does on each coulomb of charge
50
What is the equation for emf using work done and charge?
emf = W/Q
51
What is the potential difference across the load resistance?
the work done when 1 coulomb of charge flows through the load resistance.
52
What is the potential difference across the load resistance called?
The terminal p.d.
53
If there was no internal resistance, what would the terminal potential difference be?
the same as the emf
54
In real power supplies, why wouldn't no internal resistance cause the terminal p.d. to be the same as the emf?
there's always some energy lost overcoming the internal resistance
55
What are the lost volts?
The energy wasted per coulomb in overcoming the internal resistance
56
Using the conservation of energy: energy per coulomb supplied by the source =
energy per coulomb used in load resistance + energy per coulomb wasted in internal resistance | The word version of emf = I(R + r)
57
State three equations for emf, the first using terminal p.d (V) and lost volts (v). The second using current (I), load resistance (R) and internal resistance (r). The third using terminal p.d (V), current (I) and internal resistance (r).
1) emf = V + v 2) emf = I(R + r) 3) emf = V + Ir
58
How can you calculate the total emf od multiple cells in series?
adding their individual emfs together.
59
Why does it make sense that the total emf in a series circuit is all emfs added together?
each charge goes through each of the cells and so gains emf from each one
60
What is the total emf for identical cells in parallel?
the total emf of the combination of cells is the same size as the emf of each of the individual cells
61
What is an easy way to measure the emf of a power source?
connecting a voltmeter across its terminals
62
why is the value measured when connecting a voltmeter across the terminals of a power source slight less than the emf?
Voltemeters have a very high resistance, but a small current will still flow through them, so there have to be some lost volts
63
Explain why whatever current flows into a junction is the same as the current flowing out of it
Charge doesn't get used up or lost, whatver charge flows into a junction will flow out again, current is the rate of flow of charge so must follow the same logic
64
What does Kirchhoff's first law state?
The total current entering a junction = the total current leaving it
65
Energy is transferred around the circuits due to conservation of energy. Energy transferred to a unit charge is what?
emf
66
Energy is transferred around the circuits due to conservation of energy. Energy transferred from a unit charge is what?
potential difference
67
What does Kirchhoff's second law state?
The total emf around a series circuit = the sum of the pds across each component
68
What is kirchhoff's second law in symbols?
emf = the sum of IR
69
What four things must be remembered about series circuits in accordance to Kirchhoff's laws?
1) there are no junctions so same current everywhere 2) Kirchhoff's second law says the emf is split between components so emf = v1 + v2 + v3 3) V=IR, so if I is constant then IRtotal = IR1 + IR2 + IR3 3) cancelling the Is gives: Rtotal = R1 + R2 + R3
70
Which four things must be remembered about parallel circuits in accordance with Kirchhoff's laws?
1) current is split at each junction: I = I1 + I2 + I3 2) in a loop, the emf equals the sum of individual p.d.s so same p.d across all components 3) V/Rtotal = V/R1 + V/R2 + V/R3 4) cancellng out the Vs, 1/Rtotal = 1/R1 + 1/R2 + 1/R3
71
What is a potential divider?
A circuit with a voltage source (eg power supply) and a couple of resistors in series
72
How is the potential difference of the voltage source divided?
in the ratio of resistances. eg, if you had a 2ohm resistor and a 3ohm resistor, the former would get 2/5 of the p.d. across it and the latter 3/5
73
What is the divided ratio of resistances as an equation?
V1 / V2 = R1 / R2
74
Which equation proves that the current is the same through both resistors?
V1 / V2 = R1 / R2 as it can be rearranged to give V1 / R1 = V2 / R2 and I = V/r
75
How can potential dividers be used?
to choose the resistances to get the voltage you want across one of them
76
Why is it useful that potential dividers can be used to choose the voltage over a resistor?
Can be used to calibrate voltmeters which have a very high resistance
77
When does an LDR have a very high resistance?
in the dark
78
When does an NTC thermistor have a high resistance?
at low temperatures
79
What does a potentiometer do?
uses a variable resistor to give a variable voltage
80
Which way is conventional current
The opposite direction to the flow of electrons
81

A-Level Physics (13 decks)

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