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a - physics > chapter 9 - energy power and resistance > Flashcards

chapter 9 - energy power and resistance Flashcards

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1
Q

current

A

rate of flow of charge
amps
ammeter ( ideally 0 R)
in series the same
in parallel it splits

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2
Q

charge is made up of

A

electrons in wires
or ions in liquids

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3
Q

kirchoffs first law

A

total current going into a junction = total current going out due to the conservation of charge
- explains why current splits / rejoins in parallel

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4
Q

calculating current - in terms of charge use

A

I = Q/t
charge / time

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5
Q

calculating current - in terms of electron use

A

I = nAve
n - number of free electrons per m3
A - cross sectional area of the wire m2
v - average drift velocity of electrons - m/s
e - charge of an electron - C

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6
Q

charge of an electron

A

1.6x10-19 C

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7
Q

number of free electrons per m3

A

number of electrons/volume

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8
Q

using a micrometer to find the diameter and area

A
  • zero the micrometer
  • gently clamp the wire between the jaws to get diameter in mm
  • in case wire is not uniform repeat in 3 places and average
  • convert to m and use pir2
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8
Q

number of electrons

A

charge flowing / charge of 1 electron

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9
Q

measuring drift velocity in a wire

A
  • set up a circuit with a cell, ammeter and the wire
  • measure diameter using micrometer to find area
  • measure I
  • know n and e
  • use v = I/nAe
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10
Q

voltage

A

Energy that charges are carrying/transfering

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11
Q

2 types of voltage

A
  • electromotive force (emf)
  • potential difference (pd)
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12
Q

electromotive force (emf)

A
  • energy gained per unit charge from a battery/power supply (chemical > electrical)
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13
Q

potential difference (pd)

A

energy given away per unit charge as they pass through a component (from electrical > other types)

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14
Q

voltmeter

A
  • measures voltage
  • connected across a component in parallel
  • it reads out the difference in energy being carried by the coulombs of charge going in vs out
  • an ideal voltmeter has infinite R (prevents charge passing through)
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15
Q

voltage in series vs parallel

A

SERIES
- supply voltage (emf) splits between components
PARALLEL
- each branch receives the same voltage

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16
Q

kirchoffs 2nd law

A

the sum of the emf = the sum of the pd in any loop of a circuit due to conservation of energy

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17
Q

voltage equation

A

V = E/Q

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18
Q

resistance

A
  • restricts the flow of current
  • high R = low I > low R = high I
  • measured in Ohms which can be defined as Volt per Amp
  • all components have some resistance
19
Q

resistance is affected by:

A
  • type of material/ RESISTIVITY
  • LENGTH - longer wire = more resistance R ∝L
  • AREA - thinner = more resistance R ∝1/A
  • TEMP - see notes on lamp/thermistor
20
Q

resistance equations

A

R = pL /A
R = V/I (just a convenient way to calc R - V+I don’t actually affect R)

21
Q

resistors in series

A

INCREASES overall resistance
Rt = R1 + R2 + R3

22
Q

resistors in parallel

A

DECREASES overall reistance (more routes to take)
- if two identical resistors Rt is half one of them
- two non identical use “product over sum” Rt = R1R2/R1+R2
- any resistors use 1/Rt = 1/R1 + 1/R2 + 1/R3…

23
Q

experiment to measure the resistivity of a material

A
  • measure d with a micrometer (zero it, 3 times and average)
  • calculate area using pi r^2
  • measure length with a ruler (eye level) (use low I to avoid heating)
  • measure R with an Ohmeter
  • repeat for several diff lengths
  • plot R and L - R = p/A L
  • gradient = p/A
  • p = gA
24
Q

Fixer resistor

A

obeys ohms law
“Voltage is proportional to current providing temp and dimensions are constant”

25
Q

Filament

A

resistance increases due to temp increasing (current has a heating effect due to electrons colliding with atoms so they vibrate faster)
ions in the filament vibrate faster causing more freq collisions with electrons

26
Q

filament lamp graph

A

S shape graph
for each increase in voltage current goes up by a smaller amound

27
Q

LED

A
  • more efficient and robust than filament lamp
  • only allows current to flow if “forward biased” and above threshold voltage
  • otherwise has infinite R
28
Q

LED graph

A

I is zero until threshold voltage then straight line up

29
Q

thermistor / LDR

A
  • heat/light energy releases more electrons per m^3 in the semiconducting material (n increases) so more current will flow (I = nAve) and therefore resistance will decrease as R = V/I
30
Q

thermistor / LDR graph

A

downward curve (resistance over temp / LI)

31
Q

component graphs watch out for

A

switched axes

32
Q

component graphs watch out for

A

switched axes

33
Q

explain the variation of resistance with pd for the filament bulb in terms of particle behaviour

A
  • as V ^ velocity of electrons ^
  • greater energy transfer in collisions
  • increases the temp of filament ions
  • amplitude of vibrations increases
  • collision rate between ions/electros ^
  • due to R ^ current doesn’t increase in proportion to V
34
Q

explain how the graph shows this is a diode

A
  • conducts in one direction only
  • conducts when pd is beyond threshold voltage eg 0.2V
  • in reverse direction resistance is very high/ infinite
35
Q

how to get an IV graph for components

A
  • measure I and V for a range of values
  • vary V across the component
  • reverse the supply or polarity (if its a diode graph)
  • repeat and average
    (circuit with a variable power supply, ammeter, component, voltmeter)
36
Q

power

A

rate of transfer of energy
- watts or J/s
- for electrical power it’s a measure of how bright/hot / fast a component is

37
Q

power equations

A

P = E/t
P = IV
P = I^2 R
P = V^2 /R

38
Q

power of lamps experiment

A

connect 2 diff lamps (A and B) in series and parallel
measure I and V across each lamp
use P= IV
- in series the higher R lamp recieved more V so is brighter
- in parallel the higher R gets smaller I so is dimmer

39
Q

energy (big)

A

if we leave a component in it transfers electrical energy to other types

40
Q

energy equation

A

E = ItV

41
Q

kiloWatt hour

A

used instead of J as for household appliances J is too big
(use E = Pt with power in ke and t in hours)
1kWh = 3 600 000 J

42
Q

energy (small)

A

as one coulomb of charge passes through a 9V battery it gains 9J of charge

43
Q

1 volt in J/C

A

1 J/C
- for every coulomb of charge that passes though a battery of x volts it gains x joules

44
Q

electron volts

A

energy transferred to/ from an electron when it passes through a pd of 1V
eg if an electron passes though the 9V battery it gains 9eV
1eV = 1.6x10^-19 J

45
Q

electron gun

A

speeds up electrons with an accelerating pd (see diagram)
- thermionic emission causes electrons to escape the coil
- electrons are attracted to the anode
KE = 1/2mv^2
so V*e = 1/2mv^2

46
Q

max power

A

when internal resistance = R

a - physics (18 decks)

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