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Physics > unit 2 electricity > Flashcards

unit 2 electricity Flashcards

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1
Q

current

A

rate of flow of charge through a point

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2
Q

kirchhoff’s first law

A

principle conservation of charge

total current passes through a point = 0

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3
Q

I=nAqv

v velocity

n?
q?

A

n
-number of free charge carrier per unit volume
depends on material

q
- charge of free charge carrier

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4
Q

I=nAqv

v ?

A

drift velocity

rate change of distance travelled by free charge carriers along the wire per unit time

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5
Q

derivation of I = nAqv

A

pg 2

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6
Q

derivation of I = nAqv

homogeneous?

A

unit A

pg 3/4

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7
Q

why no current in plastic compare to copper

A

plastic is an insulator so n = 0

copper is a conductor n is very large

from I = nAqv , n= 0 in insulator so I = 0

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8
Q

for same current flow and same dimension,
drift speed of the charge carriers in semiconductor is much higher than metal

why

A

I = nAqv

v is proportional to 1/n

n semiconductor is much lower than metal
so drift speed of semiconductor is much higher

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9
Q

thin wire connected in series with thick wire made of same material

drift speed of electron in thin wire is higher because

A
  1. thin wire smaller cross sectional area
  2. connected in series, same current flow
  3. same material so same n and Q
  4. from I = nAqv
  5. drift speed, v is directly proportional to 1/A
  6. thinner wire higher drift speed
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10
Q

drift velocity
charged particles move faster through the wire with smaller diameter

why?

A

because it has a larger potential difference applied to it

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11
Q

explain the different in resistance of conductor with bigger diameter and conductor with smaller diameter

in drift velocity

A
  1. drift velocity greater for conductor with smaller diameter. therefore electrons gain more kinetic energy btwn collision with lattice ions
  2. more frequent collisions with lattice ions
  3. more energy lost in a given time in collision with lattice ions
  4. greater p.d required for a given current ( V =E/Q )
  5. resistance of conductor with the smaller diameter is greater than big diameter conductor ( R=V/I )
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12
Q

as the temperature of the wire increases, ions in the lattice gain higher thermal energy

what happens to drift speed

A

higher thermal energy, vibrate with larger amplitude
rate of collision of electrons with vibrating ions increase

drift speed decreases

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13
Q

motion of charged carriers in wire

A
  1. the charges on the plates of a cell
    - attract and repel charge carries in wire
  2. these forces make charge carriers accelerate ( due to electric force ) until they collide with atoms / ions within material
    - transfer of energy
    - increasing in temperature
  3. charge carrier continues to accelerate as before
  4. as a result of this cycle of acceleration and collision
    - charge carriers settle into constant average speed
  5. they are continually gaining energy from cell and losing energy to material

graph pg 5

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14
Q

potential difference

A

work done when unit charge flow from one point to another point

electrical energy converted to other form of energy per unit charge

v= work done / charge
v= power / current
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15
Q

electric potential energy

A

E = QV

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16
Q

ideal voltmeter

A

connected in parallel
infinite internal resistance
no current flows through voltmeter
potential diff across component remain constant

17
Q

ideal ammeter

A

connected in series

no resistance

no potential diff across ammeter , current remains constant

18
Q

electromotive force

A

the work done by cell to bring unit positive charge through complete circuit

e= work done by cell/ charge

unit : volt
involves change in chemical energy to electric energy per unit charge

19
Q

kirchhoff’s second law

A

principle conservation of energy

sum of emf through a complete circuit is equal to sum of potential differences

emf = IR

20
Q

resistance

A

potential diff across component per unit of current pass through the component
definition of R: V = IR ( not pd )

R= V/I

unit ohm

21
Q

ohmmeter

disadvantage

A

disadvantage

  • not very accurate for low resistance component
  • reading depends on cell in ohmmeter which may flat as use for long time

while using prevent contact resistance

  • resistance to current flow due to surface conditions
  • cause high voltage drop in system

contact corrosion leads to

  • power loss
  • heat generation
22
Q

ohmmeter

not accurate
so what else can we use

A

use ammeter and voltmeter to get readings and use formula R = V/I

advantage
- more accurate

disadvantage

  • high current heats up component
  • change resistance
  • may change few values of V and I

if plot V vs I
straight line through origin , gradient = resistance

23
Q

ohmmeter

advantage

A

advantage

  • very small current flows through the component
  • resistance remains constant
24
Q

ohmmeter

not accurate
so what else can we use

R is not gradient except for ohmic conductor

so how to get R for non ohmic

A

R=V/I
NOT R=change in V/ change in R

gradient = R=change in V/ change in R = ohmic conductor only , gradient passes through origin

25
Q

ohmic conductor
conductor that obeys the ohm’s law

define

A

ohm’s law
- potential difference across ohmic conductor is proportional to the current provided external conditions

  • eg. temp remains constant

-for object to obey ohm’s law
resistance remains constant at any values of pd / current

-copper at low current flow obeys ohm’s law cause temp constant

26
Q

filament lamp an ohmic conductor ?

A

no

working temp very high

27
Q

filament lamp NOT an ohmic conductor

explain how resistance change indicates in graph pg 10

A
  1. at low voltage the resistance is constant / constant gradient
    - when voltage increases resistance increases
  2. at low voltage, temp of lamp constant, reisistance constant
  3. as voltage increases, current increases
  4. current heats up lamp temp increases
  5. resistance of lamp increases

conclusion : as v increases , for equal increases in V leads to smaller increase in I , resistance increases

  • do not talk about gradient cause resistance not equal to gradient
28
Q

filament lamp NOT an ohmic conductor

explain in microscopic term for the increase in resistance

A

metal, temp increases resistance increases

  1. R due to collision btwn electrons and ions
  2. V increases I increases and temp increase
  3. ions in lattice more thermal energy , vibrate with larger amplitude
  4. increase frequency of collision
  5. offer more resistance to the flow of electrons so resistance increases
29
Q

filament lamp NOT an ohmic conductor

why the filament is more likely to fail when being switch on rather than at other times

A

initially temp is low , current is high

resistance of lamp increases as temp increases

current falls to steady calue when temp constant

max current, hence max heating when lamp is switched on

filament breaks due to melting caused by temp rise

30
Q

thermistor NTC

NON ohmic conductor

negative temp coefficient means

A

when temp rises

resistance of thermistor decreases

31
Q

thermistor NTC

NON ohmic conductor

graph and symbol

A

pg 11

graph is a reverse S shaped through origin

symbol rectangle and a hockey stick right through the middle

32
Q

thermistor NTC

NON ohmic conductor

explain how R change indicates in graph pg 11

A

as voltage increases, current increases

current heats up the thermistor

temp of thermistor increases

resistance of thermistor decreases

as V increases, for equal increase in V leads to larger increase in I so resistance decreases

33
Q

thermistor NTC

NON ohmic conductor

explain in microscopic term for the decrease in resistance

A

semiconductor, temp increases resistance decreases

  1. V increases I increases and temp increase
  2. ions in lattice more thermal energy , vibrate with larger amplitude
  3. more charged carrier set free , n increases
  4. I = nAqv, I increases, R=V/I resistance decreases
  5. same time rate of collision bten ions and charge carriers increase
  6. offer more resistance to the flow of charge carriers so resistance increases
  7. the decrease in R is much larger than the increase in R so overall effect is resistance decrease
34
Q

diode including LED

NON OHMIC CONDUCTOR

forward biased , current flows
reverse biased, no current flow

function and pratical use

A

only allow current to flow in one direction

usage

  1. rectification change AC to DC
  2. produce DC supply
  3. protect components
  4. power indicator light ( LED )
35
Q

extra note on ohm’s law

which 2 equations are the representation of resistance R

A

V=IR

R=V/I (non ohmic conductor ‘s resistance can be found using this ) , but value is not a constant

note: not representation of ohm’s law

36
Q

resistivity

symbol: p

defined as / formula
unit

A

( resistance times cross-sectional area )
over
length

unit : ohm metre

37
Q

resisitivity of a material is also equal to

resistivity depends on

A

the resistance btwn 2 opposite faces of a cube of the material whose sides are of unit length

resistivity depends on material

Physics (12 decks)

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