unit 2 electricity

Question 1

current

Answer 1

rate of flow of charge through a point

Question 2

kirchhoff’s first law

Answer 2

principle conservation of charge

total current passes through a point = 0

Question 3

I=nAqv

v velocity

n?
q?

Answer 3

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

q
- charge of free charge carrier

Question 4

I=nAqv

v ?

Answer 4

drift velocity

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

Question 5

derivation of I = nAqv

Answer 5

pg 2

Question 6

derivation of I = nAqv

homogeneous?

Answer 6

unit A

pg 3/4

Question 7

why no current in plastic compare to copper

Answer 7

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

Question 8

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

why

Answer 8

I = nAqv

v is proportional to 1/n

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

Question 9

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

drift speed of electron in thin wire is higher because

Answer 9

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

Question 10

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

why?

Answer 10

because it has a larger potential difference applied to it

Question 11

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

in drift velocity

Answer 11

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 )

Question 12

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

what happens to drift speed

Answer 12

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

drift speed decreases

Question 13

motion of charged carriers in wire

Answer 13

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

Question 14

potential difference

Answer 14

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

Question 15

electric potential energy

Answer 15

E = QV

Question 16

ideal voltmeter

Answer 16

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

Question 17

ideal ammeter

Answer 17

connected in series

no resistance

no potential diff across ammeter , current remains constant

Question 18

electromotive force

Answer 18

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

Question 19

kirchhoff’s second law

Answer 19

principle conservation of energy

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

emf = IR

Question 20

resistance

Answer 20

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

R= V/I

unit ohm

Question 21

ohmmeter

disadvantage

Answer 21

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

Question 22

ohmmeter

not accurate
so what else can we use

Answer 22

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

Question 23

ohmmeter

advantage

Answer 23

advantage

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

Question 24

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

Answer 24

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

Question 25

ohmic conductor conductor that obeys the ohm's law define

Answer 25

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

Question 26

filament lamp an ohmic conductor ?

Answer 26

no | working temp very high

Question 27

filament lamp NOT an ohmic conductor | explain how resistance change indicates in graph pg 10

Answer 27

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

Question 28

filament lamp NOT an ohmic conductor explain in microscopic term for the increase in resistance

Answer 28

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

Question 29

filament lamp NOT an ohmic conductor why the filament is more likely to fail when being switch on rather than at other times

Answer 29

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

Question 30

thermistor NTC NON ohmic conductor negative temp coefficient means

Answer 30

when temp rises resistance of thermistor decreases

Question 31

thermistor NTC NON ohmic conductor graph and symbol

Answer 31

pg 11 graph is a reverse S shaped through origin symbol rectangle and a hockey stick right through the middle

Question 32

thermistor NTC NON ohmic conductor explain how R change indicates in graph pg 11

Answer 32

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

Question 33

thermistor NTC NON ohmic conductor explain in microscopic term for the decrease in resistance

Answer 33

semiconductor, temp increases resistance decreases 2. V increases I increases and temp increase 3. ions in lattice more thermal energy , vibrate with larger amplitude 4. more charged carrier set free , n increases 5. I = nAqv, I increases, R=V/I resistance decreases 6. same time rate of collision bten ions and charge carriers increase 7. offer more resistance to the flow of charge carriers so resistance increases 8. the decrease in R is much larger than the increase in R so overall effect is resistance decrease

Question 34

diode including LED NON OHMIC CONDUCTOR forward biased , current flows reverse biased, no current flow function and pratical use

Answer 34

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 )

Question 35

extra note on ohm's law which 2 equations are the representation of resistance R

Answer 35

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

Question 36

resistivity symbol: p defined as / formula unit

Answer 36

( resistance times cross-sectional area ) over length unit : ohm metre

Question 37

resisitivity of a material is also equal to resistivity depends on

Answer 37

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