Courants et circuits à courant continu

Question 1

Define electric current and give equation

Answer 1

Rate of flow of charge

I=Q/t

Question 2

What does area under I-t graph represent?

Answer 2

Charge

Question 3

How to derive drift velocity?

Answer 3

Consider conductor length L, cross-sectional area A thru which current I flows. No. density of charge (no. per unit volume) of charge carriers is n & charge of each charge carrier is q. Avg drift velocity of charge carriers is v

no. of charge carriers in conductor = nAL
charge in conductor, Q = nALq
Time for charge carriers to leave the conductor, t=L/v
Hence current I=Q/t = (nALq)/(L/v) = nAvq

Question 4

What is drift velocity formula?

Answer 4

I=nAvq

where
I is current,
n is no of charge carriers per unit volume,
A is cross-sectional area of wire
v is drift velocity
q is charge of one charged particle (eg electron)

Question 5

Define potential difference and give equation

Answer 5

P.d. Btw 2 pt in circuit is electrical energy converted to other forms of energy per unit charge flow from one pt to the other

V=W/Q

Question 6

Define resistance and give equation

Answer 6

Resistance of a circuit component is ratio of p.d. across component to current flowing thru it

R=V/I

Question 7

Can potential be negative? (not potential difference) What does it mean?

Answer 7

Yes. It just means it is less than zero (just like negative temperatures are colder than 0 degrees)

Question 8

What is potential divider formula?

Answer 8

Vi = (Ri/Rt)V

where
Vi is pd to find across a component,
Ri is resistance of this component,
Rt is total resistance in series
V is total pd

Question 9

What direction does current flow?

Answer 9

• Positive to negative end of battery (long to short line)
  OR
• High to low potential

Question 10

What value potential is earth?

Answer 10

Always 0V

Question 11

What is equation connecting resistance R, length, cross-sectional area and resistivity?

Answer 11

R=ρl/A

where
R is resistance,
ρ is resistivity,
l is length (// to direct n of current),
A is cross sectional area (normal to direct n to current)

Question 12

What is electrical power formula?

Answer 12

P=VI=I²R=V²/R

Question 13

What are the conservation laws for circuits?

Answer 13

1. Total current entering a junction = total current leaving junction
2. Sum of emf round a circuit = sum of pd round the circuit

Question 14

Define electromotive force and give equation

Answer 14

E.m.f. of a source is energy converted from other form to electrical energy per unit charge

E=W/Q

Question 15

What are the formulae to find terminal potential difference across a non-ideal battery?

Answer 15

V = IR = E - Ir = (R/R+r)E

where
I is current (constant in series circuit),
R is resistance of external resistor,
r is internal resistance of battery,
E is actual e.m.f. of battery

Question 16

What is the maximum power transfer theorem?

Answer 16

Power produced is maximum when R=r,

where R is external resistor,
r is internal resistance of battery

Question 17

What happens with a galvanometer when there is balanced potential?

Answer 17

When galvanometer is connected btw 2 pt of same potential, current will not flow in galvanometer; galvanometer will show null deflection

Question 18

What is the potentiometer principle?

Answer 18

At balance length, Vxj = Vab

Question 19

Describe I-V graph of metallic ohmic conductor at constant temperature

Answer 19

• +ve straight line graph, pass thru O
• resistors for which ratio V to I const r said obey Ohm’s law (ohmic resistor)
• resistance in metal is due to collis n of e- w lattice structure of ion
• if temp kept constant, mag vibrat n of lattice ion remain same, so resistance same

Question 20

Describe I-V graph of filament lamp

Answer 20

• y = √ x on positive V, y = - √(-x) on negative V graph
• as pd across filament increase, current increase
• energy dissipated as heat fr filament increase, cause higher temp
• higher temp, greater amplitude of vibrat n of lattice ion, so increase chance collis n btw free e- & lattice ion (no. density of charge carrier slightly changed)
• resistance of filament increase (ratio V/I increase)

Question 21

Describe I-V graph of NTC thermistor

Answer 21

• y = x³ graph
• thermistor made of semiconductor material
• increase pd across thermistor increase current, cause temp rise
• temp rise cause increase no of mobile charge carriers
• effect increase in no. density charge carriers outweigh effect of increase in collis n btw free e- & lattice ion
  => resistance decrease (ratio V/I decrease)

Question 22

Describe I-V graph of semiconductor diode

Answer 22

• increasing curve +ve V, small curve towards 0 grad graph
• diode hv low resistance when in forward bias (current flow in direct n of arrow in symbol)
• diode hv vv high resistance when in reverse bias (when current flow in opp direct n to arrow in symbol)
  NOTE: for typical diode, current flow in forward bias oni when V above some threshold value

Question 23

Describe light dependent resistor (LDR)

Answer 23

• LDR r semiconductors whose resistance decrease as light intensity incident increases
• light falling onto LDR release more charge carriers, increasing ability of LDR conduct current