Courants et circuits à courant continu Flashcards
Define electric current and give equation
Rate of flow of charge
I=Q/t
What does area under I-t graph represent?
Charge
How to derive drift velocity?
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
What is drift velocity formula?
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)
Define potential difference and give equation
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
Define resistance and give equation
Resistance of a circuit component is ratio of p.d. across component to current flowing thru it
R=V/I
Can potential be negative? (not potential difference) What does it mean?
Yes. It just means it is less than zero (just like negative temperatures are colder than 0 degrees)
What is potential divider formula?
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
What direction does current flow?
- Positive to negative end of battery (long to short line)
OR - High to low potential
What value potential is earth?
Always 0V
What is equation connecting resistance R, length, cross-sectional area and resistivity?
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)
What is electrical power formula?
P=VI=I²R=V²/R
What are the conservation laws for circuits?
- Total current entering a junction = total current leaving junction
- Sum of emf round a circuit = sum of pd round the circuit
Define electromotive force and give equation
E.m.f. of a source is energy converted from other form to electrical energy per unit charge
E=W/Q
What are the formulae to find terminal potential difference across a non-ideal battery?
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
What is the maximum power transfer theorem?
Power produced is maximum when R=r,
where R is external resistor,
r is internal resistance of battery
What happens with a galvanometer when there is balanced potential?
When galvanometer is connected btw 2 pt of same potential, current will not flow in galvanometer; galvanometer will show null deflection
What is the potentiometer principle?
At balance length, Vxj = Vab
Describe I-V graph of metallic ohmic conductor at constant temperature
- +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
Describe I-V graph of filament lamp
- 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)
Describe I-V graph of NTC thermistor
- 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)
Describe I-V graph of semiconductor diode
- 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
Describe light dependent resistor (LDR)
- 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
