CURRENT ELECTRICITY
1. I = Q / t
Here I =
Electric current
Q = Electric charge
t = time
2. S.I. unit
of electric current is ampere.
3. 1 ampere
= 1 coulomb / 1 second
4. 1 ampere
= 3 x 109 stat ampere
5. 1 ampere
= 1 / 10 ab ampere
6. EMF =
Work done / charge
7. Differences between EMF and POTENTIAL
DIFFERENCE:
EMF –
§ It is the work done by a source in
taking a unit positive charge once round the complete circuit.
§ It is equal to maximum potential
difference between the two terminals of a source when it is in an open circuit.
§ It has non – electrostatic origin.
§ It is a cause.
§ It is larger than potential
differences across any circuit element.
§ It is independent of external
resistance of the circuit.
POTENTIAL DIFFERENCE –
§ It is the work done by a source in
taking a unit positive charge from one point to a circuit.
§ Potential difference may exist
between any two points of a closed circuit.
§ It exists only when the circuit is
closed.
§ It is an effect.
§ It is always less than EMF.
8. Ohm’s Law
–
It states
that at constant temperature, the current flowing through a conductor is
directly proportional to the potential difference applied across its ends.
V = R I
Here V =
Potential difference
R = Resistance
I = Electric current
9.
I
V
10.
Limitations of Ohm’s law –
·
Ohm’s
law is applicable only when the conductor is at constant temperature.
·
It
is applicable only for ohmic conductor.
11. The
conductor which obey Ohm’s law are called Ohmic conductors.
12. The
conductor which do not obey Ohm’s law are called non-ohmic conductor.
13. R = V /
I
Here R =
Resistance of conductor
V = Potential difference
I = Electric current
14. 1 ohm =
1 volt / 1 ampere
15. R α L
Here R =
Resistance
L = Length of the conductor
16. R α 1 / A
Here A = area of cross section
17. The resistance of a conductor also
depends on the nature of material.
18. S.I. unit of resistivity is Ohm
metre.
19. ρ = RA / L
Here ρ = Resistivity
20. J = I / A
Here J =Current density
I = Electric current
A = area
21. I = ʃ J. dS
22. G = 1 / R
Here G = Conductance
R = Resistance
23. S.I. unit of conductance is Ohm-1
or mho or siemens (s).
24. σ = 1 / ρ
Here σ = conductivity
ρ = Resistivity
25. S.I. unit of conductivity is Ohm-1
metre-1 .
26. J = E / ρ = σ E
27. Vd = a τ = -e E τ / m
Here Vd = drift velocity
τ
= Relaxation time
28. I = e n A Vd
Here I = electric current
N = No. of electrons
A = area of cross section of the
conductor
Vd = Drift velocity
29. J = e n Vd
30. R = m L / n e2 τ A
31. R = ρ L / A
32. ρ = m / n e2 τ
33. The resistivity of a metal
increases and conductivity decreases with increase in temperature.
34. The unit of temperature
coefficient of resistivity is 0celcius-1.
35. For metals, temperature
coefficient of resistivity is positive.
36. Resistances in Series:
If
a number of resistors are connected end to end so that the same current flows
through each one of them in succession, then they are said to be connected in
series.
R s = R 1
+ R 2 + R 3 +………..+ R n
ü Current through each resistance is same.
ü Equivalent resistance is larger than the
largest equivalent resistance.
37. Resistances in Parallel:
If
a number of resistances are connected in between two common points so that each
of them provides a separate path for current, then they are said to be
connected in parallel.
1 / R p =
1 / R 1 + 1 / R 2 + ………. + 1 / R n
ü Potential drop across each resistance is same.
38. r = R ( E – V / V )
Here r = internal resistance
R = Resistance
E = EMF
V = Potential difference
39. Kirchhoff’s laws –
ü First law /
Junction Rule / Current law –
In an electric circuit, the algebraic
sum of currents at any junction is zero.
∑ I = 0
ü Second law /
Loop rule / Voltage law –
It states that the algebraic sum
of EMFs in any loop of a circuit is equal to the sum of products of currents
and resistances in it.
∑ E = ∑ I R
Here E = EMF
R = Resistance
I = Current
ü The wheatstone bridge is said to be balanced
when the potential difference across the galvanometer is zero so that there is
no current through the galvanometer.
40. Joule’s law of electrical heating –
ü Heat produced is directly proportional to the
square of the current flowing through the conductor.
H α I2
ü Heat produced is directly proportional to
resistance of conductor.
H α R
ü Heat produced is directly proportional to the
time of flow of current.
H α
t
41. W = V I t
Here W = Electric energy
V = Potential Difference
t = Time
42. P = W / t = V I = I2 R
= V2 / R
Here V = Potential Difference
I = Electric current
R = Resistance
43. S.I. unit of electric power is
watt.
44. 1 KW = 1000 WATT
45. 1 MW = 106 WATT
46. The commercial unit of power is
horse power (h p).
47. 1 h p = 746 watt
48. W = P t = I2 R t
49. The commercial unit of electric
energy is kilowatt hour or Board of trade (B.O.T) unit.
50. 1 K W h = 3.6 x 106 joule
51. 1 W h = 3.6 x 103 joule
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