NCERT Solution Class 12th Physics Chapter – 3 Current Electricity
| Textbook | NCERT |
| class | Class – 12th |
| Subject | Physics |
| Chapter | Chapter – 3 |
| Chapter Name | Current Electricity |
| Category | Class 12th Physics Notes |
| Medium | English |
| Source | last doubt |
NCERT Solution Class 12th Physics Chapter – 3 Current Electricity
?Chapter – 3?
✍Current Electricity✍
?Notes?
The time rate of flow of electric charge is called electric constant.
S.I. unit an electric current is Ampere (A).
1A = 1 C S-1.
Although a direction is associated with the electric current, yet it is a scalar quantity.
The current density is a vector quantity directed along the direction of the flow of current.
The number density of free electrons is of the order of 1022 per cm3.
The number density of free electrons is negligible in insulators.
S.I. unit of resistance is the ohm (Ω)
The reciprocal of resistance is conductance and has S.I. unit mho or Siemen (S).
The conductor is said to be ohmic if they obey Ohm’s law. The V-I graph for such a conductor is a straight line.
The conductors are said to be non-ohmic if they don’t obey Ohm’s law. The V-I graph is not a straight line for such conductors.
When current is drawn from a cell its terminal potential difference is less than the e.m.f. of the cell.
Series combination of cells is used when the internal resistance of the cell is negligible as compared to the external resistance of the circuit.
The parallel combination of cells is used when the external resistance of the circuit is much smaller as compared to the internal resistance of the cell
The mixed grouping of cells is used when the external resistance of the circuit is of the same order as the internal resistance of the cell i.e., R ≈ r.
Wheatstone bridge is a circuit consisting of four resistances P, Q, R, and S a galvanometer and a battery connected such that
P/Q=R/S
It is said to be balanced when there is no current through the galvanometer.
Metre bridge or Slide wire bridge is the commonly used form of the wheat stone bridge.
The current in the external circuit flows from the + ve to – ve terminal of the cell or battery and is called conventional current which is opposite to the electronic current.
Current is the same through the resistors connected in series.
The pot. difference is the same through the resistors connected in parallel.
1 A = 6.25 × 1018 electrons flow per second
When a cell is short-circuited, the terminal potential diff. across it is zero.
α for most metals is 1/273K-1.
a (temperature coefficient of resistance) for insulators and semiconductors is – ve but + ve for metals.
The terminal P.D. of a cell depends on the internal resistance (r) of the cell, hence it also depends on the factors on which r depends like, the area of plates, the separation between the plates, cone, electrolyte, nature of electrodes, temperature, etc.
1 KWh = 3.6 × 106 J.
Ohm’s law: States that if physical conditions of a conductor like temperature etc. remain unchanged, then the current flowing through it is directly proportional to the potential difference applied across it.
Resistance of a conductor is defined as the opposition offered by it to the flow of current. It is equal to the ratio of P.D. (V) and current (I) through the conductor.
i.e, R = V/I
Current density (I) – It is defined as the current per unit area of the cross-section of the conductor.
i.e., J = I/A
The internal resistance of a cell – It is defined as the resistance offered by the electrolyte of the cell to the flow of current through it.
Conductance – It is defined as the reciprocal of the resistance of the conductor.
i.e., G = 1/R
Conductivity – It is defined as reciprocal of the resistivity of the conductor i.e. σ = 1/ρ
Temperature coefficient of resistance of a conductor – It is defined as the increase in resistance per unit original resistance at 0°C per unit rise in its temperature.
Principle of potentiometer – It states that when a constant current is passed through a conductor of the uniform area of cross-section, the potential drop across any part of it is always directly proportional to the length of that part.
V ∝ l
Electric energy – It is defined as the total work done by the source of energy in maintaining the electric current through the circuit for a given time.
KWh – The electric energy consumed or dissipated in the circuit is said to be 1 Kilowatt-hour if a device of 1 kW power is used for one hour. It is also called UNIT.
Electric power – It is defined as the rate of doing work by the source .of e.m.f. in maintaining the electric current in the circuit.
1 Watt – The electric power of a circuit or a device is said to be 1 watt if one ampere current flows through it on applying a P.D. of one volt.
Shunt – It is defined as a small resistance connected in parallel to the cell.
Important Formulae
Current density (J) and electric field are related as:
J = σE
R = ρ l/A
ρ = 1/σ
where ρ = resistivity or specific resistance of the conductor having conductivity σ.
internal resistance of the cell is given by
Using potentiometer r is calculated using
where l1 and l2 are balancing lengths with cell in open closed circuits respectively.
S = shunt resistance
Drift velocity is given by
υd = I/neA
or
I = neAvd.
Current in the serìcs circuit of n cells is
Is = nE/R+nr
Current in the circuit of m cells in parallel is given by
In mixed grouping of cells, I in the circuit is given by,
I due to a single cell is
I = E/R+r
The equivalent resistance and power of resistance connected in series are given by:
Rs = R1 + R2 + R3 + ……………
and 1/Ps = 1/P1 + 1/P2 + 1/P3 +…
Time required to neutralise earth’s surface,
Where R = radius of earth,
σ = surface charge density
I = current over globe
The equivalent resistance and power of resistance connected in parallel are given by and
1/RP = 1/R1 + 1/R2 + 1/R3 +… and
Pp = P1 + P2 + P3 + ………….
Electric energy is given by
E = Pt = VIt = I2 Rt = V2/R t.
Electric power is given by
P = E/t = VI = I2 R = V2/R
Variation of resistance and resistivity with temperature is given by
Rt = R0 (1 + α Δ t)
and pt = p0 (1 + αΔt)
V = kl for potentiometer.
E1/E2 = l1/l2 where E1 and E2 are emfs of two cells l1, l2 = corresponding balancing lengths.
where τ = relaxation time,
n = current density of free electron,
e = charge of an electron.