Current Electricity

Question 1

Average current

Answer 1

iₐᵥ₉ = ∆Q/∆T

Question 2

Instantaneous current

Answer 2

iᵢₙₛₜₐₙₜₐₙₑₒᵤₛ = dq/dt

Question 3

Drift velocity

Answer 3

• Vd = -eEτ/m
• Vd = eVτ/mℓ (in terms of potential)

e: charge of electron
E: electric field
τ: relaxation time
V: potential
m: mass of electron

Question 4

Current in terms of drift velocity

Answer 4

i= VₔenA

Vₔ: drift velocity
e: charge on electron
n: number of electron per unit vol
A: area of cross section

Question 5

Resistance
(in terms of relaxation time)

Answer 5

R = (mℓ) / ne²τA

Question 6

Resistivity
(in terms of relaxation time)

Answer 6

ρ= m/ne²τ

Question 7

Effect of temperature for conductors and metals

Answer 7

• Resistance and resistivity increases with increase in temp

As temp increases, vibration of electrons increases and relaxation time decreases

Question 8

Effect of temp for semiconductors

Answer 8

• As temperature increases, resistance and resistivity decreases

With increase in temperature…number of electrons increase

Question 9

Effect of temp for alloys

Answer 9

• For alloys like nichrome, manganin and constantan, resistance remains unchanged with increase in temperature

Question 10

Kirchoff’s junction rule (current law) is based on

Answer 10

Conservation of charge

Question 11

Kirchoff’s loop rule (voltage law) is based on

Answer 11

Conservation of energy

Question 12

Effect of temp for insulators

Answer 12

Resistance decreases with increase in temperature

Question 13

Standard resistors are made using

Answer 13

Manganin

Manganin has large resistivity and being an alloy, the effect of temp on resistance is negligible

Question 14

A steady current flows in a metallic conductor of non-uniform cross section. The quantity that remains constant along the length of the conductor are…

Answer 14

Current only

Question 15

Microscopic form of ohms law

Answer 15

J = σI

J: current density
σ: conductivity
I: current

Question 16

Conductivity σ

Answer 16

σ = 1/ ρ

Question 17

Temp coeff of resistivity

Answer 17

Rt = Ro (1 + α ∆T)

Rt: final resistance
Ro: initial resistance
α: temp coeff
∆T: change in temp

Question 18

Emf (ε)

Answer 18

• ε = V + Ir
• ε = I(R + r)

Question 19

Terminal potential difference (V)

Answer 19

• V = IR
• V = εR/ (R + r)

R: resistance
r: internal resistance

Question 20

Terminal potential difference during charge and discharge

Answer 20

when current is being drawn
V= ε - Ir

When cell is being charged
V = ε + Ir

Question 21

Emf and internal resistance in series

Answer 21

• εₙₑₜ = ε₁ + ε₂ +…. (depends on polarity)
• rₙₑₜ = r₁ + r₂ +…. (always adds up)

Question 22

Emf in parallel

Answer 22

• εₙₑₜ/ rₙₑₜ = ε₁/r₁ + ε₂/r₂
• 1/rₙₑₜ = 1/r₁ + 1/r₂

Question 23

Energy (U)

Answer 23

U = QV

Question 24

Power

Answer 24

• In series
  P = I²R
• In parallel
  P = V²/R or P = VI

Question 25

Joules law of heating

Answer 25

H = I²Rt H: heat I: current R: resistance t: temp

Question 26

Relaxation time (τ)

Answer 26

τ = λ / vₐᵥ₉ vₐᵥ₉: average speed of electrons λ: mean free path

Question 27

Rated voltage

Answer 27

The maximum voltage which can be applied on the bulb without it fusing

Question 28

Resistance of bulb

Answer 28

R= V²/P V: rated voltage P: rated power

Question 29

Rated power

Answer 29

The power which the bulb would consume when max (rated) voltage is applied P= V²/R P: rated power V: rated voltage R: resistance of bulb