Current Electricity Part 4 Numericals on KCL, KVL, Wheatstone & Meter Bridge Class 12 Physics

Описание: In this video we do *Current Electricity – Part 4 (Numericals)* for Class 12 Physics.

We now apply the *Kirchhoff Toolkit* (KCL & KVL sign rules) to solve:
• basic circuit numericals,
• Wheatstone bridge,
• and meter bridge questions.

Taught in clear Hinglish, aligned to **NCERT + JEE Main level**.

Topics / Problems covered:

• Quick toolkit recap (very short)
– How to assume current directions
– KVL sign rules for resistors (±IR)
– KVL sign rules for cells (±ε)
– Junction rule (KCL): ΣI_in = ΣI_out

• Problem 1: Single loop with internal resistance
– Cell: ε = 12 V, internal resistance r = 1 Ω
– External resistor: R = 5 Ω
– Find:
(a) current in the circuit,
(b) terminal voltage of the cell
– Uses: I = ε / (R + r), V_terminal = ε − Ir

• Problem 2: Two opposing cells + resistor
– Loop with cells ε₁ = 6 V and ε₂ = 4 V (opposite orientation)
– Resistor R = 10 Ω in series
– Assume I clockwise, write KVL:
+ε₁ − IR − ε₂ = 0
⇒ I = (ε₁ − ε₂) / R
– Interpretation of negative current (if it had come): actual flow opposite to assumed.

• Problem 3: Balanced Wheatstone bridge
– P = 10 Ω, Q = 20 Ω, R = 15 Ω, S = 30 Ω, cell 6 V
– Check balance using:
P/Q = R/S?
Here: 10/20 = 15/30 = 1/2 ⇒ bridge is balanced
– Hence galvanometer current I_G = 0 (midpoints at same potential).

• Meter Bridge – Problem 5: Simple case
– Known resistance R = 4.0 Ω in one gap, unknown X in the other
– Balance point at ℓ = 60 cm from R side
– Use meter bridge balance:
X / R = ℓ₁ / ℓ₂
Here wire lengths:
R side: 60 cm
X side: 40 cm
⇒ X / 4.0 = 40/60 = 2/3
⇒ X = 8/3 Ω ≈ 2.67 Ω

• Meter Bridge – Problem 6: Including resistance of wire
– Bridge wire: 1.0 Ω over 1 m (uniform)
– One gap: 3.0 Ω, other gap: unknown X
– Balance point at 40 cm from 3.0 Ω side
– First find:
R_40 = 1.0 × (40/100) = 0.40 Ω
R_60 = 1.0 × (60/100) = 0.60 Ω
– At balance:
3.0 / X = R_40 / R_60 = 0.40 / 0.60 = 2/3
⇒ X = 4.5 Ω
– Shows how to include wire’s resistance while still using Wheatstone ratio.

• Summary
– Applying KCL & KVL to simple loops
– Recognising a balanced Wheatstone (P/Q = R/S ⇒ I_G = 0)
– Using meter bridge formulas:
X / R = ℓ / (100 − ℓ)
and extended case with wire resistance

This video is ideal for:
• Class 12 CBSE / ISC / State Board students,
• JEE Main / NEET aspirants revising circuit numericals,
• Anyone wanting a solid practice set on Current Electricity basics.

Watch other parts in this series:
• Part 1: Current, Ohm’s Law & Resistance
• Part 2: Series/Parallel, EMF & Internal Resistance
• Part 3: Kirchhoff’s Laws, Wheatstone Bridge & Meter Bridge
• Toolkit: KCL & KVL sign rules for numericals
• Upcoming: Bonus – Unbalanced Wheatstone (Galvanometer Current) + Revision set

📌 Check the *Current Electricity playlist* to follow the chapter in order.