Ohm's Law, Resistance and Power
Overview
Ohm's Law, Resistance, and Power form the foundation of electrical theory for ITI Electrician Semester 1 (NCVT). These concepts explain the relationship between voltage (V), current (I), and resistance (R), and how electrical energy is converted to heat or work. Mastery of formulas, units, and practical applications is essential for both theory and practical papers. Questions frequently test formula application, unit identification, and numerical problem-solving.
Typed Notes
Ohm's Law — Statement, Formula & Conditions
Ohm's Law states: 'The current flowing through a conductor is directly proportional to the voltage across it, provided temperature and other physical conditions remain constant.' Formula: V = I × R. Rearranged: I = V/R and R = V/I. The unit of resistance is Ohm (Ω), named after Georg Simon Ohm. This law applies only to ohmic (linear) conductors like copper and aluminium wire. It does NOT apply to semiconductors, electrolytes, or gas-discharge tubes. Exam Tip: Draw the VIR triangle — cover the unknown to get the formula instantly.
Resistance — Factors & Resistivity
Resistance (R) opposes the flow of current. It depends on: (1) Length — R increases as length increases (directly proportional). (2) Cross-sectional Area — R decreases as area increases (inversely proportional). (3) Material — measured by resistivity (ρ, rho); copper has low ρ ≈ 1.7×10⁻⁸ Ω·m, making it ideal for wiring. (4) Temperature — for metals, resistance increases with temperature; for semiconductors, it decreases. Formula: R = ρL/A. Unit of resistivity: Ohm-metre (Ω·m). Memory Hook: 'LMAT' — Length, Material, Area, Temperature.
Power, Energy & Joule's Law
Electrical Power is the rate of doing work or consuming energy. Formulas: P = VI, P = I²R, P = V²/R. Unit: Watt (W); 1 HP = 746 W. Joule's Law of Heating: Heat produced H = I²Rt joules, where t is time in seconds. Electrical Energy = P × t; measured in kWh for billing (1 Unit = 1 kWh). Efficiency η = (Output Power / Input Power) × 100%. Exam Tip: In numerical problems, always check if time is in seconds (for Joules) or hours (for kWh). 1 kWh = 3.6 MJ.
Series and Parallel Circuits
Series Circuit: Same current flows through all components; total resistance R_T = R1+R2+R3; voltage divides across each resistor (V = IR). Parallel Circuit: Same voltage across all branches; total resistance 1/R_T = 1/R1+1/R2+1/R3; current divides among branches. For two resistors in parallel: R_T = (R1×R2)/(R1+R2) — called the product-over-sum rule. Key fact: Adding more resistors in parallel REDUCES total resistance. Practical use: House wiring uses parallel connection so each appliance gets full 230 V supply. Exam Tip: Series = same I; Parallel = same V.
Topics & Sub-topics
Har topic ke saath sub-topics aur example — chapter ka complete coverage.
01Voltage, Current, Resistance
- Voltage (V) = pushing force, volt
- Current (I) = flow of charge, ampere
- Resistance (R) = opposition, ohm
220 V, 100 W bulb ka resistance hot state par ≈ 484 Ω (R = V²/P).
02Ohm's Law V = IR
- Current ∝ voltage at fixed resistance
- Triangle: V, I, R
- Limitations: non-linear devices
12 V battery, 4 Ω bulb → I = V/R = 3 A. Yahi hisaab site par roz lagta hai.
03Series and Parallel Circuits
- Series: Rt = R1+R2, same current
- Parallel: 1/Rt = 1/R1+1/R2, same voltage
- House wiring = parallel
Do 60 W bulb series mein 220 V par dim jalte hain, parallel par full brightness.
04Power and Energy
- P = VI = I²R = V²/R, watt
- Energy = P × t, kWh = 1 unit
- Electricity bill = kWh
2 kW geyser 1 ghante mein 2 unit khaata hai — 30 min = 1 unit.
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01Ohm's Law: V = I × R, where V = Voltage (Volts), I = Current (Amperes), R = Resistance (Ohms Ω) — valid only for constant temperature and linear conductors.
- 02Resistance depends on four factors: material (resistivity ρ), length (L), cross-sectional area (A), and temperature. Formula: R = ρL/A.
- 03Power Formula Triangle: P = V × I = I²R = V²/R; unit of Power is Watt (W); 1 kW = 1000 W.
- 04Energy = Power × Time; unit is Joule (J) or kilowatt-hour (kWh); 1 kWh = 3.6 × 10⁶ Joules (1 Unit of electricity).
- 05Resistors in Series: R_total = R1+R2+R3; Resistors in Parallel: 1/R_total = 1/R1+1/R2+1/R3; current divides in parallel, voltage divides in series.
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
A 230 V supply is connected across a resistance of 46 Ω. Calculate the current flowing through the circuit.
A resistor carries 4 A current and dissipates 64 W of power. Find the resistance of the resistor.
State the four factors on which the resistance of a conductor depends.
Two resistors of 6 Ω and 3 Ω are connected in parallel. Find the equivalent resistance.
A 1000 W electric heater is used for 5 hours daily. Calculate the energy consumed in kWh for 30 days and find the cost at ₹6 per unit.
Ohm's Law (V=IR) links voltage, current, and resistance under constant temperature. Resistance depends on length, area, material (resistivity), and temperature. Power formulas (P=VI, I²R, V²/R) and Joule's heating law (H=I²Rt) are heavily tested numerically. Energy is measured in kWh (1 Unit). Series circuits share current; parallel circuits share voltage. These fundamentals underpin all electrical trade calculations in NCVT ITI Electrician exams.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.Which of the following is the correct expression for Ohm's Law?
Q2.The unit of electrical energy used for billing purposes is:
Q3.In a parallel circuit, which quantity remains the same across all branches?
Q4.Resistivity (ρ) of a material is measured in which unit?
Mark karo — dashboard par progress dikhega.