This lesson covers the fundamentals of electric circuits. You'll learn about the three key quantities—current, potential difference (voltage), and resistance—and the law that connects them all.

1. Short Notes: Core Concepts

A. The Three Key Players: Current, Voltage, Resistance

• Conventional Current vs. Electron Flow: By convention, we say current flows from the positive (+) terminal to the negative (-) terminal. In reality, electrons flow from negative to positive.

B. Ohm's Law: The Golden Rule of Circuits

• Statement: At a constant temperature, the current (I) flowing through a conductor is directly proportional to the potential difference (V) across it.

• Formula: V = I × R

• This is the most important formula in this chapter. It links voltage, current, and resistance.

C. Factors Affecting Resistance of a Conductor

• Length: Longer wire → Higher resistance.

• Cross-sectional Area: Thicker wire → Lower resistance.

• Material: Different materials have different abilities to resist current (e.g., copper has low resistance, nichrome has high resistance).

D. Resistors & Combining Them

• Resistors: Components made to have a specific resistance value.

• There are two main ways to connect them:

  1. Series Combination:

     • Connected end-to-end in a single path.

     • The same current flows through all resistors.

     • The total resistance is the sum of individual resistances.

     • Formula: R_total = R₁ + R₂ + R₃

     • Result: The total resistance is always greater than the largest single resistor.

  2. Parallel Combination:

     • Connected across the same two points, providing multiple paths for the current.

     • The total current splits among the branches.

     • The total resistance is found using the reciprocal formula.

     • Formula: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃

     • Result: The total resistance is always less than the smallest single resistor.

2. Tips & Tricks for the Exam

• Ammeter vs. Voltmeter: Remember A for Along the wire (Series) and V for aVross the component (Parallel).

• Ohm's Law Triangle: Draw a triangle with V at the top, and I and R at the bottom. Cover the quantity you want to find, and the formula is revealed.

• Parallel Resistance Quick Math: For two resistors in parallel, you can use the shortcut: R_total = (R₁ × R₂) / (R₁ + R₂). This is faster than the reciprocal method for two resistors.

3. Important Points & Common Exam Questions

• Simple Calculation: "A current of 0.5 A flows through a lamp with a resistance of 24 Ω. What is the potential difference across the lamp?"

  • Answer: V = IR = 0.5 A × 24 Ω = 12 V.

• Series Circuit: "Resistors of 5 Ω and 10 Ω are connected in series to a 6 V battery. What is the total current flowing from the battery?"

  • Answer: First, find total resistance: R_total = 5 Ω + 10 Ω = 15 Ω.

  • Then, find the current: I = V / R_total = 6 V / 15 Ω = 0.4 A.

• Parallel Circuit: "Resistors of 6 Ω and 12 Ω are connected in parallel. What is their equivalent resistance?"

  • Answer: 1/R_total = 1/6 + 1/12 = 2/12 + 1/12 = 3/12.

  • Flip the fraction: R_total = 12 / 3 = 4 Ω.

• Circuit Diagram Reading: You might be shown a circuit with ammeters and voltmeters and asked to calculate a reading. Use the rules for series/parallel and Ohm's Law step-by-step.