We are now entering a chapter that powers the modern world. Electricity is often considered one of the toughest topics in General Science because it is invisible; you cannot see current flowing like water. However, if you master the analogies and the core relationships (like Ohm's Law), this chapter becomes very scoring. In your exam, questions will range from numericals on power bills to conceptual questions on why fuses blow. Here is your comprehensive, dissected study guide for Electricity.

1. The Basics: Current & Potential

To understand electricity, the best analogy is water flowing in a pipe.

A. Electric Current (I)

Think of this as the Rate of Flow of water. In physics, it is the rate of flow of electric charges (electrons) through a conductor.
Formula: I = Q / t (Charge / Time).
SI Unit: Ampere (A). (1 Ampere = 1 Coulomb of charge flowing per second).
Instrument: Ammeter.

• It is always connected in Series.
• Why? Because it has very low resistance and needs to measure the flow passing through it.

B. Potential Difference (V)

Think of this as the Water Pressure or the pump that pushes the water. Without pressure (potential difference), water (current) will not flow.
Definition: The work done to move a unit charge from one point to another.
Formula: V = W / Q (Work done / Charge).
SI Unit: Volt (V).
Instrument: Voltmeter.

• It is always connected in Parallel.
• Why? Because it has very high resistance and measures the difference across two points.

2. Ohm's Law (The Golden Rule)

This is the most fundamental law of electricity.

Exam Tip: If you plot a graph of V vs I, you get a straight line passing through the origin.

3. Resistance (The Obstacle)

Resistance is the opposition to the flow of current. It is like friction for electricity or rocks inside the water pipe.

Factors Affecting Resistance

• Length (L): Longer wire = More collisions = More resistance. (R ∝ L).
• Area (A): Thicker wire = More space for electrons = Less resistance. (R ∝ 1/A).
• Nature of Material: Metals like Copper/Silver have low resistance. Insulators like Rubber have high resistance.
• Temperature: For metals, Resistance increases with temperature.

Exam Fact: Alloys (like Nichrome) have much higher resistivity than pure metals. This is why heating elements are made of alloys—they get hot but do not burn or oxidize easily at high temperatures.

4. Combination of Resistors

In circuits, we can arrange components in two main ways.

A. Series Combination (One Path)

Resistors are connected end-to-end, like a chain.

• Current (I): Same in all resistors.
• Voltage (V): Divided across resistors.
• Total Resistance (R_s): R₁ + R₂ + R₃. (The total is always higher than the largest individual resistor).
• Disadvantage: If one component fails (e.g., one bulb fuses), the whole circuit breaks. This is used in old decorative fairy lights.

B. Parallel Combination (Multiple Paths)

Resistors are connected across common points (ladder-like).

• Voltage (V): Same across all resistors.
• Current (I): Divided into different branches.
• Total Resistance (1/R_p): 1/R₁ + 1/R₂ + 1/R₃. (The total is always lower than the smallest individual resistor).

5. Heating Effect (Joule's Law)

When current flows against resistance, energy is used up to overcome friction, and this energy dissipates as heat.

Formula: H = I²Rt

• Heat (H) is proportional to the Square of Current (I²).
• Proportional to Resistance (R).
• Proportional to Time (t).

Applications (High Yield)

1. Electric Bulb: The filament is made of Tungsten.

• Why? It has an extremely high melting point (3380°C). It glows white-hot without melting.
• Gas: The bulb is filled with inactive gases like Nitrogen or Argon to prevent the filament from burning (oxidizing).

2. Electric Fuse: A safety device connected in Series.

• Principle: Heating effect of current.
• Material: Wire of Low Melting Point (usually Lead-Tin alloy).
• Action: If current exceeds the safe limit, the wire heats up, melts, and breaks the circuit, saving your expensive appliances.

3. Electric Iron/Toaster: The heating element is made of Nichrome because it has high resistance and does not oxidize at high heat.

6. Electric Power (The Bill)

Power (P) is the rate at which electrical energy is consumed.

The Commercial Unit of Energy

• SI Unit of Power: Watt (W).
• Commercial Unit: Kilowatt-hour (kWh). This is what we call "1 Unit" on our electricity bill.
• Conversion: 1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ Joules.

7. Mentor’s Final Drill (Exam-Ready Questions)

Study Strategy: Focus on the "Why" questions in the Application section (Why Tungsten? Why Nichrome? Why Parallel circuits?). For numericals, practice simple applications of V = IR and P = VI. You have now covered the essentials of Electricity!