Electricity and Magnetism

Overview

Electricity and Magnetism forms a crucial portion of the Science section in MP TET Varg-2. This topic tests your understanding of how electric current flows, how circuits work, and how electricity and magnetism are interconnected. Questions typically involve circuit diagrams, magnetic effects of current, and practical applications that students encounter in daily life.

For the exam, you must be comfortable with basic circuit calculations (Ohm's law, series-parallel combinations), understand the behaviour of magnets, and know how electric current produces magnetic effects. This topic also connects to practical teaching scenarios—demonstrating circuits, explaining household wiring, and conducting simple experiments with magnets. Expect 3–5 questions directly from this area, often application-based.

Mastery here requires clarity on definitions, familiarity with circuit symbols, and the ability to apply formulas to simple numerical problems.

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Key Concepts

• **Electric Current** is the flow of electric charge (electrons) through a conductor. It flows from positive to negative terminal in conventional terms, but electrons actually move from negative to positive.

• **Electric Circuit** is a closed path through which current flows. It requires a source (cell/battery), conducting wires, a load (bulb/resistor), and often a switch.

• **Ohm's Law** states that current through a conductor is directly proportional to voltage and inversely proportional to resistance, provided temperature remains constant.

• **Series Circuit**: Components connected end-to-end; same current flows through all; total resistance = sum of individual resistances.

• **Parallel Circuit**: Components connected across same two points; voltage same across all; total resistance is less than the smallest individual resistance.

• **Magnets** have two poles (North and South). Like poles repel; unlike poles attract. Magnetic field lines emerge from North pole and enter South pole.

• **Magnetic Effect of Current**: A current-carrying conductor produces a magnetic field around it. This is the basis of electromagnets, electric motors, and generators.

• **Electromagnetic Induction**: A changing magnetic field in a conductor induces an electric current. This principle underlies generators and transformers.

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Formulas / Key Facts

| Formula / Fact | Context | |----------------|---------| | V = I × R | Ohm's Law: Voltage = Current × Resistance | | R(series) = R₁ + R₂ + R₃ + ... | Total resistance in series | | 1/R(parallel) = 1/R₁ + 1/R₂ + 1/R₃ + ... | Total resistance in parallel | | P = V × I = I²R = V²/R | Electric Power (in watts) | | E = P × t = V × I × t | Electrical Energy consumed | | 1 kWh = 1000 W × 1 hour = 3.6 × 10⁶ J | Unit of electrical energy (1 unit) | | Right-Hand Thumb Rule | Thumb shows current direction; curled fingers show magnetic field direction around a straight conductor | | Fleming's Left-Hand Rule | Used for motors: First finger = Field, Second finger = Current, Thumb = Motion | | Fleming's Right-Hand Rule | Used for generators: Thumb = Motion, First finger = Field, Second finger = Induced current |

**Key Facts:**

• SI unit of current: Ampere (A)
• SI unit of resistance: Ohm (Ω)
• SI unit of voltage: Volt (V)
• Fuse wire has low melting point and high resistance—melts to break circuit during overload
• Earth wire prevents electric shock by providing low-resistance path to ground

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Worked Examples

### Example 1: Applying Ohm's Law **Problem:** A bulb has resistance 20 Ω and is connected to a 10 V battery. Find the current flowing through it.

**Solution:**

• Given: V = 10 V, R = 20 Ω
• Using V = I × R
• I = V/R = 10/20 = 0.5 A

**Answer:** Current = 0.5 Ampere

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### Example 2: Resistors in Series **Problem:** Three resistors of 2 Ω, 3 Ω, and 5 Ω are connected in series with a 20 V battery. Find total resistance and current.

**Solution:**

• Total resistance R = 2 + 3 + 5 = 10 Ω
• Current I = V/R = 20/10 = 2 A

**Answer:** Total resistance = 10 Ω, Current = 2 A

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### Example 3: Resistors in Parallel **Problem:** Two resistors of 6 Ω each are connected in parallel. Find equivalent resistance.

**Solution:**

• 1/R = 1/6 + 1/6 = 2/6 = 1/3
• R = 3 Ω

**Answer:** Equivalent resistance = 3 Ω

*Note: For two equal resistances in parallel, equivalent resistance = R/2*

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### Example 4: Electrical Energy **Problem:** A 100 W bulb is used for 10 hours daily. Calculate energy consumed in 30 days in kWh.

**Solution:**

• Daily energy = 100 W × 10 h = 1000 Wh = 1 kWh
• Monthly energy = 1 × 30 = 30 kWh

**Answer:** 30 units (kWh)

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Common Mistakes

| Wrong Thinking | Correct Understanding | |----------------|----------------------| | "Current gets used up in a bulb, so less current flows after it." | Current remains same throughout a series circuit. Energy is converted, not current consumed. | | "In parallel, total resistance is the sum of resistances." | In parallel, total resistance is always less than the smallest resistance. Use reciprocal formula. | | "Magnetic field lines can cross each other." | Magnetic field lines never intersect. At any point, field has only one direction. | | "Confusing Fleming's Left and Right hand rules." | Left hand = Motor (force on conductor); Right hand = Generator (induced current). Remember: "MoLe" - Motor = Left. | | "Fuse should have high melting point for safety." | Fuse must have LOW melting point so it melts quickly during overload, breaking the circuit. |

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Quick Reference

• **Ohm's Law:** V = IR (Voltage = Current × Resistance)

• **Series:** Same current, voltages add, resistances add

• **Parallel:** Same voltage, currents add, 1/R = 1/R₁ + 1/R₂ + ...

• **Power:** P = VI = I²R = V²/R (measured in Watts)

• **1 Unit of electricity** = 1 kWh = 1000 W used for 1 hour

• **Electromagnet strength** increases with: more turns, more current, soft iron core

• **Right-Hand Thumb Rule:** Thumb = current direction, fingers curl in field direction