Effects of Electric Current

Definition:

An electric motor is a device that converts electrical energy into mechanical energy.

Principle:

When a current-carrying conductor is placed perpendicular to a magnetic field, it experiences a force that causes motion (Fleming's Left-Hand Rule).

Construction:

The electric motor consists of:

1. A rectangular armature coil ABCD mounted on an axle
2. Two split rings (commutator) P and Q
3. Two carbon brushes X and Y
4. A horse-shoe shaped electromagnet (or permanent magnet)
5. A DC power supply (battery)

The coil is wound around a soft iron core and placed between the poles of the electromagnet. Ends A and D of the coil are connected to split rings P and Q, which are in contact with brushes X and Y. The coil can rotate freely, while the brushes remain fixed.

Working:

1. When current flows from the battery through brush X, it enters the coil and returns via brush Y. Current flows upward from A to B and downward from C to D.
2. According to Fleming's left-hand rule, arm AB experiences a downward force and arm CD experiences an upward force. These opposite forces create a couple that rotates the coil anticlockwise.
3. After half a rotation, split ring P comes in contact with brush Y and split ring Q with brush X, reversing the direction of current in the coil.
4. The forces on the arms reverse again, but the coil continues rotating in the same direction due to inertia.
5. This process repeats, and thus continuous rotation is achieved.

Applications:

• Electric fans, mixers, washing machines
• Industrial machines, conveyor belts
• Electric vehicles
• Computer hard drives, DVD players

Definition:

An AC electric generator (alternator) converts mechanical energy into electrical energy.

Principle:

It operates on the principle of electromagnetic induction discovered by Michael Faraday. When a coil rotates in a magnetic field, an electric current is induced in it. The direction of induced current is given by Fleming's right-hand rule.

Construction:

It consists of:

1. A rectangular coil ABCD wound on a soft iron core
2. Two slip rings R₁ and R₂ (unlike split rings in DC motor)
3. Two carbon brushes B₁ and B₂
4. A horse-shoe shaped electromagnet or permanent magnet
5. A galvanometer connected to the brushes to detect current

The coil is placed between the magnetic poles and can rotate freely about an axis perpendicular to the magnetic field.

Working:

1. When the coil rotates clockwise, arm AB moves upward and arm CD moves downward (cutting magnetic field lines).
2. Using Fleming's right-hand rule, the induced current flows in the direction A→B→C→D and passes from brush B₂ to B₁ in external circuit.
3. After half a rotation (180°), the positions of the arms interchange (AB moves down, CD moves up).
4. This causes the direction of induced current to reverse to D→C→B→A. Now current flows from brush B₁ to B₂.
5. Thus, the direction of current changes after every half rotation, producing alternating current (AC).

Applications:

• Power plants (thermal, hydro, nuclear)
• Automobile alternators
• Wind turbines
• Portable generators

Key Insight:

Both AC and DC generators work on the principle of electromagnetic induction. The crucial difference is in the arrangement of the commutator:

• AC Generator uses slip rings that maintain continuous contact, allowing the alternating current from the coil to reach the external circuit unchanged.
• DC Generator uses a split-ring commutator that reverses connections every half-cycle, converting AC from coil to DC in external circuit.

Most large-scale power generation is AC because it can be transmitted over long distances with less energy loss (using transformers to step up voltage).

What is a Short Circuit?

A short circuit occurs when the live wire (phase) comes into direct contact with the neutral wire or earth wire, providing a very low-resistance path for current.

How it Occurs:

1. Damaged insulation: Wires rubbing against each other, insulation worn out
2. Loose connections: Exposed wire ends touching
3. Faulty appliances: Internal wiring defects
4. Water exposure: Moisture reducing insulation resistance
5. Accidental contact: Metallic objects bridging wires

Why it's Dangerous:

According to Ohm's Law: I = V/R

When R becomes very small (approaching zero), current I becomes extremely large.

Example: If 220V mains shorted through 0.01Ω resistance: I = 220/0.01 = 22,000 A (dangerously high!)

Effects of Short Circuit:

Protection Against Short Circuits:

• Fuses: Melt when current exceeds rating, breaking circuit
• Circuit Breakers (MCB): Automatically trip when fault detected
• Proper insulation: Regular inspection of wiring
• Earthing: Provides safe path for fault current
• Residual Current Devices (RCD): Detect imbalance and cut power

Reason:

Tungsten has several properties that make it ideal for bulb filaments:

1. Very high melting point (3380°C): Can withstand the high temperatures needed for incandescence (around 2500-3000°C) without melting.
2. High tensile strength: Can be drawn into very thin wires (filaments) without breaking.
3. Low vapor pressure: Doesn't evaporate quickly at high temperatures (though some evaporation still occurs, blackening bulb over time).
4. Good electrical conductivity: Allows current to flow while generating heat through resistance.
5. High resistivity: Generates sufficient heat to glow white-hot when current passes.

Modern bulbs often have tungsten filaments coiled to increase surface area and efficiency.

Reason:

Nichrome (80% Ni, 20% Cr) and similar heating alloys have ideal properties for heating elements:

High resistivity means nichrome produces more heat for same current (P = I²R). High melting point prevents melting during operation. Oxidation resistance prevents burning out quickly.

Reason:

Copper and aluminium are preferred for transmission lines because:

1. High electrical conductivity: Low resistivity minimizes power loss (I²R loss). Silver is better but too expensive.
2. Good mechanical strength: Can withstand tension in overhead lines.
3. Ductility: Can be drawn into thin wires.
4. Malleability: Can be shaped as needed.
5. Cost-effective: Reasonably priced and abundantly available.
6. Corrosion resistance: Form protective oxide layers.

Comparison:

• Copper: Better conductor but heavier and more expensive
• Aluminium: Lighter and cheaper; used for long transmission lines despite slightly lower conductivity

For very high voltage transmission, aluminium with steel core (ACSR) is common for strength-weight balance.

Reason:

While both are units of energy, kilowatt-hour (kWh) is more practical for electricity billing:

Why kWh is preferred:

1. Practical magnitude: 1 joule is tiny (lifting 100g 1m high). Household appliances consume millions of joules daily. kWh gives manageable numbers (e.g., 300 kWh/month vs 1.08 × 10⁹ J).
2. Easy calculation: Electricity meters measure power (kW) × time (h) directly.
3. Relates to appliance ratings: Appliances rated in watts/kW, so energy = power × time in hours.
4. Standard for billing: Electricity companies worldwide use kWh for tariffs.
5. Consumer understanding: Easier for consumers to relate usage to appliance power and time.

Example: A 100W bulb used 10 hours daily for 30 days:

Energy = 0.1 kW × 10 h/day × 30 days = 30 kWh (easy to calculate and understand)

In joules: 30 × 3.6 × 10⁶ = 1.08 × 10⁸ J (cumbersome number)

Solenoid Definition:

A solenoid is a long coil made of many turns of insulated copper wire wound closely in cylindrical form. When current flows through it, it produces a magnetic field similar to a bar magnet.

Comparison: Solenoid vs Bar Magnet

Similarities:

1. Both produce similar magnetic field patterns (field lines emerge from N pole, enter S pole)
2. Magnetic field lines are continuous closed curves
3. Both attract magnetic materials (iron, nickel, cobalt)
4. Like poles repel, unlike poles attract
5. Both can be used to magnetize other magnetic materials

Magnetic Field Strength of Solenoid:

Where: B = magnetic field inside solenoid, μ₀ = permeability, n = turns per unit length, I = current

With iron core: B = μ₀ μ_r n I (μ_r = relative permeability of iron ~ 5000)

iii. Electric Motor

Use: Converts electrical energy into mechanical energy for various applications.

Types & Applications:

Principle: Fleming's Left-Hand Rule - force on current-carrying conductor in magnetic field.