Electric Current & Its Effects

Cheatsheet Content

1. Heating Effect of Electric Current (Joule Heating) Definition: When current flows through a conductor, electrical energy is converted into heat energy due to resistance. Joule's Law of Heating: The heat produced ($H$) is directly proportional to the square of the current ($I$), the resistance ($R$), and the time ($t$). $$H = I^2 R t$$ where $H$ is in Joules (J), $I$ in Amperes (A), $R$ in Ohms ($\Omega$), and $t$ in seconds (s). Power Dissipated: $P = I^2 R = V I = \frac{V^2}{R}$ Advantages of Heating Effect: Electric Heaters: Used in room heaters, water heaters (geysers), toasters, electric kettles. Electric Bulbs (Incandescent): Filament (usually tungsten) heats up to a very high temperature, emitting light. Electric Fuses: Melt and break the circuit when current exceeds a safe limit, protecting appliances. Electric Iron: Heating element warms up to press clothes. Soldering Iron: Melts solder for joining electronic components. Disadvantages of Heating Effect: Energy Loss: In electrical circuits and transmission lines, a significant amount of energy is wasted as heat, reducing efficiency. Damage to Appliances: Excessive heating can damage electrical components, reduce their lifespan, or cause malfunctions. Fire Hazards: Overheating due to short circuits or overloading can lead to fires. Need for Cooling: Electronic devices often require cooling systems (fans, heat sinks) to dissipate unwanted heat. Electric Fuses Purpose: A safety device that protects electrical circuits and appliances from damage due to overcurrent. Principle: Made of a wire with a low melting point (e.g., tin-lead alloy). When current exceeds the fuse's rating, it melts and breaks the circuit. Placement: Always connected in series with the live wire of the circuit. Ratings: Specified in Amperes (e.g., 5A, 10A). A fuse with an appropriate rating must be used for a specific appliance. Types: Cartridge Fuses: Cylindrical, enclosed in a glass or ceramic body. Rewirable Fuses: Common in older installations, can be rewired after blowing. MCBs (Miniature Circuit Breakers): Modern alternative to fuses. They trip (switch off) automatically on overcurrent and can be reset. 2. Magnetic Effect of Electric Current Discovery: Oersted observed that a current-carrying wire produces a magnetic field around it. Right-Hand Thumb Rule: If you hold a current-carrying conductor in your right hand with your thumb pointing in the direction of the current, your curled fingers indicate the direction of the magnetic field lines. Magnetic Field Lines: Form concentric circles around a straight current-carrying wire. Their direction reverses if the current direction reverses. Factors Affecting Magnetic Field Strength: Directly proportional to the current ($I$). Inversely proportional to the distance from the conductor ($r$). Electromagnets Definition: A temporary magnet produced by passing electric current through a coil of wire (solenoid) wrapped around a soft iron core. Construction: A coil of insulated copper wire wound around a ferromagnetic material (like soft iron). Working Principle: When current flows through the coil, it creates a magnetic field. The soft iron core gets magnetized, concentrating the field and making the electromagnet strong. When the current is switched off, the core loses most of its magnetism. Factors Affecting Strength: Number of turns: More turns, stronger magnetic field. Current: Higher current, stronger magnetic field. Nature of core material: Soft iron core makes a strong electromagnet. Length of solenoid: Shorter solenoid for a given number of turns concentrates the field. Practical Uses of Electromagnets Electric Bells: Utilizes the make-and-break of a circuit to produce sound. Cranes: Used to lift heavy magnetic materials like scrap iron. Relays: Electrically operated switches. Loudspeakers: Convert electrical signals into sound waves. Motors and Generators: Fundamental components in their operation. Medical Applications: MRI scanners, removing metallic foreign objects from the eye. Maglev Trains: Use strong electromagnets for levitation and propulsion. Electric Bell Components: Electromagnet: Coil of wire around a soft iron core. Armature: A soft iron strip with a hammer attached. Contact Screw: Touches the armature to complete the circuit. Gong: A bell-shaped metal object that the hammer strikes. Battery/Power Supply: Provides current. Switch: To operate the bell. Working: When the switch is pressed, current flows from the battery through the electromagnet. The electromagnet gets magnetized and attracts the armature. As the armature moves towards the electromagnet, the hammer strikes the gong, producing sound. Simultaneously, the contact screw breaks contact with the armature, interrupting the circuit. The electromagnet loses its magnetism, and the armature springs back to its original position due to a spring. This re-establishes contact with the screw, completing the circuit again, and the process repeats rapidly, causing continuous ringing.