Overview
This lecture explains the heating effect of electric current, factors affecting the heat produced, and practical applications in everyday electrical devices.
Basics of Electric Current
- Electric current is the flow of electric charge, defined as charge divided by time.
- The unit of current is ampere (A).
Effects of Electric Current
- Electric current produces three effects: heating, magnetic, and chemical.
- Heating effect example: filament in a toaster gets hot and toasts bread.
- Magnetic effect example: creation of electromagnets.
- Chemical effect example: electrolysis of water.
Heating Effect of Electric Current
- The heating effect refers to heat produced when current passes through a high resistance wire.
- Wires with high resistance (nichrome, tungsten) heat up, while low resistance wires (copper) do not.
- Heat is produced because electrons collide with atoms in the wire, transferring kinetic energy.
Joule's Law of Heating
- Joule’s Law: Heat energy (H) = i²RT, where i=current, R=resistance, T=time.
- Heat produced increases with square of current, resistance, and duration.
- If current is doubled, heat produced increases four times.
- If resistance or time is doubled, heat produced doubles or increases fivefold, respectively.
Applications of the Heating Effect
- Electrical heating appliances (iron, kettle, toaster, oven, heater) use coils of high resistance (nichrome) as heating elements.
- Nichrome is chosen for its high resistivity and melting point (~1400°C).
- In electric bulbs, a tungsten filament glows when heated (~2500°C); tungsten is chosen for its high melting point (~3400°C) and stability.
- Bulbs are filled with inert gases (argon or nitrogen) to prevent the filament from burning.
- Incandescent bulbs convert >95% energy to heat, <5% to light, making them inefficient.
Electric Fuse and Safety
- Fuses are safety devices with a thin, high-resistance, low melting point wire (solder: tin and lead alloy).
- When excessive current flows, the fuse wire melts, breaking the circuit and protecting appliances.
Key Terms & Definitions
- Electric Current — Flow of electric charge; measured in amperes.
- Heating Effect — Production of heat as electric current passes through resistance.
- Joule’s Law — Formula: H = i²RT, heat generated in a resistor.
- Resistance — Opposition to current flow; higher resistance means more heat.
- Incandescent Bulb — Bulb emitting light from a heated tungsten filament.
- Electric Fuse — Safety device that melts to break a circuit if current is too high.
Action Items / Next Steps
- Review Joule’s Law of heating and related calculations.
- Identify household devices that use the heating effect of electric current.
- Prepare for next lessons on magnetic and chemical effects of current.