7. Explain why a bulb glows on passing current.

A bulb glows when an electric current passes through it due to the conversion of electrical energy into light and heat energy, a process primarily facilitated by the bulb's filament. The most common type of bulb used to illustrate this principle is the incandescent bulb, although the basic idea of converting electrical energy into light (and heat) applies to other types of bulbs, like LEDs and fluorescent lights, albeit through different mechanisms. Here’s how it works in an incandescent bulb:

Incandescent Bulb:

1. Filament and Resistance: At the heart of an incandescent bulb is a thin filament typically made of tungsten. When electrical current is passed through the filament, the filament's resistance to the electrical current causes it to heat up.

2. Heat and Light: The resistance causes the filament to get so hot that it glows, emitting light. The temperature of the tungsten filament in an incandescent bulb can reach up to about 2,500 to 3,000 degrees Celsius (about 4,532 to 5,432 degrees Fahrenheit), producing a broad spectrum of light that includes visible light, infrared (heat), and some ultraviolet light.

3. Thermal Radiation: The glowing of the filament is a result of thermal radiation. Objects at high temperatures emit electromagnetic radiation across a spectrum. For the temperatures achieved by the tungsten filament, a significant portion of this radiation is in the visible spectrum, making the filament visibly glow.

Energy Conversion:

• The electrical energy supplied to the bulb is converted into heat due to the resistance offered by the filament. This is described by Joule's law, which states that the heat generated in a conductor by an electric current is proportional to the square of the current, the resistance of the conductor, and the time for which the current flows.

• A portion of this heat then further converts into light energy, making the bulb glow. However, it's worth noting that incandescent bulbs are not very energy-efficient, as the majority of the electrical energy (about 90-95%) is converted into heat rather than light.

Other Types of Bulbs:

• LED (Light Emitting Diode): In LED bulbs, electrons recombine with electron holes within the device, releasing energy in the form of photons. This process is called electroluminescence, and the color of the light (corresponding to the energy of the photons) is determined by the specific energy band gap of the semiconductor material used in the LED. LEDs are more energy-efficient than incandescent bulbs because a greater portion of the electrical energy is converted directly into light.

• Fluorescent Lights: These contain a mercury vapor that emits ultraviolet (UV) light when an electric current causes the gas to glow. The UV light then stimulates a fluorescent coating on the inside of the bulb to emit visible light. Fluorescent lights are more energy-efficient than incandescent lights but less so than LEDs.

In summary, a bulb glows on passing current due to the conversion of electrical energy into light (and heat), with the specific mechanism of conversion depending on the type of bulb.