How an electric bulb gives light
A very hot filament that glows.
Heat or light?
A filament bulb gets very hot while it is switched on, and it also gives light. We use it to get light.
Most of the power consumed by the filament appears as which of these?
What this lesson covers
The idea
In an electric bulb, a thermally isolated filament of a strong, high-melting-point metal retains the heat produced until it becomes very hot and emits light; most of the power appears as heat and only a small part as light.
Heat or light?
A filament bulb gets very hot while it is switched on, and it also gives light. We use it to get light.
Most of the power consumed by the filament appears as which of these?
- Heat, with only a small part as light
- Light, with only a small part as heat
- Equal parts of heat and light
Inside the bulb
Open every tab to learn what the filament needs.
A hot wire that does not melt
The filament has to get very hot to glow, so it keeps its heat and is made of a metal that will not melt. Even so, most of the power ends up as heat.
In an electric bulb a thermally isolated filament of a strong, high-melting-point metal retains the heat produced until it becomes very hot and emits light. Most of the power appears as heat and only a small part as light.
Notes
The filament of a bulb is thermally isolated and made of a strong, high-melting-point metal; it retains heat until it is very hot and emits light, but most of the power appears as heat.
Check yourself
Why is the filament of an electric bulb made of tungsten?
Why is the filament thermally isolated as much as possible, using an insulating support?
What happens to most of the power consumed by the filament?
What is the melting point of tungsten, according to the chapter?
- It is a strong metal with a high melting point, so it does not melt when very hot — correct. Yes!
- It has the lowest melting point of all metals. The filament must not melt at the high temperature it reaches, so a metal with a high melting point is used.
- It is an insulator. Tungsten is a metal. A strong metal with a high melting point is needed so the hot filament does not melt.
- So that it can lose its heat quickly. The filament must retain as much of the heat generated as possible, so that it gets very hot and emits light.
- So that it retains as much of the heat as possible and gets very hot — correct. Yes!
- So that no current can flow through it. The current must flow through the filament. The insulating support only keeps the heat from leaking away.
- It appears as light. Most of the power appears as heat. Only a small part is radiated as light.
- It is stored in the filament for ever. The power consumed appears as heat and a small part as light.
- It appears as heat — correct. Yes!
- 3380 °C — correct. Yes!
- 338 °C. The melting point of tungsten is 3380 °C, ten times larger than this.
- 33800 °C. The melting point of tungsten is 3380 °C.