Electricity: Circuits and their Components · Lesson 2 of 6
Electric Lamps and LEDs
“See how a filament lamp and an LED produce light and identify the connections each one needs.”
• Identify the glass bulb, filament, supports, and terminals of a small incandescent lamp. • Explain that the filament gets hot and emits light when current passes through it. • Identify the positive and negative leads of the untrimmed LEDs used here. • Compare incandescent lamps and LEDs without assuming that they work in the same way. • Explain why separate terminals matter when connecting a lamp.
Looking Closely at a Lamp
A torch may contain a tiny glass lamp with a visible wire, or a different kind of lamp called an LED. Both can give us light, but their structures are different. Before building a circuit, we need to know where its wires should connect and which part of the lamp produces the light.
Use a torch and a small lamp that your teacher has identified as incandescent. Observe the lamp while it is switched off, and then watch which part glows when the torch operates. Do not look directly into a bright torch beam. Switch off the torch before removing a lamp, and let a hot lamp cool before it is handled.
A Thin Wire That Glows
The glass bulb encloses a very thin wire. This thin wire is the filament. When electric current passes through it in a suitable circuit, the filament gets hot enough to emit light. The light comes from the hot filament, rather than from the glass itself.
The thin wire inside an incandescent lamp that becomes hot and glows when electric current passes through it.
A lamp that produces light by making its filament hot enough to glow.
Two thicker wires support the filament and connect it to the outside terminals. One support connects to the metal case around the base. The other connects to the central metal tip at the bottom. A piece of insulating material separates the case from the tip. These details are small, but they determine the route that current must take through the lamp.
Imagine tracing a path from the central tip up one support, through the thin filament, and back down the other support to the metal case. The filament lies between the two terminals. Connecting wires to both correct terminals gives the circuit a route through the light-producing part. The two support wires do not simply join each other directly and skip the filament.
Problem
A student says, “The bulb is glowing, so the glass must be making the light.” How would you improve this explanation?
- 1.Observe the small lamp closely while it operates safely. The thin filament inside is the bright part.
- 2.The current makes the filament hot, and the hot filament emits light.
- 3.The glass surrounds the filament and lets its light pass out. Calling the whole object a bulb does not mean that its glass is the light source.
Problem
You can touch the metal case at two different spots on a small lamp. Does that give you its two terminals?
- 1.Both spots are on the same metal case, so they belong to one terminal.
- 2.The second terminal is the separate central metal tip.
- 3.A correct pair of external connections uses the case and tip. Two contact positions are not necessarily two different terminals.
The metal case is one terminal even though it is quite large. Connecting both circuit wires to different spots on that case does not make current pass through the filament. Identify the separate central tip as well.
A Different Kind of Light: the LED
Many modern torches use a Light Emitting Diode, shortened to LED. An LED does not contain the glowing wire filament of an incandescent lamp. It is a different electrical component that produces light when current passes through it in the permitted direction.
A Light Emitting Diode: a component that emits light when current passes through it in the permitted direction.
Observe an ordinary LED from the classroom kit before connecting it. It has two wire leads that act as terminals. For the untrimmed LEDs used here, the longer lead is positive and the shorter lead is negative. These signs describe the connection the LED needs; they do not mean that the LED is a cell or that it supplies energy on its own.
LEDs are available in different colours and shapes. A torch may use one LED, several LEDs, or a complete LED module. The complete torch lamp may not be removable. Observe it without forcing it out. The individual lead-length rule is useful for the simple untrimmed LEDs in our investigation; if leads have been cut, use the component markings or a teacher-verified identification instead of guessing.
Problem
A red LED and a green LED each have one long lead and one short lead. Should the red LED be connected with the opposite terminal rule?
- 1.Colour tells us something about the light produced, not that the terminal signs have been exchanged.
- 2.For both ordinary untrimmed LEDs, the long lead is positive and the short lead is negative.
- 3.Use the same identification rule, while still using the approved source and kit suitable for each LED.
Problem
An LED has a + terminal and a − terminal. Why does it still need a cell or battery?
- 1.Terminal signs show how to connect the component correctly.
- 2.The cell or battery supplies the electrical energy. The LED uses that energy to emit light in a suitable circuit.
- 3.Having signed terminals does not make every component an electrical source.
Compare the Structures Before Connecting Them
The useful comparison is more than “both give light”. Their light-producing parts and terminal behaviour explain why a connection that works with one may fail with the other. Keep these differences in mind when we build and reverse their connections in the next lesson.
| Feature | Small incandescent lamp | Ordinary untrimmed LED |
|---|---|---|
| Light-producing part | Thin filament that gets hot and glows | No glowing wire filament |
| Terminals | Separate metal case and central tip | Long positive lead and short negative lead |
| Direction in the simple circuit | Either terminal can connect to the cell positive terminal | Must be connected in the permitted direction |
| Possible appearances | Small glass bulb with metal base | Different colours and shapes; may be part of a module |
The two terminals must remain distinguishable in both cases. In a filament lamp, their separation gives us a path through the filament. In an LED, the signs also tell us which connection direction is required. Simply touching wires to a component without tracing its proper terminals is not enough to explain whether it will glow.
An LED can be damaged by an unsuitable source or too much current. Use a teacher-approved LED kit or module and follow its connection instructions. The fact that a torch contains several cells does not mean that any bare LED can be connected directly to those cells.
Problem
You are asked to compare an incandescent torch lamp and an LED without yet building a circuit. What should you record?
- 1.Draw the visible structure of each component. Label the filament and glass where present.
- 2.Identify both connection terminals: tip and case for the lamp; verified positive and negative leads for the LED.
- 3.Record similarities and differences. Both use two connections and produce light, but only the incandescent lamp has a wire filament.
Quiz
Which part produces light in an operating incandescent lamp?
Which pair identifies the two terminals of the small lamp described here?
What is the insulating separator at the base useful for?
For an ordinary untrimmed LED in the classroom kit, which lead is positive?
Which statement correctly compares these lamps?
Both LED leads have been shortened to the same length. What should a student do?
Practice Problems
- Draw and label the glass, filament, support wires, metal case, central tip, and insulating separator of a small incandescent lamp.
- Explain why the word bulb does not mean that the glass itself produces the light.
- Two circuit wires touch two different places on the same metal case. Explain why this does not give a path through the filament.
- List two similarities and two differences between an incandescent lamp and an LED.
- An LED has long and short untrimmed leads. Label them and explain what the labels are used for.
- Explain why an LED with + and − terminals cannot light a room simply by being placed on a table.
The filament joins the two supports; one support reaches the central tip and the other the case. Keep case and tip separated by insulation.
Key Takeaways
• An incandescent lamp produces light from a hot, glowing filament. • The filament lies on the path between the separate metal-tip and metal-case terminals. • An LED produces light without a glowing wire filament. • For the ordinary untrimmed LED shown here, long means positive and short means negative. • Both lamp types need a suitable source and correct connections; an LED also requires the permitted direction.