Electricity: Circuits and their Components · Lesson 3 of 6
Making a Circuit Work
“Build, predict, and explain complete circuits for filament lamps and LEDs.”
• Prepare secure connections to the correct cell and lamp terminals. • Predict which of six arrangements provides a complete path through a lamp. • Explain an electrical circuit and the stated direction of current in its external path. • Distinguish a filament-lamp connection from an LED connection. • Investigate simple faults systematically and relate a solar-powered fan to the circuit idea.
From Separate Parts to a Working Path
A cell on a desk and a lamp beside it do not automatically produce light. Their terminals need to be joined in a particular way. We will begin by making reliable connections, then predict whether each arrangement gives a path through the lamp. This approach makes an observation useful: we can explain a result instead of merely memorising which picture glows.
Use a small cell, a compatible incandescent torch lamp, a cell holder, a lamp holder, and four prepared lengths of wire. The teacher should help expose about a centimetre of metal at each wire end. The covered middle of the wire stays intact. Connect the wires to the holder terminals so the exposed metal touches metal securely, rather than leaving the insulating cover trapped under a connection.
Place the cylindrical cell in the holder according to its signs; in the holder shown in the source, the negative end faces the spring. Attach the lamp-holder wires to its two terminal screws and fit the lamp securely. A holder carries the outside connection to the correct component terminal. It is not an extra energy source. If a holder is unavailable, a teacher may use electrical tape to hold appropriate wire ends against the correct cell terminals, or the lamp case and separate tip.
Do not join the two cell terminals directly with a wire. That bypasses the lamp and can heat the cell and wire. Assemble and change the teacher-approved circuit with the source disconnected, then connect briefly to observe.
Predict First, Then Observe
Call the positive cell terminal P and the negative terminal N. Call the two filament-lamp terminals A and B. These letters are labels for connection points, not new electrical quantities. Each of the six arrangements below changes the links between those points. Trace each one before looking at the observation table.
| Arrangement | Connections to inspect | Expected observation with working, compatible parts | Reason |
|---|---|---|---|
| 1 | P to A; N to B | Lamp glows | Both source terminals join different lamp terminals. |
| 2 | P to A; gap in the N-to-B link | No glow | One necessary connection is interrupted. |
| 3 | Gap in the P-to-A link; N to B | No glow | The other necessary connection is interrupted. |
| 4 | Both A and B join P; N is unconnected | No glow | There is no path through the lamp between both source terminals. |
| 5 | Both A and B join N; P is unconnected | No glow | The positive source terminal is absent from the useful path. |
| 6 | P to B; N to A | Lamp glows | The same complete connection is made with the lamp terminals exchanged. |
For an investigation, make a blank record with arrangement, prediction, and observation columns. Write a prediction for all six before testing. Then build them with the teacher-approved components, observe briefly, and compare the results. Arrangements 1 and 6 provide a complete useful route; 2 and 3 contain gaps; 4 and 5 leave one source terminal out. If a predicted working arrangement does not light, first check the apparatus rather than changing the rule.
Problem
Both lamp terminals are connected by separate wires to the cell positive terminal. Why does the lamp remain dark?
- 1.Identify the connections rather than only counting wires. Both lead to P.
- 2.The negative terminal N has no connection to the lamp.
- 3.There is no complete path through the lamp between the two source terminals, so the lamp does not operate.
Problem
In arrangement 6, the two wires cross in the drawing without being joined. Does the crossing stop the lamp glowing?
- 1.A crossing in a picture is not automatically a contact. These wires connect P to B and N to A.
- 2.Trace each wire separately to its destination. Both different lamp terminals connect to the two source terminals.
- 3.The route through the filament is complete. The incandescent lamp glows with the connections exchanged.
What Makes This an Electrical Circuit?
A working arrangement supplies a continuous route through the lamp. Current passes through the filament as part of that route, and the filament gets hot and shines. A visible line drawn around a lamp is not enough: the real connections and the route inside the lamp must also be continuous.
An arrangement that provides a complete conducting path through electrical components and back to the source so that current can flow.
For these circuits, the direction of electric current in the external path is taken from the positive terminal of the cell or battery, through the connected components, to its negative terminal. This agreed direction helps us describe a circuit consistently. Current does not need to cross an air gap, and the energy source does not remove the need for two connections.
An incandescent lamp has no required positive or negative orientation in this simple circuit. Exchanging its case and tip connections still gives a path through the filament. This is why both arrangements 1 and 6 work. The result is about this lamp type; it must not be transferred automatically to an LED.
Problem
A working lamp has its case connected toward source + and its central tip toward source −. What happens if these two lamp connections are exchanged?
- 1.The two different lamp terminals still connect to the two different source terminals.
- 2.The current passes through the filament in the opposite direction.
- 3.The compatible incandescent lamp still glows; it does not require one particular orientation.
A Broken Filament Is a Hidden Gap
Sometimes every outside wire appears to be joined but the lamp stays dark. The path also runs through the thin filament inside the glass bulb. If that filament is broken, the path is open inside the lamp, even though the outside arrangement looks complete.
An incandescent lamp whose filament has broken, interrupting the current path and preventing the lamp from glowing.
To investigate, keep the rest of the circuit unchanged and replace the suspect lamp with a known-working compatible lamp. If the replacement glows, that supports the conclusion that the original lamp is faulty. If it does not, check the source, contacts, and wire ends. A dark lamp alone does not identify which one of those parts is at fault.
Problem
A connected lamp is dark. A known-working replacement shines in the same circuit. What can you conclude?
- 1.The unchanged source and connections can operate the replacement lamp.
- 2.The difference between the two trials is the lamp.
- 3.The original lamp is faulty; a broken filament is a likely reason. Inspect it safely rather than assuming that all dark lamps have exactly the same fault.
The LED Adds a Direction Rule
Now use the approved LED kit that is designed for its specified two-cell source. The cells must join positive-to-negative inside the holder. Identify the free battery positive terminal by following the contact to the positive end of the end cell; identify the free negative terminal in the same way. A wire colour can help after it has been checked, but does not replace tracing the actual connections.
With the source disconnected, connect battery positive to the LED positive lead and battery negative to its negative lead. For an ordinary untrimmed LED, these are the long and short leads respectively. The correctly assembled compatible kit lights when connected. Reverse only the LED connections for the comparison, and it does not light. Restore the correct arrangement after the observation.
Within its intended operating conditions, the LED permits current in one direction and blocks the reverse direction. It can therefore remain dark even when all outside wires seem joined. To make the LED glow, the circuit needs a suitable working source, correct contacts, and the permitted LED direction. The lead signs describe that direction; they do not show that the LED creates electrical energy.
Problem
The same approved source makes an LED shine with its long lead connected toward +. It goes dark when its leads are exchanged. Has the source necessarily failed?
- 1.Only the LED orientation was changed.
- 2.The LED blocks the reverse direction, unlike the incandescent lamp.
- 3.Restore the correct orientation and observe again. The paired observation is explained by the LED direction rule, rather than by an automatic source failure.
A visible loop is not a promise of light. A broken filament, a loose contact, an unsuitable or exhausted source, or a reversed LED can prevent operation. State which parts are assumed to work before making a prediction.
A Solar Panel as the Source
The source does not have to be a cell. A small educational solar panel can supply electrical energy to a compatible toy motor when enough light falls on it. The motor turns a fan instead of producing light, but it still needs the proper connections to its source.
With a teacher, use a matched small panel-and-fan kit. Join the panel to the motor using both specified connections, keep fingers away from moving blades, and compare the fan in useful illumination with the panel shaded. Record what happens without expecting every indoor light level to run the motor. A complete circuit is necessary, and the panel must also receive enough light to supply the device.
Problem
A solar-powered toy fan runs in suitable light. It stops when the panel is covered while the wires remain joined. Is an open wire the only explanation?
- 1.The wire arrangement did not change; the illumination did.
- 2.The panel may no longer supply enough electrical energy for the motor to run.
- 3.An unchanged, complete connection can still produce no motion when the source cannot operate the device under those conditions.
Quiz
Which arrangement can light a working compatible incandescent lamp?
Why do arrangements 1 and 6 both light the filament lamp?
What is the stated current direction in the external circuit?
All outside wires are joined, but the filament is broken. What happens?
A compatible LED is correctly identified. Which connection permits it to glow?
Which test most clearly checks whether a suspect lamp is faulty?
A solar-panel toy fan stops when its panel is shaded. Which explanation is reasonable?
Practice Problems
- Explain, using P, N, A, and B, why arrangement 4 in the six-arrangement diagram does not glow.
- Predict what changes when a gap in the N-to-B wire of arrangement 2 is securely closed.
- Describe why the cell holder and lamp holder each need two outside connections.
- A dark lamp has joined outside wires. List three checks that go beyond looking at the overall loop.
- A correct LED circuit goes dark when only the LED leads are reversed. Explain the result and the correction.
- Plan a prediction-and-observation record for a solar-panel toy fan in suitable light and in shade.
A and B both join P. N is unconnected, so the lamp has no complete useful path between the two source terminals.
Key Takeaways
• A working lamp circuit needs contacts to both source terminals and both different lamp terminals. • Only arrangements 1 and 6 in the six-arrangement investigation provide the required filament path. • The stated external current direction is from source positive through the components to source negative. • A filament lamp can connect either way, but a broken filament interrupts its path. • A suitable LED circuit also needs the permitted LED direction. • Investigate faults by checking contacts and changing one suspect part at a time.