CBSE Class 10 Physics – Electric Current and Circuit: Long Answer Questions
Medium Level (Application & Explanation)
Q1. Define electric current and explain its direction. Use everyday examples to show how the concept works.
Answer: Electric current is the flow of electric charge (generally free electrons) through a conductor like a copper wire. By definition, the conventional direction of current is taken from the positive terminal to the negative terminal of a source in the external circuit, even though electrons actually move from negative to positive. The amount of current is given by the formula I = Q/t, where I is current, Q is charge, and t is time. In a torch, when you close the switch, a complete path (circuit) is created and current flows, making the bulb glow. When you charge a phone, the charger allows charge to flow into the battery. Turning on a ceiling fan completes the circuit so current goes through the motor and rotates the blades. In all these, current flows only when the circuit is closed.
Q2. A device transfers 300 C of charge in 5 minutes. Calculate the current and explain what this value means in practical terms.
Answer: We use the formula I = Q/t. Here, Q = 300 C and t = 5 minutes = 300 s. So, I = 300/300 = 1 A. This means that every second, 1 coulomb of charge passes through any cross-section of the conductor. In practical terms, a current of 1 ampere is a steady and safe level for many small devices powered by cells. For example, a small bulb or a toy motor can operate near such currents, provided the circuit is closed and components are correctly connected. This calculation also shows how the same total charge spread over more time would give less current, and if the same charge is delivered in less time, the current increases. Always remember, without a closed circuit, even with a good battery, the current remains zero.
Q3. What is an electric circuit? Distinguish between open and closed circuits with components and examples.
Answer: An electric circuit is a closed conducting path through which current can flow from the source, through a load, and back to the source. Its essential components are: a cell or battery (source of energy), connecting wires (path), a switch/key (to control on/off), and a load like a bulb or fan. In a closed circuit, the path is complete and current flows; for example, when you turn on a doorbell, pressing the button closes the circuit and the bell rings. In an open circuit, there is a break—due to an open switch, broken wire, or blown fuse—so no current flows and devices won’t work. The classroom activity of wiring a battery–switch–bulb clearly shows the bulb glows only when the switch is closed. Thus, continuity is the key condition for current.
Q4. Explain the role of a switch in a circuit. How does it control current in devices like torches, doorbells, and toy cars?
Answer:
A switch is a simple but vital control element that makes a circuit open or closed. When the switch is open, there is a break in the circuit and no current flows; when it is closed, the circuit becomes continuous, allowing current to pass. In a torch, sliding the switch connects the battery to the bulb, so it glows. In a doorbell, pressing the push-button closes the loop, and the bell rings only while the button is held. In a toy car, a button switch enables the car to move only when you want it to—by completing the path momentarily. The switch thus provides safety, control, and
Q5. Why do we use circuit symbols and diagrams? Describe how they help in building and troubleshooting circuits.
Answer: Circuit symbols provide a simple, standardized language to represent components like cells, switches, bulbs, resistors, ammeters, and voltmeters. Using symbols, a circuit diagram becomes quicker to draw, easier to read, and universally understood—reducing confusion. Diagrams help you check essentials: Is there a closed loop? Is the switch closed? Are the meters connected correctly (ammeter in series, voltmeter in parallel) as indicated in the symbol list? For building circuits, the diagram acts like a map: you follow the connections and ensure proper joining of terminals. For troubleshooting, the diagram shows possible points of open circuit, helps verify the direction of current (conventional), and reveals missing elements. In class experiments or home repairs, a clear diagram guides you to test continuity and component function systematically and safely.
Q6. A bulb in a simple battery–bulb–switch circuit does not glow. Outline a step-by-step troubleshooting plan using concepts from this chapter.
Answer:
- Confirm the circuit forms a closed loop: follow the path from the battery’s positive terminal, through the closed switch, to the bulb, and back to the negative terminal.
- Check the switch position. If open, close it and test again.
- Inspect wire connections for looseness or breaks. A single loose end creates an open circuit.
- Test the battery: ensure correct polarity and that it is not weak or discharged. Try a known good cell if possible.
- Examine the bulb: a broken filament means an open circuit inside the bulb. Replace with a working bulb to confirm.
- Clean contact points (terminals, clips) to ensure good conductivity.
- If still not working, rebuild the circuit from scratch, following the circuit diagram and ensuring the loop is continuous.
- Use a simple continuity test by replacing the bulb with a test lamp setup; if it glows, the path is good.
High Complexity (Analytical & Scenario-Based)
Q7. A string of fairy lights stops glowing when one bulb’s internal filament breaks. Use the idea of closed paths to explain why the whole string may go off and how you would locate the faulty bulb.
Answer: Many fairy lights are wired so that all bulbs share the same current path. If even one bulb’s filament breaks, it creates an open circuit, interrupting the closed loop and stopping current everywhere—so the entire string goes off. To locate the fault, start with a systematic test: gently press each bulb into its holder to restore contact in case of a loose fit. If you have a small battery–bulb tester, you can take out each bulb and test it individually; a working bulb completes the loop and glows. Alternatively, divide the string into sections by checking for any segment where wiggling restores light—this indicates a poor connection near that point. Replace the faulty bulb or fix the loose contact to re-establish a continuous path, allowing current to flow and all bulbs to glow again.
Q8. Design a simple circuit for a toy car that runs only when a button is pressed, and explain where you would place an ammeter to measure current.
Answer: Use a battery (source), push-button switch (control), motor (load), and wires (path). Connect the battery’s positive terminal to one terminal of the push-button. From the other terminal of the button, connect to one terminal of the motor. Connect the motor’s other terminal back to the battery’s negative terminal. This loop is a closed circuit only when the button is pressed, so the car moves on demand. To measure current, place the ammeter in series with the motor, for example between the switch and the motor. Series placement ensures all current flowing to the motor also flows through the ammeter, giving a correct reading. The conventional current goes from positive to negative through the loop. Ensure tight connections and keep wire lengths short to maintain a reliable closed path and accurate measurement.
Q9. Two students draw torch circuits. In Diagram A, the switch is open but all other connections are correct. In Diagram B, the switch is closed, the ammeter is in series, and a voltmeter is drawn across the bulb. Explain which circuit will make the bulb glow and why.
Answer: In Diagram A, despite correct wiring of battery and bulb, the open switch creates an open circuit, so no current flows and the bulb will not glow. The essential condition—having a closed path—is missing. In Diagram B, the switch is closed, so the path is complete and current can flow from the battery, through the ammeter, bulb, and back to the battery. The ammeter in series will correctly measure the current through the bulb because all charge must pass through it. The voltmeter across the bulb is correctly connected in parallel, so it measures the potential difference without disturbing the circuit significantly. Therefore, Diagram B will make the bulb glow and also allow both current and voltage measurements to be taken. This shows how correct symbol use and placement guide circuit success.
Q10. Without stripping insulation, how would you test a long wire for an internal break using only a cell, a small bulb, and two short test leads? Explain the logic behind your method.
Answer: Make a simple continuity tester: connect the cell and bulb in series using the two short test leads, leaving two free probe ends. Touch one probe to one end of the long wire and the other probe to the other end. If the bulb glows, the wire forms part of a closed circuit, indicating it is continuous. If it does not glow, the wire is likely broken internally. To locate the break more closely, shift one probe along accessible endpoints (for example, metal plugs or exposed terminal points) or test section by section if connectors are available. Wherever the bulb stops glowin...