Electric Current and Circuits
Electric Current and Circuits
Electricity gives 2 questions in the Part-III paper. The chapter is examined through circuit vocabulary, Ohm's law and safety devices, and it connects to the chemical and heating effects covered in the next topic, so treat the two together.
Circuit Basics
An electric current is a flow of charge. A closed circuit is needed for it to flow; an open circuit carries no current.
| Quantity | Symbol | SI unit | Measured by |
|---|---|---|---|
| Current | I | Ampere | Ammeter, in series |
| Potential difference | V | Volt | Voltmeter, in parallel |
| Resistance | R | Ohm | — |
| Charge | Q | Coulomb | — |
| Power | P | Watt | — |
An ammeter is always connected in series and a voltmeter always in parallel. Reversing them is the standard practical error and a standard question.
✗ A voltmeter is connected in series in a circuit | ✓ A voltmeter goes in parallel; only the ammeter goes in series
Ohm's law: V = I × R | Power P = V × I
Conventional current is taken to flow from the positive to the negative terminal, while electrons actually move the other way. Both statements are correct and questions sometimes test whether you know they coexist.
Series and Parallel
| Point | Series | Parallel |
|---|---|---|
| Current | Same through all components | Divides between branches |
| Voltage | Divides across components | Same across all branches |
| If one fails | The whole circuit stops | Others keep working |
| Total resistance | R₁ + R₂ + ... | Less than the smallest resistance |
| Used in | Fairy lights of the old type | House wiring |
House wiring is in parallel so that each appliance gets the full supply voltage and one failing bulb does not switch off the house. This is asked directly and the reason matters as much as the fact.
Conductors and Insulators
- Conductors allow current through — copper, aluminium, silver, and also acidic or salty water.
- Insulators do not — rubber, plastic, glass, dry wood, dry air.
- Distilled water is a poor conductor, while ordinary tap water conducts because of dissolved salts.
- This is why wet hands make an electric shock more likely, and why electricians use rubber-handled tools.
The distilled-water point is a reliable discriminator, because most candidates assume all water conducts.
Safety Devices
| Device | Function |
|---|---|
| Fuse | A thin wire that melts and breaks the circuit on overload |
| MCB | A switch that trips automatically; reusable |
| Earth wire | Carries leaked current safely into the ground |
| Switch | Opens or closes the circuit deliberately |
A fuse is made of a material with a low melting point and is connected in series with the live wire. Its purpose is to protect the appliance and the wiring from excessive current, not to control the appliance. The three wires in household supply are live, neutral and earth, and the earth wire is the one that prevents a metal appliance body from giving a shock.
घरों की वायरिंग समांतर क्रम में होती है, श्रेणी क्रम में नहीं।
TRE pointer: Three items repeat here: ammeter in series and voltmeter in parallel, why house wiring is in parallel, and the fact that distilled water is a poor conductor. The last is the best discriminator because it runs against intuition. Since a science teacher is expected to handle laboratory safety, a question on the fuse or the earth wire is entirely likely. With −1/3 negative marking, note whether a question asks for the device or its purpose.
60-Second Recap
- Current needs a closed circuit; ammeter goes in series, voltmeter in parallel.
- V = IR and P = VI; conventional current runs positive to negative, electrons the other way.
- In series the current is the same and voltage divides; in parallel the reverse.
- House wiring is parallel so each appliance gets full voltage and failures stay local.
- Distilled water is a poor conductor; tap water conducts because of dissolved salts.
- A fuse melts to break the circuit on overload; the earth wire carries leaked current away.
Frequently Asked Questions
Why is house wiring done in parallel?
So that every appliance receives the full supply voltage and can be switched on or off independently. In a series circuit the voltage would divide between appliances and a single failure would break the whole circuit, switching off the entire house at once.
Does distilled water conduct electricity?
No, distilled water is a poor conductor because it contains almost no dissolved ions. Ordinary tap water conducts well because of dissolved salts and minerals. This is why wet hands increase the risk of shock, even though pure water by itself would not.
How should an ammeter and a voltmeter be connected?
An ammeter is connected in series, because the current to be measured must pass through it. A voltmeter is connected in parallel across the component, because it measures the potential difference between two points. Reversing them is a standard laboratory and examination error.
What is the function of a fuse?
To protect the circuit and appliances from excessive current. It is a thin wire of low melting point placed in series with the live wire, and it melts and breaks the circuit when the current rises too high. A miniature circuit breaker performs the same task but is reusable.