Electricity — Class 7 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 7 Science, Term 2 — Chapter 2. Circuits, current and its effects.
1. About this chapter
This chapter covers the electric circuit, electric current and its unit, conductors and insulators, series and parallel circuits, and the effects of electric current.
2. Electric current and circuits
- Electric current is the flow of electric charge; it is represented by the symbol I and measured in amperes (A) with an ammeter. Current = charge ÷ time — if 10 units of charge pass a point each second, the current is 10 A.
- Small currents are measured in milliamperes (mA): 1 A = 1000 mA, so 0.25 A = 250 mA.
- A circuit needs a cell/battery, connecting wires, a switch and a device (such as a bulb). Conductors (copper) allow current; insulators (wood, plastic) do not. (Analogy: water flows through a pipe as current flows through a wire.)
3. Series and parallel circuits
| Series circuit | Parallel circuit |
|---|---|
| single loop connection | connected in branches |
| current is the same in all parts | voltage is the same across each component |
| bulbs share the power (dimmer) | each bulb is fully powered |
| if one bulb fails, all go out | if one bulb fails, the others stay lit |
4. Effects of electric current
- Heating effect: current through a wire changes electrical energy into heat. Heating appliances use a heating element of high melting point — for example nichrome (an alloy of nickel, iron and chromium).
- Current also has a magnetic effect (electromagnet) and a chemical effect (electroplating).
5. Worked examples
Example 1. How many milliamperes are there in 0.25 A? 0.25 × 1000 = 250 mA.
Example 2. In which circuit do all bulbs go out if one fails? A series circuit.
Example 3. Which alloy is used as a heating element? Nichrome (nickel, iron, chromium).
6. Book-back questions (Samacheer Kalvi)
I. Choose the correct answer
- If 10 units of charge pass a point each second, the current is — (a) 10 A / (b) 1 A. Ans: (a) 10 A.
- The number of milliamperes in 0.25 A is — (a) 25 mA / (b) 250 mA. Ans: (b) 250 mA.
- The heating element of an appliance is made of — (a) copper / (b) nichrome. Ans: (b) nichrome.
II. Analogy (fill in) 4. Water : pipe :: electric current : wire. 5. Copper : conductor :: wood : insulator. 6. Length : metre scale :: current : ammeter.
III. True or False 7. In a parallel circuit, the components are divided into branches. — True. 8. The representation of electric current is A. — False (it is I).
IV. Answer briefly 9. Differentiate series and parallel circuits. — In series, the current is the same and all bulbs go out if one fails; in parallel, the voltage is the same and the other bulbs stay lit. 10. What is the heating effect of current? — Electrical energy is converted to heat when current flows through a wire.
7. Common mistakes
- Mistake: Writing the symbol of current as "A". Fix: Current is represented by I; the ampere (A) is its unit.
- Mistake: Saying parallel bulbs all go out if one fails. Fix: In parallel, the others stay lit; in series, all go out.
- Mistake: Confusing the milliampere conversion. Fix: 1 A = 1000 mA, so 0.25 A = 250 mA.
8. Quick revision
- Term 2 · Ch 2 · electricity.
- Current (I) = charge ÷ time, measured in amperes by an ammeter; 1 A = 1000 mA.
- Circuit: cell + wires + switch + device; conductors (copper) vs insulators (wood).
- Series: same current, all go out if one fails; parallel: same voltage, others stay lit.
- Heating effect → nichrome (nickel, iron, chromium); also magnetic and chemical effects.
