NCERT Solutions Curiosity Chapter 3 Activity 3.7: Let us experiment — An electrical circuit
Book page 31 Updated on2026-09-05
Q1.
Insert two cells in the holder, taking care that for each cell, its negative terminal is towards the spring side of the holder (Fig. 3.10b) and the battery is ready to use. How will you decide which is the positive terminal of this battery?
Answer
Look at which end of each cell the two ends of the holder touch.
The end of the holder that is connected to the positive terminal of one cell is the positive terminal of the battery.
The end of the holder that is connected to the negative terminal of the other cell is the negative terminal of the battery.
Holder end A — touches the + cap of cell 1 → + terminal of the battery
Holder end B — touches the – disc of cell 2 → – terminal of the battery
Tip: In a cell holder the spring always presses against the flat disc, so the spring side of each cell is its negative side. Follow the springs and you can never get the polarity wrong.
Q2.
Now, connect the free end of the battery positive terminal wire to the longer wire of LED, and the free end of the second wire to the shorter wire of LED (Fig. 3.10c). Does the LED glow?
Answer
Yes, the LED glows.
+ of battery → longer wire (+ terminal) of LED
– of battery → shorter wire (– terminal) of LED
Current can pass in this direction → LED lights up
Why it happens: An LED allows current in one direction only. Here that direction agrees with the direction in which the battery pushes the current, so current flows and the LED emits light. Two cells are used because a single cell usually does not give enough push to light an LED.
Q3.
Repeat the above step but interchange the wires connected to the LED (Fig. 3.10d). Does the LED glow again?
Answer
No, it does not glow this time.
+ of battery → shorter wire (– terminal) of LED
– of battery → longer wire (+ terminal) of LED
Current cannot pass in this direction → LED stays dark
Why it happens: Electric current can pass through an LED in one direction only. When the connections are interchanged, the battery tries to push current the wrong way through the LED and the LED blocks it. So always take care to connect an LED correctly in a circuit.
Compare: An incandescent lamp behaves differently — it glows whichever way round it is joined, because the current simply heats its filament and heating does not depend on direction.