Observations In Step 2 with the cables on A and B the red LE

Observations In Step 2 with the cables on A and B the red LED went on and with C and D the green LED went on but with B and C neither light went on

Questions

1. The LEDs used require 1.5 V applied in order to light up. If the battery source provides 3 V, explain why one LED easily lights, while two in series do not light noticeably.

2. Why doesn’t the red LED light up when you connected points C and D in Procedure 1.3? Explain this in terms of open and closed circuits.

3. Did the brightness of the red LED change much when you added the green LED to the circuit in Procedure 1? What does this tell you about the voltage across each parallel branch?

4. Do you think the same batteries could power 10 LEDs in parallel? What would prevent the circuit from operating very long? (Hint: the batteries can supply the same voltage to each branch, but the total power used up by the circuit increases as you add LEDs.)

Procedure 1 1. Using two LEDs and a 100 n resistor, construct the series circuit shown in Figure 7 . 2. Turn the switch to \"on\" and connect the jumper ends to points A and B, noting which LED lights up. 3. Now try connecting the jumper wire to the points C and D. Think about where the current is flow ing in each case, and why each LED lights up. 4. Finally, connect the jumper between points B and C. Answer question 2 before moving on. a ON of US Figure 7: Configuration for Experiment 2, Procedure 1 Figure 8: Configuration for Experiment 2, Procedure 2

Solution

1. When one LED is connected, assuming 1.5V drop across one LED and a finite voltage drop across the resistor, the voltage across the other LED is not sufficient to switch it to ON state. Hence, only one LED glows.

2.Red light does not glow as there is no current flowing through the LED when C and D are connected.

3. When the RED and the GREEN led are connected in parallel, the voltage across them are same. Hence they glow with same intensity.

4. Yes they can power 10 batteries as volatge drop is same for parallel circuits. But the current requirement is now additive instead of being same for series combinations. One problem with parallel circuits is thermal runaway,  When one LED is a parallel combination draws more current than another, the LEDs being overdriven will get heated up and their voltages will change more, resulting in more unequal current sharing.