[SPM Physics Electric Solution] Chapter 3

How to solve this question?


SPM Physics Electric Question

Diagram 17 shows an electric circuit.
Rajah 17 menunjukkan satu litar elektrik.

What is the voltmeter reading if the resistance of the light bulb is 12 \Omega ?
Apakah bacaan voltmeter jika rintangan mentol itu ialah 12 \Omega ?
A 8 \mathrm{~V}
B 12 \mathrm{~V}
C 18 \mathrm{~V}
D 36 \mathrm{~V}

SPM Physics Electric Solution


First we will solve the parallel section of the circuit, which is:

1/R = 1/3 + 1/6 => This gives us Effective Resistance, R of 2 ohm.

Then, we solve the series section of the circuit, combining 2 ohm and 12 ohm of the bulb gives us a total resistance of 14 ohms.

Now, we distribute the voltage in proportional to the resistance of each portion,

The voltage across the light bulb is:
V = 12 * 12/14 = 10.3 V

I believe the options of the answer there are wrong. However by logic, you can directly eliminate 18 V and 36 V directly from your answer options. And since there is an additional voltage component present other than the light bulb, it is impossible for the voltage across the light bulb to be 12 V.

So by elimination, 8 V sounds possible for the light bulb, assuming there is also voltage drop across the ammeter. But regardless, the correct answer should be 10.3 V

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Thank you very much for your explanation!

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