F.5 physics electric circuits

2014-10-13 9:23 am

Picture:
http://postimg.org/image/59mdmjztr/
In the figure shown, the voltmeter which has a very high resistance, reads 0.4V.
a. What is the value of R?
b. If switch K is closed, what will the voltmeter read?
Please help! Thankyou!

回答 (1)

用戶10012

2014-10-13 3:34 pm

✔ 最佳答案

Since the voltmeter has very high resistance, current flowing through it is negligible ( = zero).
Since reading is 0.4V, so current flowing through the 2 ohms resistor
= 0.4/2 = 0.2A.
The same current flow through all resistors,
so (2 + 3 + R)(0.2) = 1.5V
1 + 0.2R = 1.5
0.2R = 0.5
R = 2.5 ohms.
(b)
When switch K is closed, the 3 ohm resistor is shorted. So resistance of the circuit becomes 2 + R = 4.5 ohms.
So current becomes 1.5V/4.5 ohms = 1/3A
So reading of voltmeter = voltage across the 2 ohm resistor = (1/3)(2) = 2/3V.

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