# Electric Current and High Resistance

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• Published : March 9, 2013

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1. Is it desirable for an ammeter to have a high resistance or low resistance? Why?

➢ Yes, it is desirable for an ammeter to have a high resistance or low resistance because the lower the resistance, the better the ammeter as it is connected in series and so its resistance will affect the current flow and thereby affect any calculations to be obtained.

2. An ammeter of resistance Ra is connected through a rheostat to a cell of negligible internal resistance. When the rheostat is adjusted to a value R1, the ammeter reads 0.75 of the full-scale value. When it is adjusted to a value R2, the ammeter reads 0.25 of full-scale value. Compute the value of Ra in terms of R1 and R2.

➢ V = RI1
V = (Ra + R1) 0.75A

V = RI2
V = (Ra + R2) 0.25A

V=V
(Ra + R1) 0.75A = (Ra + R2) 0.25A
(Ra + R1) 3 = Ra + R2
3Ra + 3R1 = Ra + R2
2Ra = R2 – 3R1
Ra = 0.5(R2 – 3R1)

3. In a simple series circuit a battery is connected through an ammeter to an electric lamp. A voltmeter is connected parallel to the lamp. The ammeter reads 0.75 A and the voltmeter 50 V. What is the resistance of the lamp neglecting the current drawn by the voltmeter? If the current in the voltmeter is 0.001 A for each volt it indicates, what is the actual current in the lamp and the corrected value of the resistance?

➢ Without voltmeter current:

R = V/I
R = 50V/0.75A
R = 66.67ohms

With voltmeter current:

Ivoltmeter = (0.001A/V)(50V)
Ivoltmeter = 0.05A
Ilamp = I - Ivoltage
Ilamp = 0.75A – 0.05A
Ilamp = 0.7A

R = V/Ilamp
R = 50V/0.7A
R = 71.429ohms