Question Video: Reading a Measurement from a Moving-Coil Galvanometer Scale | Nagwa Question Video: Reading a Measurement from a Moving-Coil Galvanometer Scale | Nagwa

Question Video: Reading a Measurement from a Moving-Coil Galvanometer Scale Physics • Third Year of Secondary School

The diagram shows a moving-coil galvanometer. The arm of the galvanometer points to the center of the scale. The maximum current that the wires connected to the galvanometer can carry is 240 𝜇A. Which of the following must be true of the current 𝐼 through the galvanometer coil? [A] 𝐼 = 0 𝜇A [B] (120 > 𝐼 > 0) 𝜇A [C] (240 > 𝐼 > 0) 𝜇A [D] 𝐼 = 120 𝜇A [E] 𝐼 = 240 𝜇A

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Video Transcript

The diagram shows a moving-coil galvanometer. The arm of the galvanometer points to the center of the scale. The maximum current that the wires connected to the galvanometer can carry is 240 microamperes. Which of the following must be true of the current 𝐼 through the galvanometer coil? (A) 𝐼 equals zero microamperes. (B) 120 is greater than 𝐼 is greater than zero microamperes. (C) 240 is greater than 𝐼 is greater than zero microamperes. (D) 𝐼 equals 120 microamperes. (E) 𝐼 equals 240 microamperes.

Let’s begin by taking a closer look at the scale in the diagram. Notice that the galvanometer is capable of measuring a maximum magnitude, so positive or negative, of 240 microamps. We can recall that the sign of the current simply indicates its direction in the coils. But in any case, the scale is marked to measure values between negative 240 and positive 240 microamps. This is in contrast to many measuring devices we use in everyday life, such as a ruler or car speedometer. Both of these have scales with a minimum value of zero and can therefore only measure positive values.

But since the galvanometer scale can measure an equal magnitude of both positive and negative values, zero must be marked directly in the center. Notice that this is where the galvanometer arm is pointing. Recall that when there is no current present in the coils, there is no torque on the coils by either the magnetic field or the torsion springs. Thus, the coils and arm are not displaced from their resting equilibrium position, as shown in the diagram.

So, because the galvanometer coils have not rotated and the arm is indicating a value of zero microamperes on the scale, there must be no current present in the coils. Answer choices (B) through (E) all indicate a positive current value and therefore cannot be correct. Option (A), which says 𝐼 equals zero microamperes, is the correct answer.

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