Question Video: Determining the Rotation Direction of the Pulley in a Hot Wire Ammeter | Nagwa Question Video: Determining the Rotation Direction of the Pulley in a Hot Wire Ammeter | Nagwa

Question Video: Determining the Rotation Direction of the Pulley in a Hot Wire Ammeter Physics • Third Year of Secondary School

The diagram shows a hot-wire ammeter. A platinum–iridium alloy wire is connected to an alternating-current circuit and a silk thread is tied around the platinum–iridium alloy wire. When the current in the platinum–iridium alloy wire increases, the wire expands due to an increase in its temperature. In which of the rotation directions shown does the pulley rotate when the wire expands?

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Video Transcript

The diagram shows a hot-wire ammeter. A platinum–iridium alloy wire is connected to an alternating-current circuit and a silk thread is tied around the platinum–iridium alloy wire. When the current in the platinum–iridium alloy wire increases, the wire expands due to an increase in its temperature. In which of the rotation directions shown does the pulley rotate when the wire expands?

When we look at a hot-wire ammeter, the part that we use to read the current is the dial. For this diagram, when looking at the dial from this direction, it should look something like this, with rotation I moving the needle to the right or clockwise and rotation II moving the needle to the left or counterclockwise.

Now, this is a hot-wire ammeter. This entire setup here is designed so that the needle won’t fling back and forth. We use hot-wire ammeters when we want to measure the magnitude of the current, not the direction, which is ideal for getting a reading with an alternating-current circuit. This means that when we read a hot-wire ammeter dial, the current readings on the left side of the dial will be of lower magnitude, while the current readings on the right side of the dial will be of higher magnitude, not negative and positive like we would expect of an ammeter, which measures current direction.

So when the question asks us “Which of the rotation directions shown does the pulley rotate when the wire expands?,” rotation I means that the current has increased and will indicate a higher magnitude, while rotation II means the magnitude has decreased. Remember that this happens because the pulley is directly attached to the dial of the ammeter. So when it rotates, the dial will as well. And the answer to this rotation is actually partially given in the question text.

When the current in the wire increases, the wire expands due to an increase in its temperature. So the expansion of the wire occurs due to an increase in current in the wire. So to accurately represent this, we should expect the dial to move to the right or clockwise. This means that the correct rotation direction for the pulley when the wire expands should be rotation direction I.

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