Worksheet: Alternating-Current Ammeters

In this worksheet, we will practice describing the construction of ammeters using the thermal expansion of conducting wires to measure alternating currents.

Q1:

Which of the following would help reduce developing zero errors in a hot-wire ammeter?

  • AUsing the ammeter to measure a direct current immediately after using the ammeter to measure an alternating current
  • BRapidly cooling the hot wire immediately after using the ammeter
  • CMounting the hot wire on a plate that has a coefficient of thermal expansion the same as the hot wire
  • DMounting the hot wire on a plate that has a coefficient of thermal expansion that is very different from that of the hot wire

Q2:

Which of the following diagrams most correctly shows the divisions of the scale of a hot-wire ammeter corresponding to equal changes in current?

  • A
  • B
  • C

Q3:

A hot-wire ammeter is calibrated by using it to measure a direct current source that is also measured by a direct-current ammeter. Which of the following most correctly explains why the divisions on the scale of a hot-wire ammeter would not be spaced equivalently to the divisions on the scale of a direct-current ammeter that had the same resistance?

  • AThe electrical power dissipated by a wire is proportional to the square of the current in the wire, whereas the deflection of a galvanometer is proportional to the square root of the current.
  • BAn alternating current dissipates twice the power of a direct current as it is bidirectional.
  • CThe electrical power dissipated by a wire is proportional to the square of the current in the wire, whereas the deflection of a galvanometer is proportional to the current.
  • DThe electrical power dissipated by a wire is proportional to the current in the wire, whereas the deflection of a galvanometer is proportional to the square of the current.

Q4:

Which of the following conditions must apply for a hot wire ammeter to give a constant reading for an alternating current?

  • AThe electrical power dissipated by the wire must equal the power with which the wire heats its surroundings.
  • BThe electrical power dissipated in the wire must be greater than the power with which the wire heats its surroundings.
  • CThe electrical power dissipated in the wire must be zero.
  • DThe wire must heat its surroundings with the same power that it is heated by its surroundings.

Q5:

The diagram shows a hot-wire ammeter. Which of the components shown is attached to electrically conductive components but is not itself electrically conductive?

  • AIV
  • BII
  • CI
  • DIII

Q6:

The diagram shows a hot-wire ammeter. Which of the components shown in the diagram is made of a platinum–iridium alloy?

  • AIII
  • BIV
  • CII
  • DI

Q7:

The platinum–iridium alloy wire in a hot-wire ammeter expands when its temperature increases and contracts when its temperature decreases. The temperature of the wire is dependent on the current in the wire. A hot-wire ammeter using such a wire will give a constant reading for an alternating current that has a particular peak value. Which of the following most correctly explains how an alternating current with a frequency of 50 Hz in the wire can produce a constant hot-wire ammeter reading?

  • AThe wire expands when its temperature increases much faster than it contracts when its temperature decreases, so the wire never reduces in temperature for a sufficient time to contract noticeably.
  • BThe wire heats a hot-wire ammeter’s other mechanical components. The expansion and contraction of these components are out of phase with each other, so the reading on the ammeter remains constant.
  • CThe frequency at which the wire can undergo a cycle of expansion and contraction is much smaller than the frequency of the alternating current, so the expansion of the wire corresponds to the effective value of the current.

Q8:

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?

  • AII
  • BI

Q9:

The diagram shows a hot-wire ammeter. Which of the components shown in the diagram provides a force that balances the force produced by the effect of an alternating current?

  • AII
  • BIV
  • CIII
  • DI

Q10:

A galvanometer is connected to an alternating current source that has a low frequency. Terminal a of the galvanometer is positive when terminal b is negative and vice versa. The polarities of the terminals alternate periodically. Which of the following most correctly describes the deflection of the galvanometer arm?

  • AThe arm oscillates between two positions, one of which is the zero-deflection position.
  • BThe arm oscillates between two positions equidistant from the zero-deflection position.
  • CThe arm remains at the zero-deflection position.
  • DThe arm undergoes uniform circular motion.

Q11:

The diagram shows a hot-wire ammeter. Which of the components shown in the diagram includes a shunt?

  • AII
  • BI
  • CIV
  • DIII

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