Video Transcript
The diagram shows a hot-wire
ammeter. Which of the components shown in
the diagram includes a shunt?
In this question, we’re asked to
identify the shunt in a hot-wire ammeter. First, let’s remind ourselves how a
hot-wire ammeter works.
A hot-wire ammeter is a device used
to measure an alternating electrical current. In this diagram, we have a circuit
consisting of an alternating-current source and a resistor. We also have an ammeter. The ammeter is designed so that the
current in the circuit is split across two parallel branches. One of these branches contains a
resistor, called a shunt resistor. The other branch does not contain
any components. But it does contain a section of
wire that is made out of a special material, platinum–iridium alloy. This is referred to as a hot
wire.
A piece of string, which does not
conduct electricity, is tied to the hot wire. The string runs across a pulley and
is tied to a spring at the other end. The spring holds the string taut,
so that it pulls on the hot wire. When the alternating current in the
circuit reaches the ammeter, the charges either flow through the branch that
contains the shunt resistor or through the branch with the hot wire.
When charge flows through the hot
wire, the temperature of the hot wire increases, and the wire expands, becoming
longer. When the hot wire becomes longer,
the spring is able to pull the nonconducting string towards the right. As the string moves over the
pulley, it changes the position of a needle, which points to a measurement
scale. This allows the value of the
alternating current in the circuit to be measured.
Now we know how a hot-wire ammeter
works, identifying the shunt in this diagram will be fairly simple. Let’s go through each of the
components labeled.
Component I consists of some wire
and a resistor. This is connected in parallel to
the wire in component II. Component II must be the hot wire;
we can see it is being pulled on by the nonconducting string, component III, which
is connected to a spring, component IV.
To answer this question, we simply
need to recall that the term “shunt” is short for shunt resistor. The shunt resistor in this diagram
would be located in component I of the diagram. This resistor is also connected in
parallel to the hot wire, which is exactly how a shunt should be connected. So, we can be confident that this
resistor is the shunt. The answer to this question is,
therefore, component I.
