Video Transcript
Each of the following diagrams
shows a circuit containing a cell, a bulb, and an ammeter. Which one shows how the ammeter
must be connected to the circuit in order to measure the current in the circuit?
Okay, so in order to answer this
question, we should first recall that an ammeter is in fact used to measure the
current in a circuit. However, it has to be connected in
a very specific way into the circuit in order for it to work. We’ve been told that each one of
these circuit diagrams (A), (B), (C), and (D) shows us a cell, a bulb, and an
ammeter. Let’s recall that the circuit
symbol of a cell looks like this, a short line to represent the negative terminal of
the cell and a long line to represent the positive terminal. A bulb is drawn like this, a circle
with a cross through it. And an ammeter is drawn this way, a
circle with a capital A inside it. And we can see that each one of
these circuit diagrams does, in fact, have a cell, a bulb, and an ammeter in it.
Now, let’s also recall that for an
ammeter to work, it must be connected in series in our circuit. So let’s go through each circuit
and see if our ammeter is indeed connected in series. Let’s start with circuit (A). We can imagine charges flowing from
the positive terminal of our cell. In other words, we’re considering
the current in our circuit. And in this particular case, as
soon as we get to this junction, some of the charges must be going off in this
direction, whereas the others go off in this direction. And then we see the ammeter
connected on one of these branches. Therefore, we can say that our
ammeter is not connected in series in this circuit.
An additional problem is that the
circuit is not even complete. We can see that the end of this
wire here is not connected to anything; it’s just sort of left there. Therefore, we can definitively say
that circuit (A) is not the correct way in which to connect an ammeter in order to
measure the current in our circuit.
Moving on to circuit (B) then, we
can once again begin at the positive terminal of our cell and consider the charges
flowing in this direction. We can see that all of the current
does indeed pass through our ammeter and then through the bulb before continuing
clockwise and arriving at the negative terminal of the cell. At which point we’ve gone around
the entire circuit and all of the current has passed through our ammeter. It is connected in series. Therefore, option (B) is a good
candidate for a correct circuit.
Moving on to option (C) though, we
can once again begin at the positive terminal of the cell, going this way
clockwise. Then we can see that all of the
current does pass through the bulb. But as soon as we get to this
junction here, the current must split. Some goes in this direction and the
rest in this direction. And then we see that the ammeter is
again on one of these branches. Additionally, we’ve got the same
problem as in circuit (A). The circuit is actually
incomplete. This end of the wire is not
connected to anything. So because our ammeter is not
connected properly in circuit (C) and it’s also an incomplete circuit, we can say
that this is not a correct answer.
Finally, moving on to circuit (D)
again, beginning at the positive terminal, we go around clockwise until we get to
this junction here. Now, at this point, we may have
been able to say that our ammeter was at least connected in parallel if the circuit
had been completed, for example, if we had a wire that looked like this dotted
line. And in that case at least, the
ammeter would have been connected in parallel with our circuit, and the circuit
would have been a complete circuit. That still wouldn’t mean that the
ammeter was functioning because again it needs to be connected in series. And in fact, for the given circuit,
neither is the ammeter in series nor is the circuit complete. So, option (D) is out of the
question as well. At which point we can say that
option (B) shows us how to correctly connect an ammeter in order to measure the
current in a circuit.
