1
\$\begingroup\$

I am currently learning about calculating the power for different elements within a circuit and found the following question in a book of which I am not sure how to reason about and solve. I have included my thought process below.

The figure shows an ammeter (AM) and
voltmeter (VM) connected to measure the
current and voltage, respectively, for circuit element A. When current actually enters
the + terminal of the ammeter the reading is
positive, and when current leaves the + terminal the reading is negative. If the actual
voltage polarity is positive at the + terminal
of the VM, the reading is positive; otherwise,
it is negative. (Actually, for the connection
shown, the ammeter reads the sum of the current in element A and the very small current
taken by the voltmeter. For purposes of this
problem, assume that the current taken by
the voltmeter is negligible.)

Find the power for element A and state whether energy is
being delivered to element A or taken from
it if:

a) the ammeter reading is +2 A and the voltmeter reading is −25 V;

b) the ammeter reading is −2 A and the voltmeter reading is +25 V;

c) the ammeter reading is −2 A and the voltmeter reading is −25 V.

circuit_figure

My try:

a) The current is flowing through the AM from + to -, as such when we want to calculate the power for element A the path begins from a negative terminal, goes through the element A to a positive terminal on the right side of the VM. So we use P = VI = (-25)(2) = -50W (negative => element absorbs the energy).

Side note about my above try: Is the polarity really flipped in the VM? It said in the question "If the actual voltage polarity is positive at the + terminal of the VM, the reading is positive; otherwise, it is negative. We have VM = -25V in a) and as such am I correct to say that + and - should be switched on the figure?

b) Here I am split on how to think. I am thinking that since the AM reads a negative current, it is flowing from - to + through the AM. So if current from element A is flowing to a negative terminal, it must've come from a positive terminal no? In that case I can again use P = VI = (25)(-2) = -50W (absorbed by the element)

c) Same as b) but different sign on measured voltage by VM. P = VI = (-25)(-2) = 50W (supplied to the element).

\$\endgroup\$
2
  • \$\begingroup\$ Just multiply volts and amps and take into account signs. \$\endgroup\$
    – Andy aka
    Mar 21, 2022 at 13:35
  • \$\begingroup\$ @Andyaka Sure, that's the idea. What I am obviously having trouble with is reasoning about the signs and the theory behind them, which is why I asked the question. \$\endgroup\$
    – NoName123
    Mar 21, 2022 at 13:36

1 Answer 1

1
\$\begingroup\$

enter image description here

All I've done in my redraw is move the ammeter from one side to the other but, not switched its polarity. For the voltmeter that was reading negative 25 volts, I've redrawn it connected in reverse to the original diagram and made the voltage reading positive 25 volts. Hence, power is positive 50 watts i.e. element A is a 25 volt generator feeding an undisclosed load that is taking 2 amps.

Does this help? It always helps me to make a redraw on many problems.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.