1
\$\begingroup\$

Circuit diagram

Hi there, I am learning the kirchhoffs-laws, according to the passive sign convention if a positive current enter through the positive terminal of an element and exits out of its negative terminal, then the element is dissipating power.

So, in this diagram the current enter through the negative terminal element with 3 ohms resistance. The calculation shows that the voltage across it is the same sign as the voltage across the 2 ohms resistor. Why do they have the same voltage sign although the current enter through different polarity of terminal, is there any element that has such properties in real life?

\$\endgroup\$
2
  • \$\begingroup\$ Conventional current enters on positive side of resistor, but KVL can be done by just picking polarities at random. This is the case here. Answer will be negative, which just means polarity assumptions were wrong. \$\endgroup\$ Mar 31 at 13:48
  • \$\begingroup\$ By the way, this would be the situation if the "element" was a true negative "resistor" (circuit) with -3 ohm resistance (the so-called VNIC converter). It would act as a proportional negative voltage source producing voltage v2 = -3i (adding it to 20 V) instead consuming a voltage drop v2 = 3i (subtracting it from 20 V). \$\endgroup\$ Mar 31 at 17:46
0
\$\begingroup\$

Resistors aren't polarised. The positive and negative are simply labels, and someone has written the v2 labels the wrong way up.

Alternatively, this is a homework question asking for a negative value for v2. If you put the negative lead of a voltmeter to the -v2 and the positive lead to +v2, you will read a negative vale on the voltmeter. If you swap the leads you read a positive value.

\$\endgroup\$
3
  • \$\begingroup\$ I would not say that the v2 labels are wrong, just that they will result in a negative value for v2. If an alternating current passes through a resistor we wouldn't say that its labeled voltage is wrong half of the time. \$\endgroup\$ Mar 31 at 14:59
  • \$\begingroup\$ @ElliotAlderson Can you think of any element or electrical component that can be represented by V2 (based on the way it is connected in this diagram) ? or maybe they are just simply made up as for homework. \$\endgroup\$
    – user281667
    Mar 31 at 15:16
  • \$\begingroup\$ The component is a resistor. The labels don't change that. \$\endgroup\$
    – pjc50
    Mar 31 at 15:23

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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