# resistive circuit question

#### Alex1700

Joined Jan 12, 2020
106

#### ericgibbs

Joined Jan 29, 2010
18,234
Hi Alex,
Is this question a Homework or College assignment?
Moderation.

#### Alex1700

Joined Jan 12, 2020
106
Hi Alex,
Is this question a Homework or College assignment?
Moderation.
personal study

#### Alec_t

Joined Sep 17, 2013
14,009
The steps in the calculation of Is3 are all set out above, but not in the clearest fashion.

#### Alex1700

Joined Jan 12, 2020
106
The steps in the calculation of Is3 are all set out above, but not in the clearest fashion.
able to explain to me why able to use KCL

#### Jerry-Hat-Trick

Joined Aug 31, 2022
450
Why Gustav was given credit for making Georg's elegantly simple law more complicated has always been a mystery to me.

Use Ohm's law. Va = 20 volts. Vb = 20 - 5 = 15 volts, Vc = 8 Volts.
So I1 = (Va-Vc)/R1 = (20-8)/10 = 1.2A and I2 = (Vb - Vc)/R2 = (15-8)/4 = 1.75A
Since the arrow for IS3 is drawn pointing the wrong way the current is the negative sum of 1.2 and 1.75 = -2.95

If you are being taught to do unrealistic problems using Kirchoff's laws I'd recommend looking for a better course. I've interviewed so many new graduate engineers who can do this kind of problem but have no idea how to address real world problems.

• Alex1700

#### Alec_t

Joined Sep 17, 2013
14,009
able to explain to me why able to use KCL
KCL can be applied at any circuit node. In this case the node labelled Vc is the one being considered.

#### Alex1700

Joined Jan 12, 2020
106
Why Gustav was given credit for making Georg's elegantly simple law more complicated has always been a mystery to me.

Use Ohm's law. Va = 20 volts. Vb = 20 - 5 = 15 volts, Vc = 8 Volts.
So I1 = (Va-Vc)/R1 = (20-8)/10 = 1.2A and I2 = (Vb - Vc)/R2 = (15-8)/4 = 1.75A
Since the arrow for IS3 is drawn pointing the wrong way the current is the negative sum of 1.2 and 1.75 = -2.95

If you are being taught to do unrealistic problems using Kirchoff's laws I'd recommend looking for a better course. I've interviewed so many new graduate engineers who can do this kind of problem but have no idea how to address real world problems.
may I know there are 3 voltage sources how is the KCL able to apply ?

#### Alec_t

Joined Sep 17, 2013
14,009
Why do you think the number of voltage sources is important? KCL doesn't care.

#### Alex1700

Joined Jan 12, 2020
106
Use Ohm's law.
Why do you think the number of voltage sources is important? KCL doesn't care.
why are these cases able to apply KCL based on nodes?

#### Jerry-Hat-Trick

Joined Aug 31, 2022
450
why are these cases able to apply KCL based on nodes?
Alex, you may not have understood my point. KCL is, in my view, not worth your time. Honestly, since I've never seen the point of KCL, at least in real world problems, I'm not the best person to give you a sensible answer. Why do you care? The circuit in your post is completely artificial, which is why I recommended finding a better course which teaches and tests real world problems.

• Alex1700

#### Alec_t

Joined Sep 17, 2013
14,009
why are these cases able to apply KCL based on nodes?
Because "Kirchhoff's Current Law, often shortened to KCL, states that “The algebraic sum of all currents entering and exiting a node must equal zero.”

• Alex1700