Why is the resistance figures different in the second part of the circuit?

 

Hello, Pickles
Superposition also applies in your field

The effect of applying one 15kN load is the same as the effect of applying two separate loads one of 10kN and one of 5kN to some object.

This is because the system response is linear.

So the extension of increasing the load from say 5kN to 10 kN is the same as the extension of increasing it from 10kN to 15kN in a spring.

So in electrical terms

The effect of increasing the voltage across your resistor (increase in current) from 5 volts to 10 volts is the same increase in current as you would get if you increased it from 10 volts to 15 volts.

Does this help?

 

Don't understand why
R2 ll R3 is 1.33

Also in the diagram the 7volts is on the right of the diagram so does this change which resistor it hits first?

 

The presentation in the Ebook is rather snappy, and meant for (would be) electrical engineers, and therefore assumes some basic electrical stuff.

First did you understand my analogy from strength of materials?
In other words if you had a 10m cantilever wing with known loads and deflections at 2, 5 and 10 metres, could you use a table of deflections and the superposition theorem to find the overall deflections?

 

Why is the resistance figures different in the second part of the circuit?

Which part of which circuit? There are a lot of circuits floating around in this discussion, so please indicated exactly which figure you are talking about and what you mean by "second part". The best thing is to take a screen shot of the figures you are referring to and put them directly in your post.

 

Don't understand why
R2 ll R3 is 1.33

R2 || R3 is NOT 1.33Ω

With only the 28V source "on", R2 is in parallel with R3. But R2=2Ω and R3=1Ω, so the parallel combination, R2||R3, is 0.667Ω.

With only the 7V source "on", R2 is in parallel with R1. Here R2=2Ω and R1=4Ω, so the parallel combination, R1||R2, is 1.333Ω

Also in the diagram the 7volts is on the right of the diagram so does this change which resistor it hits first?

Each wire is a "node" and the devices that connect to it are all at the same voltage (in reality, a wire has resistance, but this can usually be ignored).

It's like with a hydraulic system in an aircraft. If the tubing connecting the pump to two actuators is large enough, then it doesn't matter where the three items are relative to each other. But if the tubing is small, then it might matter. The same with the wire in an electrical system. If it's too small, then the resistance has to be taken into account and that has an effect. In the kind of diagrams you are working with, you assume that the wires are large enough not to matter.

 

http://www.allaboutcircuits.com/vol_1/chpt_10/7.html

In this link in table 2
Can you explain why the resistance has changed to 1.333 and 2.333

As in table 1
They were 0.667 and 4.667

 

The superposition theorem is common to most branches of Engineering and is also important in Physics and Chemistry, so it would be a good idea to get comfortable with its concepts before you use it in detail in any examples.

This is what I am trying to do first. To separate the superposition theorem and the electrical circuit detail and complete the first before the second.
This way explanations about the finer points of electrical circuits will come much more easily.
The reason why the tables are different are fundamental to the Superposition Theorem.
So you can't understand this without first understanding the theorem itself.

 

The reason things change is because we are trying in a step by step fashion to calculate the total resistance in a circuit. We start with three resistances arranged in a particular topology where two of them are in parallel. We transform the parallel pair into a SINGLE equivalent resistance. Now the topology is two resistors in series and we transform that into a SINGLE equivalent resistance by adding the two resistors in series together to get a total equivalent resistance. This total equivalent resistance allows us to solve for the current from the 28V source. Now we work backwards from that result to the individual currents and voltage drops due to the 28V source.

Now we apply the same technique to the 7 volt source after replacing the 28V source with a short. Again we have two resistors in parallel which are in series with a third and the mothod of solution is identical, but you should not be surprised that the numbers are different.

Last we apply the superposition theorem using the tables to get the final result.

 

Ok I've got that worked out now!! Is that what I need to do for the question I put up now?

 

Ok I've got that worked out now!! Is that what I need to do for the question I put up now?

It will be very similar.

Take your best shot at it and show your work. That way we can see where you start going wrong (if you do) and address that specific area.

 

Ok I've started and I'm not sure if I'm on the right track. Can you have a look? Thanks

 

Ok I've started and I'm not sure if I'm on the right track. Can you have a look? Thanks

When you short the left hand supply, that does not place R2 and R3 in parallel.

Remember the definition of parallel components -- they must have the exact same voltage across them.

Identify the nodes in your diagram and label them or color them or somehow make them distinctive. For instance, you have three nodes in your modified circuit, so color them red, green, and blue. If two components are both connected to the red node and the blue node, then they are in parallel. With that in mind, which two resistors ARE in parallel in the modified circuit?

 

When you short the left hand supply, that does not place R2 and R3 in parallel.

Remember the definition of parallel components -- they must have the exact same voltage across them.

Identify the nodes in your diagram and label them or color them or somehow make them distinctive. For instance, you have three nodes in your modified circuit, so color them red, green, and blue. If two components are both connected to the red node and the blue node, then they are in parallel. With that in mind, which two resistors ARE in parallel in the modified circuit?

Is it R1 and R3

 

Is it R1 and R3

Correct.

 

"By George, I think she's got it"

 

How's this?

 

Saying that the current through Rx is Ix doesn't provide a meaningful answer unless you indicate what your reference direction for Ix is. That's like saying that the load in a particular structural member on an airframe is 12kN and not mentioning whether that is in tension or compression. Aside from that your work looks good.

This is particular important with superposition problems because you must make sure that all of the reference directions are the same for each of the various voltages and currents so that you can sum them correctly.

You also need to learn how to properly track your units throughout your work -- and that is every bit as critical in mechanical engineering as in electrical engineering (more so, since there are a lot more compatible units in ME than in EE). Most of the mistakes you will make -- and you will make lots of them, just as we all do -- will mess up your units. If you are tracking your units throughout your work -- and not just tacking on the units that you want the final answer to have at the end -- then you place yourself in a good position to catch most of your mistakes almost immediately. Not after you have spent page after page of work cranking out a bunch of stuff that was guaranteed to be wrong from the second line of the first page. Not after you've been named in a wrongful death suit when the accident investigation finds that you should have caught the error had you only tracked your units.

 

I applied the same process for the same circuit with the 6V BATTERY. Sorry I'm not sure what you mean. Can you have a look at this working out and tell me if I've went wrong anywhere?
Thanks

 

You completely ignored what I said both about units and about the need to define the polarities of your voltages and currents.


So you have the following results:

Currents through R1 are 0.48A and 0.52A.
Currents through R2 are 0.76A and 0.24A.
Currents through R3 are 0.28A and 0.28A.

Superposition says that the total current through each resistor is the sum of the currents due to each separate source.

So what are the total currents in all three resistors?