Shock Value B

[ichaelm]

I have a question. How do you find total resistance and total current in a bridge circuit or similar circuit?

There are many ways to do it. My preferred method is to rearrange one side using a delta-wye transform, solve for all the unknowns, and then transform it back. Just make sure you're given enough information first. The delta-wye formulas are here.

I don't fully understand the explanation. Please may you explain it a little bit more? In addition, do the formulas work for finding the total current?

Basically, if you take one side of a bridge circuit, you have three resistors that are all connected in a circle - that is, a circuit path goes through R1, R2, and R3 and ends up back at R1 again. That is called a delta formation, because it is like a triangle, or the greek letter delta. You can use the delta-Y formulas to convert a delta formation into a Y formation, which is usually easier to solve. You use the formulas to find a equivalent circuit where instead of having the resistors connected in a triangle, you have resistors of different value connected in a Y formation. The formulas are designed so that the circuit will still work exactly the same way with either formation, with respect to the three nodes involved. Once you convert to a Y, you can solve the circuit and find the total resistance, and given an applied voltage you can find the total current. This will give you a solution for how the whole bridge circuit works with respect to its two ends: the total resistance. If you need to find more specific information about nodes within the bridge circuit, such as the individual voltages across each resistor, then you can convert back to a delta and use the information you just got for the whole circuit to help determine the individual voltages.

There are other ways to solve the circuit, like by using Kepler's voltage law, but I think this is the most systematic for a question on a test. I found an example for you on this webpage

[space scientist]

Thank you ichaelm. The information that you posted is useful.

[andrewwski]

Kepler's voltage law? I think you mean Kirchoff's?

Depending on the circuit you may or may not be able to apply KVL to find what you're looking for.

If not, yeah, apply a delta-wye transform. Then, in the basic bridge circuit, you end up with a simple series-parallel combination you can solve.

[sean9keenan]

Regarding the delta-wye transformation, is that technically covered in the rules? I would consider it covered under general circuit knowledge... Perhaps for a more "difficult" section to differentiate teams. On the same note, do you think KVL and KCL should be considered part of the tests?

Any thoughts from competitors or anyone else?

[Slothface]

Hey I'm a little bit confused about the whole 'internal resistance of a battery' concept. Can anybody help me out?

[Frogger4907]

Anyone from the NE KS regional have any opinions about the Shock Value event that was held there? thanks

[Schrodingerscat]

Hey I'm a little bit confused about the whole 'internal resistance of a battery' concept. Can anybody help me out?

The way I understand it is it is effectively the limit to the current a battery can produce. Such as if a battery was 10V and had 1 Ohm of internal resistance, you could short circuit it and only produce a 10 ohm current, where it would be extremely high if the battery did not have internal resistance, and I think it also adds the resistance to all circuits.

[Slothface]

Hey I'm a little bit confused about the whole 'internal resistance of a battery' concept. Can anybody help me out?

The way I understand it is it is effectively the limit to the current a battery can produce. Such as if a battery was 10V and had 1 Ohm of internal resistance, you could short circuit it and only produce a 10 ohm current, where it would be extremely high if the battery did not have internal resistance, and I think it also adds the resistance to all circuits.

Thanks a bajazillion!!!!!M

[space scientist]

Hey I'm a little bit confused about the whole 'internal resistance of a battery' concept. Can anybody help me out?

From what I know, every object (except for superconductors) has resistance and this includes batteries. Therefore, the internal resistance of a battery is the resistance of the materials that the batteries are made of, and this resistance can be altered by the placement of the cells in the battery. For more information, I would recommend going to http://www.allaboutcircuits.com

[sean9keenan]

On this topic of batteries and resistance: A battery can be modeled (rather well) as a voltage source, in series with a small resistor. Because of this when you add batteries(or cells) in series you increase this resistance and when you add them in parallel you decrease this resistance. This general idea is how batteries in your car while only at 12V can supply very large amounts of current (Does anyone know how car batteries start your car?), and why shorting a car battery is a much worse idea then shorting a AAA battery which has a much higher resistance.

As a note, almost all components have some sort of resistance in series, and in parallel with the ideal component (except ie.superconductors). So a better model for a battery would have some resistance in parallel with the battery, but this resistance is very large which means it does not play much of a role in many circuits, it does play some role in draining your battery over time though! Another thing to point out is that the resistance of batteries changes dramatically as they discharge, which is why you can't draw many amps from a battery that is almost dead.

Another interesting problem you might want to try is this: If you have a battery that supplies a voltage of V and has a internal resistance of r, what value resistor R do you hook up in series with the battery to get the maximum power dissipation across the resistor R