Simple Conceptual Circuit Question

Question

I have been looking back at circuit problems and I couldnt understand something.

Imagine a simple circuit with 10V battery and 5 ohms resistor. The current would be 2.0 A in this case. If 20V battery was used with the same setup, the current would double to 4.0A since the resistance doesnt change for a resistor. If this is correct, voltage drop across the resistor would have to be 10V and 20V respectably.

1. Do all the voltage always drop and occurs across a resistor in a simple series circuit? (Does the resistor always take in every potential energy when current goes through a resistor? if so what happens in a series circuit with 2 resistors?)

2. Why is it that all the voltage drops (10V and 20V) when resistor's resistance is the same?

3. Do resistors resist only against current not voltage?

Answer to these desperate questions would be very appreciated

Thanks

Answer 1

For a series circuit the potential drops must add up to the supply voltage.  For a given supply voltage, the current will change depending on the total resistance of the circuit.

Usually the very first step in series circuit analysis is adding the resistance values together to get Rt.  For a circuit with two resistors:

Rt = R1 + R2

Then the current is found from Ohms Law:

i = V/Rt

Where V is the supply voltage.  The voltage drops across the resistors can be added together and must equal supply voltage:

V1 + V2 = V

Each voltage drop is found using Ohm's Law:

V1 = i(R1)

V2 = i(R2)

Resistors "resist" against current.  If you apply a nonzero potential (voltage) across a finite resistance there will always be some current.  The amout of current depends on the value of V and R.

Answer 2

Kevin, i'm not sure i know what you mean.

1: it does but it's defined that way. your battery tells you the voltage. regardless if it's connected to resistor, wire, or cow manure. the resistor doesn't "do" anything, it just slows the current. you can connect a wire to your battery, in which case, you sort of have a short circuit. then the battery will push an infinite current. but in real life, this doesn't really happen. the wire has resistence, and the battery can only produce a finite amount of current and has internal resistence

2 resistors in series is like one bigger resistor (add them up), it slows the current even more.

2: see 1. it doesn't do anything. resistors don't change voltage. it's the battery's chemistry that determines it. image a fish tank, the higher the fish tank, the higher the voltage. imaging a siphon hose, the bigger the hose, the smaller the resistance. imagine you siphoning the fish tank water and it's spilling on your carpet, all that water is current.

so, if your fish tank is 10 feet off the ground, when you siphon, with a given hose, it'll flow at a certian rate. the drop is still 10 feet. if you have your tank at 20 feet, you'll get much more flow using the same hose but the drop *has* to be 20 feet, cause, that's where your tank is at.

fish tank's height, voltage

siphon hose, resistor

water spilling on your new carpet, current

size of your fish tank: capacity of the batter, you can have a goldfish bowl at 12 feet, or you can have a 100 gallon tank at 12 feet. the differnce is like a 12V camera battery versus a 12 V car batter.

3: huh? =)

Answer 3

In these idealized circuits, regard the battery as a "black box" which always maintains its specified voltage across its two terminals. Then you just use Ohm's Law.  I = E/R .  Answer 1.: The total voltage across 2 resistors in series is the sum of the voltages across each individual resistor. Ans. 2: Rearrange Ohm's Law: E=IR hence the voltage drop is proprtional to the current while the resistance stays constant. Ans. 3 "Resistance" is just the constant of proportionality between "voltage across" and "current through" a resistor.

Answer 4

Visualise ANY circuit as follows.

Positive side of voltage source X is X feet above sea level. Negative side is zero feet. Start at the +ve side. Any connecting wire is a level path, any component (resistor) is a step down. Therefore in going from +X volts at the +ve source terminal to 0 volts at the -ve source terminal you have to drop (go down) X volts. If there is only 1 resistor the full X volts is across it. If there is more than one then each will give a particular voltage drop (V=IR) where I is the series current.

If you extend this concept to a parallel circuit then it becomes obvious (given no series resistor) that the full source voltage must appear across each parallel branch. Simply treat them as separate paths from X feet high to zero feet.  

Answer 5

Imagine a simple circuit with 10V battery and 5 ohms resistor. The current would be 2.0 A in this case. If 20V battery was used with the same setup, the current would double to 4.0A since the resistance doesnt change for a resistor. If this is correct, voltage drop across the resistor would have to be 10V and 20V respectably.

1. Do all the voltage always drop and occurs across a resistor in a simple series circuit?

Essentially, yes. That is the only thing across the battery.

Does the resistor always take in every potential energy when current goes through a resistor?

"take in every potential energy" meaningless statement

if so what happens in a series circuit with 2 resistors?

some of the voltage is across one resistor, some across the other, and they add up to the battery voltage.

2. Why is it that all the voltage drops (10V and 20V) when resistor's resistance is the same?

Meaningless question, sorry

3. Do resistors resist only against current not voltage?

Again, meaningless. You have to use the correct terminology.

Just learn ohm's law, it's all there.