Shock Value B

[blue cobra]

What would the voltage be in a circuit if you had a 9v battery and a 1.5v battery wired in parallel?

[andrewwski]

BAD IDEA.

The 9V is going to be feeding current into the 1.5 V until their voltages equalize. But the 1.5 V battery is not designed to have that much of a charge, so it will likely get very warm, or possibly catch on fire. It would be very dangerous.

The total voltage would be somewhere in between 1.5 V and 9 V, but it would depend on the capacity of the batteries themselves. Remember a 9 V is actually 6 small 1.5 V cells in series.

But don't ever connect them that way. It would be very dangerous.

[Hamtown009]

How do you calculate voltage drop? I've looked at some things but I can't seem to get the grasp on it.

[blue cobra]

Voltage drop was a difficult concept for me to grasp as well. But as I understand, it is as follows:

I like to think of calculating voltage drop as applying ohm's law to one resistor. Ohm's law tells us that voltage=current x resistance. So suppose you have a resistor circuit with a 9v battery and two 100 ohm resistors. Use ohm's law to find the current in the whole circuit.
I=E/R
I=9/200
I=0.045 amps
Current is the same everywhere in the circuit, but voltage is not (which is why we can have voltage drops). So now that we know the voltage before the resistor, the current at the resistor, and the resistance of the resistor, we can calculate voltage drop, using Ohm's law.
E=IR
E=0.045 x 100
E=4.5v (That's the voltage drop!)
Since both of our resistors are the same- 100 ohms- we'll find their voltage drops are the same.
E=IR
E=0.045 x 100
E=4.5v
Something else to notice is that all of the voltage drops in a circuit add up to the voltage source. 4.5+4.5=9.

Some of the more knowledgeable people on here may be more helpful than me, but if you haven't already, check out Episode V of the Wiki. It's about voltage drops, and it links to another sight about it as well.

[Hamtown009]

Ohhhhh. ok so you take your total resistance and use ohm's law to figure out the current. then you use the current and multiply it by one of the resistors to get the voltage drop?

[Hamtown009]

Wait a sec... could you have more than one voltage drop or not?

[blue cobra]

Ohhhhh. ok so you take your total resistance and use ohm's law to figure out the current. then you use the current and multiply it by one of the resistors to get the voltage drop?

Yup, that sounds correct.

Wait a sec... could you have more than one voltage drop or not?

Yes, there is a voltage drop across everything giving resistance. If you have two resistors, there is a voltage drop across one as well as a voltage drop across the other. Remember that resistors aren't the only thing with resistance- if they tell you the resistance of, say, a light bulb, you could find the voltage drop across that too.

[Hamtown009]

ok i get it, but what if you have a resistance of let's say 55 on ONE of the known resistances and the voltage is 24. How would you figure out the current to figure out the current to figure out the voltage drop??? (answers: current is .289 A and the voltage drop is 15.9)

Resistor Resistance in ohms
R1 55
R2 18
R3 10
R4 75
R5 100
R6 15
R7 22

The total voltage as I said is 24 and I need to figure out what the current and voltage drop are.

I'm just trying to figure out what I did wrong on my practice test.

[andrewwski]

Can't tell you without seeing a schematic, you have to know which resistors are hooked up in series and parallel.

Voltage drops are fairly easy to determine. You need to know Ohm's Laws, Kirchoff's Voltage and Current Laws, and how to calculate resistance in series and parallel.

Ohm's Law states that E=I*R or Voltage = Current * Resistance. You can use this anywhere - for an entire circuit or just one component. Kirchoff's Voltage Law states that the sum of all voltages in series equals the total voltage. Thus, all components wired in parallel have the same voltage, but components wired in series have different voltage drops. Kirchoff's Current Law states that the total current draw of a circuit is equal to the sum of the individual current draws in parallel. Thus, all components wired in series have the same current running through them, but components wired in parallel have different current draws.

Total resistance in series is obtained by adding the resistances together and total resistance in parallel is obtained by using the formula 1/(1/R1+1/R2+...+1/RN).

Applying all of these will allow you to calculate the voltage, current, and resistance of each resistor of a series-parallel circuit.

[Hamtown009]

How do you find theoretical current in a circuit?