Parallel Circuit

[biferi]

I think I understand a little bit of Ohms Law but I want to check.

If I have a 20 Volt Battery and a Mottor that can only take 5 Volts I know I can use one Resister of 10 Ohm and one Resister of 5 Ohm in Series befor the Mpotter.

This will give me a 15 Ohm Resistance and my Motter will get the 5 Volts it needs.

Now if I put the one Resister of 5 Ohm and the one Resister of 10 Ohm in Parallel with the Motter I know the Resistance will not ad up.

And yes I know my Resistance will be very Low.

But if I want to findout what my Resistance is I think I do this?
1 Divided by 5 and 1 Divided by 10 and this is were I get lost.

How do you Divid 1 by 5 and how do you Divid 1 by 10?
Or is my Bad Eyes not seeing it right?

I think I see One Divided by 5 is this i Divided by 5 and is it i Divided by 10?

Thanks you.

 

[MrAl]

Hi,

If you have a motor that takes 5v and a 20v source, that's not enough information to find out what resistor you need in series.
You need to run the motor at 5v and measure the current, then calculate the resistance which is 5/i if 'i' is the current. Then you can calculate the series resistance.
Example:
Motor draws 100ma at required load.
Resistance of the motor is 5/0.1 which equals 50 ohms. So you need a resistance of 150 ohms in series. Now the 150 ohms resistor drops 15 volts at 100ma so the motor only sees 5v.

However, motors draw different current with different loads, so this assumes that the SAME load will ALWAYS be used.
Otherwise, you should use a voltage regulator that can handle the motor current. A voltage regulator will always provide the right voltage to the motor.

 

[biferi]

I am not doing anything I am just making it up.

I never got how you figure out Resistance side by side.

So lets take the Motor out of my figure.

You have a 20Volt Battery and 2 Resisters.
Resister one is 5 Ohm and Resister two is 10 Ohm.

I know if you conect a Metters Leads to the Top and Bottom of each Resister you will see the Voltage is the same.

But the Current will be iferant on each Resister.

And if you want to see the Total Resistance you do some math like this
One over Five Plus One over 10 or some math like this.

This is what I never got can you help?

 

[MrAl]

Hi,

I'd be happy to help here.

We are dealing with what is called a parallel circuit, which i think you already know. In a parallel circuit, the voltage is the same across every element, which i think you already know too. And the current splits, which i see you already know. This is different from a series circuit where the current is the same and the voltage is different across every element.

When we have to resistors in parallel we can calculate the "total resistance" RT by using the formula:
RT=R1*R2/(R1+R2)

or the equivalent formula:
RT=1/(1/R1+1/R2)

The first formula is a little nicer looking, but the second formula generalizes to more than two resistors so it is good to know too:
RT=1/(1/R1+1/R2+1/R3+...+1/Rn)
where Rn is the last resistor.
So what this means is we just have to take the reciprocal of each resistor and add them together, and then when we are done with all the additions we take the reciprocal of the final result and that gives us the total resistance.

So for example, say we have two resistors, R1=3 ohms, R2=6 ohms.
First we take the reciprocal of R1 and we get 1/3=0.33333333 aprpoximately.
Next we take the reciprocal of R2 and we get 1/6=0.16666666 approximately.
Now we add those two results together:
0.33333333+0.16666666
and we get:
0.5
Now last we take the reciprocal of that 0.5 and we get:
1/0.5=2 ohms
So the total resistance is 2 ohms.

If we had three resistors in parallel then we would do the same except then we would have three to add up before taking the last reciprocal.

Another way to state this is that we add up all the conductances, then take the reciprocal.
The conductance is simply the reciprocal of the resistance so for a 2 ohm resistor the conductance is 1/2=0.5 and that's about it.

 

[Mike odom]

you can also take a 1V source, figure the current in each resistor, then add the currents (i1 + i2 = it)... the equivalent resistance would be 1/it.

 

[Dr_Doggy]

resistance in series: CURRENT is the same everywhere, and Voltage is different on each resistor

resistance in parallel: VOLTAGE is the same everywhere, and Current is different on each resistor