Calculating Resistors in series and parallel

[kiwiNCFC]

Im having trouble doing this. Is series basically when resistors are below one another and parallel is when they are next to each other?

Can anyone tell me the formulas for calculating resisitors in s&p (level 2)

Thanks

 

[ianmacd]

Series is where they are next to each other so the electrons pass though one and then the other. Resistance is R1+R2

Parallel is when they overlap each other so the electrons pass though both at the same time. Resistance is (R1*R2)/(R1+R2)

Ian.

 

[kiwiNCFC]

The following resistances are connected together in parallel. 2ohm, 5ohm, 10ohm and 5ohm. What is the total resistance?

10ohm

1ohm

0.22ohm

22ohms

Ok so im trying do this. Im struggling to do this via R1 X R2 / R1+R2

I got 0.226

 

[Theorysparky]

1/RT = 1/R1 + 1/R2 + 1/R3 + 1/R4

1/RT = 1/2 + 1/5 + 1/10 + 1/5

1/RT = 0.5 + 0.2 + 0.1 + 0.2

1/RT = 1

RT = 1/1 = 1 ohm

 

[kiwiNCFC]

Thanks!! Its so easy!

 

[kiwiNCFC]

Got stuck again, This time on an example in a book.

For example....A parallel connection

1/R1 + 1/R2 + 1/R3

1/6ohms + 1/6ohms + 1/6ohms

Then it says Total Resistance = 1+1+1 / 6 ohms = 3/6ohms

Where do they get the 1+1+1 from this is really bugging me! Its on page 106 on basic electrical installation work by trevor linsley

 

[Theorysparky]

the common denominator is 6

1 sixth + 1 sixth + 1 sixth is 3 sixths 3/6

as the lower number is the same you can just add the top numbers

http://www.mathleague.com/help/fractions/fractions.htm#addingandsubtractingfractions

 

[kiwiNCFC]

Ok i now get it when the denominator is the same but when it is different i am getting confused.

Another example in my book says the following:

1/2ohms + 1/3ohms + 1/6ohms

3+2+1 / 6 ohms = 6/6ohms

Rp = 6 ohms/6 = 1 ohms

The bit in bold is where i am stuck. I cant for the life of me see where that has come from?!

 

[kme]

Right. If the denominators are different:

1/2 +1/3 +1/6 ohm

is the same as:

3/6+2/6+1/6 ohm

i.e. you need to make all the denominators the same, by multiplication. Whatever you have to multiply a particular denominator by you mulyiply its numerator by the same amount....

as above:

1/2 (half) of something, is exactly the same amount as 3/6 (three sixths).

The easiest way to get a common denominator is multiply them all together. It won`t give you the lowest common denominator, but it will work:

if your example was:

1/7 + 1/12 + 1/3 ohms

the 1/3 would go straight into the 1/12 (i.e. 4/12), but the seven wouldn`t.

if you do 7 X 12, you get 84. So:

12/84 + 7/84 + 28/84

would give 12+7+28 / 84

=47/84 (Rt)

=0.559 (Rt)

1/0.559 (1/Rt)

Gives 1.787 ohms.

HTH

KME

 

[kiwiNCFC]

Thanks, it clears it up a little bit. Will keep practising it