how to work out total resistance of a parallel circuit

[georgeo5664]

hi i am new to learning about electrics i have just started an electricians course and have a question. The question is : Two heating elements of 70 ohms & 30 ohms are connected in parallel the total resistance of this circuit would be......

i have tryed all differnt web sites i have worked it out using (proof sum) r1 x r2 210
(r1 + r2) = 100 = 2.1ohms

but the formula that i was given to work with was as follows example:

1 = 1 1
- + - -
Rt R1 R2

sorry if this is not the best explained description but i am new and trying

 

[morph]

i make it 21 ohms

 

[georgeo5664]

if possiable could you please explain how you got to that answer many thanks

 

[i=p/u]

i thought 70-40 = 30

 

[georgeo5664]

sorry still dont understand

 

[Guest123]

Taking only the pure resistance into account, I get 25ohms......

1/70 + 1/30 = 0.04

1/0.04 = 25.

 

[georgeo5664]

Taking only the pure resistance into account, I get 25ohms......

1/70 + 1/30 = 0.04

1/0.04 = 25.

what does the 1/ mean in front of the 70 & 30

 

[morph]

depends on how many decimal points you go to
1/ 0,0476 = 21

 

[Guest123]

depends on how many decimal points you go to
1/ 0,0476 = 21

Sorry, yes, I only went to 2dp.:glasses2:

 

[i=p/u]

one over... i remebr now its in my year 2 stuff in a bok about 4 meters west and 8 meters south

 

[georgeo5664]

i feel so stupid i am sorry i just dont understand the working out of this parallel circuit

 

[Des 56]

Take the fun out of doing the calcs
PARALLEL RESISTOR CALCULATOR

 

[i=p/u]

thats quick des

 

[Guest123]

[h=3]Parallel circuits[/h]A parallel circuit is a circuit in which the resistors are arranged with their heads connected together, and their tails connected together. The current in a parallel circuit breaks up, with some flowing along each parallel branch and re-combining when the branches meet again. The voltage across each resistor in parallel is the same.
The total resistance of a set of resistors in parallel is found by adding up the reciprocals of the resistance values, and then taking the reciprocal of the total:
equivalent resistance of resistors in parallel: 1 / R = 1 / R[SUB]1[/SUB] + 1 / R[SUB]2[/SUB] + 1 / R[SUB]3[/SUB] +...

A parallel circuit is shown in the diagram above. In this case the current supplied by the battery splits up, and the amount going through each resistor depends on the resistance. If the values of the three resistors are:

 

[georgeo5664]

right all i have to do is make sure i fully understand the workin out of the formula 1/70 am i right in saying its a fraction