Help with My sons test paper

[ICE]

Q5. One hundred resistances each of 200 ohms are connected in parallel. (3)
a. What is their combined resistance?
b. What is the combined resistance if two sets are connected in series, each set consisting of 100 resistances in parallel?

Q6. A circuit is wired as shown in Figure 1.




If R1 = 6 ohms
R2 = 4 ohms
When an electromotive force of 10 volts is applied to the circuit a current of 1 ampere flows through R1.
What is the value of resistance Rx? (5)

 

[sjm]

For n resistances in parallel 1/Rtotal=1/R1+1/R2+....+1Rn.
For N equal Resistances this becomes 1/Rtotal=1/RxN or Rtotal=R/N. In series resistance Rtotal=R1+R2+...+Rn
5a: 200/100=2ohms 5b: 2+2=4ohms


Think this is right
Hope it helps

 

[sjm]

Question 6 using ohms law I=V/R
Voltage across R1 must be 6volts for 1 amp to flow. So current through R2 = 6/4 = 1.5 amps. Total current 2.5 amps for the series circuit.
Voltage across Rx= 4volts
Therefore Rx= 4/2.5 = 1.6 ohms

Long time since I did this type of thing

 

[Merl]

Resistances in parallel:

1
1 + 1 + 1 + ----------- 1
R1 R2 R3 Rn


Resistances in series:

R1 + R2 + R3 ----------- Rn



It took ages to type in and it didn't look like this!!

 

[ICE]

Hiya and thanks for your answer, but do you mean I or 1 where you have put: 1/Rtotal=1/R1+1/R2+....+1Rn.

QUOTE=sjm;162522]For n resistances in parallel 1/Rtotal=1/R1+1/R2+....+1Rn.
For N equal Resistances this becomes 1/Rtotal=1/RxN or Rtotal=R/N. In series resistance Rtotal=R1+R2+...+Rn
5a: 200/100=2ohms 5b: 2+2=4ohms


Think this is right
Hope it helps[/QUOTE]

 

[Sintra]

Resistances in parallel:

1
1 + 1 + 1 + ----------- 1
R1 R2 R3 Rn


a) = 1
100
20000

= 1
0.005

= 200 Ohms.


Resistances in series:

R1 + R2 + R3 ----------- Rn

b) = 200 + 200

= 400 Ohms.


It took ages to type in and it didn't look like this!!

1/R = 1/R x n = 1/Rt

1/200 = 0.005 x 100 = 0.5 1/0.5 = 2 ohms

 

[Merl]

1/R = 1/R x n = 1/Rt

1/200 = 0.005 x 100 = 0.5 1/0.5 = 2 ohms

Cheers Sintra!

But where did I go wrong?

The way I worked it out the total resistance came out at 100/20000 or 1/200 which equaled 0.005

This was then divided into 1 which came out at 200.

 

[Sintra]

Each resistor is 200

so 1/200 = 0.005

There are 100 of these so 100 x 0.005 = 0.5 = Rt

So 1/Rt = 1/0.5 = 2

You went wrong on the part 100/200000 as you only add top line and bottom line stays the same

so it should have been 100/200 = 0.5

 

[Merl]

OK! Got it now.

Thanks Sintra!

Glad somebody was paying attention at college!!

 

[kung]

you find the peripheral eg 1000ohm = 0.001 and 500ohm = 0.002 = 0.003 = 333.33333 ohm etc etc or if in series 1500ohm