Question about significant figures

Diddums x · 28 Dec 2017 at 16:34

I understand how these work but one thing I am struggling to get my head around: if asked to calculate something and round it off to 2sf, when do I round? Do I calculate all the equations and then round off, or round off first and then calculate?

Help pls, head maek expload

EDIT: Here's the actual question:

"Resistors R2, R3, and R4 could be replaced by a single resistor. What would be the value of that single resistor? Give your answer to 2 significant figures."

I've got it all worked out but rounding before or after the calcs gives two different answers.

m@rty · 28 Dec 2017 at 16:41

I would assume after the calcs

Firestar_3x · 28 Dec 2017 at 16:41

Never mind.

dowie · 28 Dec 2017 at 16:51

generally a good idea to maintain as much precision as is feasible within calculations and then round the result

Diddums x · 28 Dec 2017 at 17:07

Ja, got it, thanks

Dis86 · 28 Dec 2017 at 17:08

dowie said:
generally a good idea to maintain as much precision as is feasible within calculations and then round the result

That's exactly what I do with mine. It's amazing how far something can get thrown out by over a decent length calc just by rounding early.

Diddums x · 28 Dec 2017 at 17:15

This is the question for those interested:

My answer is R=0.64Ω

Dunno if it's correct or not but as according to my username, this paper is due at midnight and I've still got a bit to do :rolleyes:

Swanster · 28 Dec 2017 at 17:37

I don't think r=0.64 can be right... there's essentially two parallel resistances: 27Ω (R2+R3) and 18Ω (R4). 0.64Ω is far too low....

Diddums x · 28 Dec 2017 at 17:52

You're right, I took R2 and R3 as being on a different line altogether, when they simply need to be added up. What an idiot :rolleyes:

4K8KW10 · 28 Dec 2017 at 17:54

10.8Ω

lude1962 · 28 Dec 2017 at 18:04

rubberduck · 28 Dec 2017 at 19:15

I rate this thread 0.00/10.00

Russinating · 28 Dec 2017 at 19:43

Came thinking thread would be about heads of states.

lunar · 30 Dec 2017 at 20:30

What us going on here?
R2 and R3 in series, 15 + 12 = 27 ohms

And This 27 ohm is in parallel with R4
So, 1/27 + 1/18 = 1/R,
R = 35.18 ohms = 35 ohms (2 sig fig)

You need not consider R1, as it was not asked about.

Kol · 30 Dec 2017 at 20:41

Nope the answer is 10.8ohms. Remember in parallel the total resistance will.always be lower than the lowest parallel resistor.

There's the quick and dirty method too when only dealing with two resistors in parallel:

27*18 / 27+18 = 486/45 = 10.8ohms

The long way would be:

1/27 + 1/18 (lowest common denominator is 54) so 5/54 is 10.8ohms

lunar · 30 Dec 2017 at 21:28

Sorry it is 10.8 ohms.
S0 it is 11 ohms (2 sig fig)

lunar · 30 Dec 2017 at 22:59

AndyT · 31 Dec 2017 at 01:50

lunar · 31 Dec 2017 at 06:17

Resistance questions are generally straightforward, but here is a teaser.

A lamp and three identical resistors form a circuit as shown below. The lamp is shining brightly. A fourth identical resistor is added to the circuit in such a way that the lamp goes off (no current in lamp).
Where should this extra resistor be inserted?