a strange question

[j.mawer]

Think you got your maths wrong. (700/2000) *230 = 80.5v

 

[j.mawer]

I would take a 2.8 (primary to secondary )ratio as 2.8 primary turns per secondary turn as apposed to a 2:8 ratio (maybe that's just me)
so I would have thought it would be 230/2.8 = 82V

However with a 2:8 ratio 8/2 *230 = 980V

 

[IQ Electrical]

Yes, there's some ambiguity in the question, is that exactly how it was written?

Lofty84, are you feeling better on AC theory now?

It just 'clicks' eventually and you see the relationship between many of these calculations.

 

[lofty84]

yep thats exactly how it is written.

I think im getting better IQ but only a bit

can either of you show me how to do this one ?

When a current of 200mA flows into a capacitor for 40mS, the charge developed in the capacitor would be ???

I dont suppose anyone knows the answer for this

 

[j.mawer]

Current = charge /second
So 200e-3=charge /40*10-3
Charge = 200e-2 * 40e-3
=0.08 Coulomb

Correction must concentrate
that should have bee
Charge = 200e-3 * 40e-3
=8mC

 

[lofty84]

Calculate the total resistance (rounded up value) from the following information -
100 m copper cable
1.5 sq mm with a resitivity of 17.5 x 10 ^9 ohms
any ideas ?


think this it might be but I might need to times the 17.5 e-9 by 0.100 but im not sure

17.5 x e-9 x 0.0100 / 1.5 = 1.166667
so 1.17

 

[lofty84]

Current = charge /second
So 200e-3=charge /40*10-3
Charge = 200e-2 * 40e-3
=0.08 Coulomb

charge = 200 x e-3 / 40 x e-3

on my calculator this ='s 5

how do I get to the next part where youve said

Charge = 200e-2 * 40e-3 as I dont understand the e-2 as it says its e -3

when I type 200e-2 * 40e-3 i do get the answer 0.08 but ive been told that it is 8mC

 

[lofty84]

Calculate the resistance of 50m of copper conductor with CSA = 4mm/sq. Take resistivity value as 1.78x10-8 ohms per metre

is it

1.78 x e-8 x 0.050 / 4

 

[j.mawer]

current = charge / time
so rearrange
charge = current * time

Then I typed wrong cos I was talking to someone else and not paying attention then copied from what I typed to the calculator.

charge = 200e-3 * 40e-3
= 0.008
=8mC
sorry!

 

[lofty84]

Determine the resistance of 100m of aluminium conductor. CSA is 120mm sq. Use resistivity of aluminium = 2.84x10-8 ohms m.


so I would do

2.84 x e-8 x 100 000 = 2.3666667

the thing the answers supposedly 0.0237 ohms

anyone know why my answer is not starting with 0.0

 

[j.mawer]

Resistance = (resistivity * length)/CSA
= (2.84e-8 * 100)/ 120e-6
= 0.0237

remember that 120mm^2 = 120e-6m^2 because there are 1,000,000 mm2 in a m2 (1000mm *1000mm)

 

[lofty84]

A resistor of 4 ohms and pure inductor with inductive reactance of 3 ohms are connected in series to 200v AC supply. Calculate the current.


square root of 4sq + 3 sq = 5

200 / 5 = 40 A

 

[lofty84]

this is really frustrateing me as I cant work them out

determine the power in a three phase star wired balanced load circuit when the line current is 15a the phase voltage 230 v and the power factor is 0.8

also doing my head in is

a 15kw three phase delta connected balanced load has a power factor 0f 0.85 determin the line current (i think 400 v has been left out of this question somewhere)

please can anyone help

 

[lofty84]

Thanks to everyone who helped me get a credit, nice one

 

[IQ Electrical]

Well done !