Questions

[lofty84]

Hiya hopefully this is ok to ask in here.

If anyone can show me how to work out this question it would be much appreciated.

Q. A magnet has a flux of 40uWB (10-6 in otherwords but I cant do the exact symbol on here). What is the flux density if the pole area is 2.5 x 1.5 cm.

a. 0.11T
b. 0.000011T
c. 11x10-6T
d 11T

I know that the formula is

Flux density B (tesla) = magnetic flux (webers)
---------------------------
CSA

but I cant seem to get any of those answers so im obviously doing something wrong.

Thank you

 

[j.mawer]

Lost the context of the question when I answered the last one!
so 5cm = 5e-2 m
but 5cm² is 5e-4 m²
So there are 100cm in 1m but if you imagine a 1m square it will be 100cm wide and 100cm high i.e. there will be 100x100=10000=1e4

 

[Knobhead]

You are obviously not one of these people so Im glad im not an apprentice working with you as I would imagine you would not pass on to much knowhow and would be a bit of a grumpy bugger to work with.

I could be a grumpy bugger, but the apprentices I’ve had have all come through their time unscathed and qualified. I’ve always been in heavy plant maintenance, to help a lad to learn “fault finding” I’ve deliberately sabotaged a plant so he had the chance to do it in “real time”. I even arranged for the plant manager to bend his ear to get the plant going. (How the PM and I kept our faces straight I’ll never know, the plant wasn’t due to start for a couple of hours). It gave real experience. My last apprentice failed his HNC finals over a question I felt to be totally unfair, it was about DC motor starters that were out of date when I was an apprentice. I’d be surprised if many on this board has worked on “face plate” starters. I arranged for him to go to another company to see one still in use. He passed on the retake.

I’ll help anyone if they deserve it.

 

[lofty84]

Cheers J.Mawer, I think your suggestion about haveing a go is spot on and its what im going to do from now on. Thanks again for the help

 

[lofty84]

If the current in a coil of inductance of 0.15H rises from 0 to 8 A in 0.02 seconds, calculate the induced e.m.f

a. 120v
b. -0.024v
c. 24v
d. -60v

I would do 0.15 x 8 / 0.02 which gives me 60v but the option is for d. -60v

why dont I get -60v

 

[lofty84]

If the current in a coil of inductance of 0.15H rises from 0 to 8 A in 0.02 seconds, calculate the induced e.m.f

a. 120v
b. -0.024v
c. 24v
d. -60v

I would do 0.15 x 8 / 0.02 which gives me 60v but the option is for d. -60v

why dont I get -60v

I think I should have done

-0.15 x 8 / 0.02

nice one mr faraday

 

[lofty84]

A coil of 20 turns is placed in a magnetic field of 6 x E-4 webbers. what is the emf induced if the flux is reduced to 1 x E-4 webers in 0.2 seconds

a. -5v
b. -0.05v
c. -0.1v
d.-0.005v

is it d

-20 x 6xE-4 / 0.2 = - 0.06v

-20 x 1xE-4 / 0.2 = - 0.01v

(-0.06 ) - ( -0.01) = -0.05

 

[lofty84]

I could be a grumpy bugger, but the apprentices I’ve had have all come through their time unscathed and qualified. I’ve always been in heavy plant maintenance, to help a lad to learn “fault finding” I’ve deliberately sabotaged a plant so he had the chance to do it in “real time”. I even arranged for the plant manager to bend his ear to get the plant going. (How the PM and I kept our faces straight I’ll never know, the plant wasn’t due to start for a couple of hours). It gave real experience. My last apprentice failed his HNC finals over a question I felt to be totally unfair, it was about DC motor starters that were out of date when I was an apprentice. I’d be surprised if many on this board has worked on “face plate” starters. I arranged for him to go to another company to see one still in use. He passed on the retake.

I’ll help anyone if they deserve it.

Fair point Tony

 

[j.mawer]

looks correct to me although not quite the right working
V=-N * δFlux/δtime
=-20 * (6e-4 - 1e-4)/0.2
=-20* 5e-4 / 0.2
=-0.05

 

[lofty84]

Ive seriously got no idea how to work this out so if anyone can show me i sure would be greatfull.

A 15 kw 400v balanced three phase delta connected load has a p.f of 0.65 lagging

calculate the line and phase currents

I just dont no where to start or what to do here

 

[malcolmsanford]

I could be a grumpy bugger, but the apprentices I’ve had have all come through their time unscathed and qualified. I’ve always been in heavy plant maintenance, to help a lad to learn “fault finding” I’ve deliberately sabotaged a plant so he had the chance to do it in “real time”. I even arranged for the plant manager to bend his ear to get the plant going. (How the PM and I kept our faces straight I’ll never know, the plant wasn’t due to start for a couple of hours). It gave real experience. My last apprentice failed his HNC finals over a question I felt to be totally unfair, it was about DC motor starters that were out of date when I was an apprentice. I’d be surprised if many on this board has worked on “face plate” starters. I arranged for him to go to another company to see one still in use. He passed on the retake.

I’ll help anyone if they deserve it.

Worked on quite a few down pit, but in all honesty not got a clue, know what they are but that is about it. I left them to the tatties underground.

 

[lofty84]

Ive seriously got no idea how to work this out so if anyone can show me i sure would be greatfull.

A 15 kw 400v balanced three phase delta connected load has a p.f of 0.65 lagging

calculate the line and phase currents

I just dont no where to start or what to do here

Can anyone show me how I can work this out