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L

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
 
Have you tried keeping all your units in standard SI units. So Tesla is actually 1 Wb / m^2
So the pole area is 2.5cm = 0.025m x 1.5=0.015m
so Flux density = flux / area
=40e-6 / ( 0.025 * 0.015)
=0.11
 
Thankyou J.mawer thats a good way of doing it my teacher hasnt shown us that before so nice one.

If not converting it though how would I do it ?
 
I have another well, I have loads im sure that im going to get stuck on but heres one for now.

Q. The direction of motion of a current carrying conductor in a magnetic field is indicated by ?

is it flemmings left hand rule
 
The easiest way (in my mind) is to keep everything in the standard base units i.e. m and Wb if you don't what you will be getting is going to be in uWb/cm2 which are a bit confusing!

Now 1 T = 1 Wb/m2.

There are 1e6 uWb in 1 Wb
and 10000cm2 in 1m2
so to get from uWb/cm2 to T you would need to divide your result by (1e6 /10000)=100
so
40 / (2.5 *1.5) = 11 uWb/cm2 = 0.11Wb/m2 = 0.11T

Easier to just work in base units if you can less prone to mistakes!
 
Your very helpfull mate, cheers.

Ive a few more if anyone knows them.

Q. A magnetic flux density at the pole of 2 T and a magnetic flux of 70 x 10-6 Wb. What is the area of the pole.

a. 3.5 mm2
b. 3.5 x 10-5 m2
c. 140 x 10-6 m2
d. 0.28 m2
 
You know from earlier
Flux density B (tesla) = magnetic flux (webers) /CSA
In Question flux density=2 and flux=70e-6
2 = 70e-6 / area

rearrange this to get area
2* area =70e-6
area = 70e-6/2
area =35e-6 m2 (35mm²)
so area =3.5x10-5m²

How much homework have you got :D
 
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Hi J thnaks for your time mate. Ive got about 16 weeks worth of homework as ive got to get my self ready for my 301 which is comeing up fast.

Do you know the question above do I basically type in to the calculator 70xE-6 (E being 10 to the) and then / 2

if the question had said 3T would I have divided by 3 ?
 
Hi J thnaks for your time mate. Ive got about 16 weeks worth of homework as ive got to get my self ready for my 301 which is comeing up fast.

If you’ve neglected your homework for 16 weeks it’s your own fault. You’re not going to learn anything by getting it done here. What are you going to do during the exam? Somehow I don’t think “phone a friend” is an option.

Have a look at CR4 - Thread: Electrical Engineering to see the reaction on other boards
 
Hiya Tony

Although I can see where you are comeing from m8 some people dont actually mind helping people out and probably quite like to teach them what they know in a sense I suppose. 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 can see your point but the web is a tool that I have at my disposal and seeing as my 301 is just around the corner I am going to use it to my advantage as it makes sense. The things I am posting is not homework its just questions I have aqquired from my tutor over the months and I am working through the ones I have no idea how to do - even with the use of my notes and electrical instilation level 3 book - sometimes other people can explain it so that it goes in and stays in mate and this is what im after.
 
Nope 5cm = 0.05m = 5E-2 where E is 10 or exp on most scientific calculators
So its basically E2 = 10^2 = 100
whilst E-2 = 10^(-2) = 1/ (10^2) = 0.01

E6 = 1000000 e.g. MOhm
E0=1
E-1=0.1
E-2=0.01 centi prefix on unit e.g centimeter
E-3 = 0.001 milli prefix on unit e.g millimeter
E-6 = micro as in microfarad
E-9 = nano a
E-12 = pico
etc
 
Last edited:
Have a crack at the questions when you post them with a bit of reasoning, doesn't matter if you get them wrong. I guess most people don't mind correcting / helping if they see you are making an effort, remember they don't know how many questions you have done and not asked about!
Also if you show workings and get it wrong someone can offer better help on what you mistakes are.
Likewise I would hope that people would offer working/reasoning with the answer as then you will actually learn how to do the questions not just the answer!
 

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