a strange question

[lofty84]

When the emf in a conductor 0.15m long moving at right angles through a magnetic field, at a velocity of 2 m/s is 0.5 V, the magnetic flux density would be


Thanks to the guys in the general electricains forum I know know that the formula is

B = e / l x v

so 0.5 / 0.15 x 2

= 1.66 T

the problem is that I have been told the the answer is 3.17 T

is there something else that I need to do to the original formula ?

 

[IQ Electrical]

E=BlV

0.5=BX0.15X2

Transpose B= 0.5/0.15X2

= 1.66 T

 

[lofty84]

thankyou IQ

could I trouble you with this one

24. If a three phase, four pole, 50Hz induction motor has a slip of 4%, the actual speed of the motor would be
a. 1240 rpm
b. 1550 rpm
c. 1440 rpm --> CORRECT ANSWER
d. 1000 rpm

I cant seem to work it out tho

 

[IQ Electrical]

f=n X p

50=n X 2

n=50/2

n= 25 r/second

= 1500 rpm

4% slip = 1500X.96

=1440 rpm

 

[IQ Electrical]

Remember the poles are calculated in PAIRS hence the use of 2 and not 4 in the equation.

 

[lofty84]

thankyou IQ

could I trouble you with this one

24. If a three phase, four pole, 50Hz induction motor has a slip of 4%, the actual speed of the motor would be
a. 1240 rpm
b. 1550 rpm
c. 1440 rpm --> CORRECT ANSWER
d. 1000 rpm

I cant seem to work it out tho

think i might have worked it out

ok

50/ 2 = 25 rps

25 rps x 60 = 1500

1500 / 100 = 15

15 x 96 = 1440 prm

 

[lofty84]

thankyou very much IQ

 

[IQ Electrical]

No problem, don't forget the 'thanks' button though, it shows members your appreciation

Any others, just post them up, it's good practice for all of us!

 

[lofty84]

one more ?

a circuit with a capacitor of capacitive reactance 5 ohms an induction of 8 ohms inductive reactance and a resistor of 4 ohms conected in series to a 230v 50hz supply what would the total impedance be
a 5 ohms
b 10 ohms
c 3 ohms
d 6 hms

is it 2 x pie x f x l

 

[lofty84]

a capacitor of 25 (funny U) F is connected i series with a resistor in circuit with with a 50hz 200v a.c supply. determine the capacitive resistance.

I know the formula is XC = 1 / 2 x pie x f x l

but i cant get the answer which is 127 ohms

can anyone help

 

[SirKit Breaker]

Wow I am impressed........... A bit ( a big bit) out of my league though

Cheers..........Howard

 

[IQ Electrical]

one more ?

a circuit with a capacitor of capacitive reactance 5 ohms an induction of 8 ohms inductive reactance and a resistor of 4 ohms conected in series to a 230v 50hz supply what would the total impedance be
a 5 ohms
b 10 ohms
c 3 ohms
d 6 hms

is it 2 x pie x f x l

Z = square root of R^2 + X^2

X = Xl - Xc

X = 3

Z = square root of 16 + 9

Z = 5 Ohms

 

[IQ Electrical]

Where there is capacitive reactance and inductive reactance, the value of X is always the larger value minus the smaller which in this case is 8-5 = 3

 

[reedsinthewater]

1/2x3.142x50x25x10 to the power of minus six. The 'funny u' represents micro farads, hence to the power of minus six.

 

[reedsinthewater]

1/2x3.142x50x25x10 to the power of minus 6, the 'funny u' represents micro farads, hence to the power of minus six.

 

[lofty84]

an a.c load has an output power of 12 kw and an input of 15 kva. determine the reactive power

can anyone work this out it looks simple but i cant remember what to do again

 

[IQ Electrical]

KVAr^2 = KVA^2 - KW^2

Or use KVAr = sqare root of KVA^2 - KW^2

KVAr = sqare root of 225 - 144

= 9 KVAr

 

[lofty84]

you know your stuff mate, nice one

 

[IQ Electrical]

No problem, it keeps my AC theory fresh too!

 

[lofty84]

Heres a weird one

what are the copper losses in the primary winding of a step up tansformer, where the resistance of the winding is 0.3 ohm and the transformer is drawing 8 amps from the supply ?

any Ideas IQ ?