Help on an exam question

[westos-electric]

Hi

I need help working this cal. Please can you confirm the answer and how it's worked out?

Question

A conductor 500mm long is placed in at right angles to a magnetic field with a magentic flux density of 0.25 tesla. What is the current required in the conductor to create a force of 15N on the conductor.

Hope you can help
Wayne

 

[pushrod]

Hi

I need help working this cal. Please can you confirm the answer and how it's worked out?

Question

A conductor 500mm long is placed in at right angles to a magnetic field with a magentic flux density of 0.25 tesla. What is the current required in the conductor to create a force of 15N on the conductor.

Hope you can help
Wayne

F = BIL

rearrange to I= F ÷ (B x L)



Edit : remember length must be in metres

 

[chris_b_52]

We are doing your homework here aren't we......

 

[westos-electric]

No just trying to get some extra help, I've the text books and done the revision but still finding it hard.

You arn't doing my homework, this was my last call.

 

[pianoman]

if its any consolation I seem to remember there was a similar question at the end of one of the chapters in my b & b course notes for the 202, but when i looked back over the chapter they had not actually covered that particular equation. however i eventually found it in Scaddan's electrical installation work.

 

[ezzzekiel]

write a list of all the formulas you have learned
carry it around with you
read it often
think of ways to help you remember them:
for example the Force formula would be the police force is the old bil so F=BIL
then learn transposition with that you can easily rearrange most formulas to find what you need.

 

[westos-electric]

Hi

Is anyone able to check my answer on this?

A 12v battery is connected across a set of resistors in series, values being 60, 30, 100, 45ohm the current in the circuit is?

60+30+100+45 = 235 12/235 = 0.053x1000 = 52mA

is this correct?

also one more thing Power factor is at unity when?

Cheers
Wayne

 

[pushrod]

Hi

Is anyone able to check my answer on this?

A 12v battery is connected across a set of resistors in series, values being 60, 30, 100, 45ohm the current in the circuit is?

60+30+100+45 = 235 12/235 = 0.053x1000 = 52mA

is this correct?

also one more thing Power factor is at unity when?

Cheers
Wayne

51mA

PF is at unity (1) when voltage and current are exactly in phase. This occurs in purely resistive circuits or when an inductance exactly counterbalances a capacitance in the same circuit.

edit : 12/235 = 0.05106

 

[westos-electric]

Hi

wondered if anyone could point me in the right direction re-

A heater supplied from a 230volt/50hz star connected system with a load current of 19.5A. Calculate the power used

&

a 3kw is supplied for a 230 volt/50hz system with a 0.7 cos calculate the supply current.

 

[pushrod]

Hi

wondered if anyone could point me in the right direction re-

A heater supplied from a 230volt/50hz star connected system with a load current of 19.5A. Calculate the power used Single phase? just use P = I x V

&

a 3kw is supplied for a 230 volt/50hz system with a 0.7 cos calculate the supply current.

Single phase
power = I x V x PF

I = power / (V x PF)

 

[westos-electric]

Question
A capacitor of capacitive reactance 5ohm an inductor of 8ohm inductive reactance and resistor of 4ohm are connected in series to a 230v 50hz supply. The total impedance of the circuit will be?

5ohm

This is a question on a sample paper but i can't seem to work out how to do the working out

Can you help?

Cheers
Wayne

 

[pushrod]

Question
A capacitor of capacitive reactance 5ohm an inductor of 8ohm inductive reactance and resistor of 4ohm are connected in series to a 230v 50hz supply. The total impedance of the circuit will be?

5ohm

This is a question on a sample paper but i can't seem to work out how to do the working out

Can you help?

Cheers
Wayne

the capacitor and the inductance are producing opposite effects to each so you just want the difference between them.

look at your impedance triangle and you have sides of 4Ω (resistance) and 3Ω (reactance 8-5) you just need to find the hypotenuse

 

[westos-electric]

Hi

I've tried working this question out but getting something wrong.

Question

A 30kW 3 phase delta connected motor has a power factor of 0.86 lagging. calculate the line current

I thought it would be

30,000 / (400x0.86) but doesn't match the correct answer which is 50.35A

Could you confirm where i'm going wrong?

Thanks
Wayne

 

[Guest123]

You need also to divide by root 3 which is 1.732.

So....30,000 / (400 x 1.732 x 0.86) = 50.35A.

 

[westos-electric]

cheers knew i'd missed something!

 

[dichroic]

You may find this electrical maths pack useful.

Regards

Dichroic

 

[westos-electric]

Hi ive still had issues with this question

A small heating circuit draws 1.5kw of power and has an internal resistance of 0.5ohms what is the current drawn?

 

[Guest123]

This is an ohms law question........


Current (I) = square root of, power (P) / resistance (R)

= Sqrt of 1500 / 0.5 = 54.77A.

 

[westos-electric]

I've been reading about transformers and need help on this question

A transformer has an output of 12kva and a power factor of 0.9. the total losses in the transformer are 600w. What is the efficiency of the transformer

now i know / think the formula is output/ output power + losses

I'm not sure how this works out, can anyone help

cheers

 

[dichroic]

I've been reading about transformers and need help on this question

A transformer has an output of 12kva and a power factor of 0.9. the total losses in the transformer are 600w. What is the efficiency of the transformer

now i know / think the formula is output/ output power + losses

I'm not sure how this works out, can anyone help

cheers

The formula you require is:

input power = output power + losses

However you need to use:

Efficiency = output power / (output power + losses)

so: Output power = KVA x Power factor

12kva x 0.9 = 10.8kw

Efficiency = 10800 / (10800+600) = 0.947

0.947 x 100 = 94.7% efficient.

Regards

Dichroic