Help with questions please

[jonnymc11]

Q1. The current in a coil changes from 5 amperes to 2 amperes in 50mS and induces a voltage of 30v into the coil. Calculate the value of the inductance of the coil?

Q2. Calculate the power dissipated by resistor 2 in the circuit here

(R1 = 5ohms, R2= 10ohms, supply voltage = 20v)

Q3. Calculate the synchronous speed of the rotating magnetic field in a 4-pole synchronous motor?

Q4. An inductive load with 10 ohms inductive reactance is connected to a 100v 50 Hz AC supply. Calculate the power dissipated in the circuit (ignoring any conductor resistance.)

Q5. Calculate the inductive reactance of a coil with inductance of 0.1H, connected to a 50Hz AC supply.

Thanks for any help.

 

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Q1. The current in a coil changes from 5 amperes to 2 amperes in 50mS and induces a voltage of 30v into the coil. Calculate the value of the inductance of the coil?Inductance = induced voltage x time / change in current

L = 30 x 0.05 / 3

L = 0.5 H (or 500 mH)

Q2. Calculate the power dissipated by resistor 2 in the circuit here

(R1 = 5ohms, R2= 10ohms, supply voltage = 20v)

Depends...

If in series then add resistors together ( giving Rt) and divide answer into voltage, this gives your current = 1.333 A

P = I x I x R

So P = 1.333 x 1.333 x 10 = 17.76 W

If in parallel then divide voltage by R2 = 20 / 10 = 2 Amps

Then multiply this by the voltage = 40 W

Q3. Calculate the synchronous speed of the rotating magnetic field in a 4-pole synchronous motor?

Motors aren't my strong point but I think we need the frequency for this calc.

If we assume 50 Hz then:

Synchronous speed = frequency / pole pairs

Synchronous speed = 50 / 2 = 25 revs per second

Q4. An inductive load with 10 ohms inductive reactance is connected to a 100v 50 Hz AC supply. Calculate the power dissipated in the circuit (ignoring any conductor resistance.)

This could be a trick question - with a purely inductive load there won't be any power dissipated - so answer 0 Watts

Q5. Calculate the inductive reactance of a coil with inductance of 0.1H, connected to a 50Hz AC supply.

Inductive Reactance = 2 x pi x frequency x inductance

Inductive Reactance = 2 x 3.142 x 50 x 0.1 = 31.42 ohms

Thanks for any help.

I think they're right:good luck:

 

[jonnymc11]

Thats very useful mate, thank you very much, appreciated!

yeah the resistor one was in series.

 

[jonnymc11]

A circuit connects a 28.8 ohm resistor and a capacitor with capacitive reactance of 38.4 ohms. Calculate the total impedance of the circuit

38 ohms

24 ohms

48 ohms

44 ohms

(Z=?)

 

[Andy™]

Z= √ R

 

[jonnymc11]

thats what i tried, but the answe doesnt match dto the 4 given on swindon massive, which is in my post.

 

[Andy™]

what answer do you get?

i get 47.76

 

[jonnymc11]

got 48ohms now

its how you put it down on the calc - is the problem

had to do: Z = √( 28.4

 

[Andy™]

brackets are very important...

 

[a1]

I got the same as Andy.

Here is a Good, Free, online Scientific Calculator...

http://web2.0calc.com/

 

[Sparklychris10]

Q1. The current in a coil changes from 5 amperes to 2 amperes in 50mS and induces a voltage of 30v into the coil. Calculate the value of the inductance of the coil?Q2. Calculate the power dissipated by resistor 2 in the circuit here

(R1 = 5ohms, R2= 10ohms, supply voltage = 20v)

Q3. Calculate the synchronous speed of the rotating magnetic field in a 4-pole synchronous motor?

Q4. An inductive load with 10 ohms inductive reactance is connected to a 100v 50 Hz AC supply. Calculate the power dissipated in the circuit (ignoring any conductor resistance.)

Q5. Calculate the inductive reactance of a coil with inductance of 0.1H, connected to a 50Hz AC supply.

Thanks for any help.

I remember having to do this in my online exams for my Level3 2330, i HATED this paper. I failed it 4 times before I finally got it, they ask you to remember so many equations and sequences for every different unit!! Hope it goes ok for you though!!!

 

[philbas]

A circuit connects a 28.8 ohm resistor and a capacitor with capacitive reactance of 38.4 ohms. Calculate the total impedance of the circuit38 ohms

24 ohms

48 ohms

44 ohms

(Z=?)

28.8 not 28.4 that everyone is working to, answer exactly 48

 

[jonnymc11]

Q2. Calculate the power dissipated by resistor 2 in the circuit here

(R1 = 5, R2= 10, supply voltage = 20v) (series)

17.8 watts

19.9 watts

10 watts

14.4 watts

 

[ADS]

Q2. Calculate the power dissipated by resistor 2 in the circuit here(R1 = 5, R2= 10, supply voltage = 20v) (series)

17.8 watts

19.9 watts

10 watts

14.4 watts

Hi jonnymc11,

Something wrong with my calculator in my post above - all the figures and formulas for this question are right.....but the answer is wrong!!!

It's A, by the way, 17.8 watts.

I've altered it in my first post now:|

 

[jonnymc11]

cheers mate, also:

A circuit consisting of an inductance of 0.35H connected to a 50Hz has an inductive reactance of:

a) 9.1 mΩ B) 0.09 Ω c) 109.9 Ω d) 1.099 kΩ

im getting 0.09, is there a power to go with the inductance?

as im doing:

1 / 2 x 3.142 x 50 x .35

= 0.009

 

[philbas]

for inductance

2*PI*frequency*inductance

=

2*PI*50*0.35 = 109.95

so 109.9

 

[jonnymc11]

i was doing capacitive reactance then?

 

[philbas]

cheers mate, also:A circuit consisting of an inductance of 0.35H connected to a 50Hz has an inductive reactance of:

a) 9.1 mΩ B) 0.09 Ω c) 109.9 Ω d) 1.099 kΩ

im getting 0.09, is there a power to go with the inductance?

as im doing:

1 / 2 x 3.142 x 50 x .35

= 0.009

If it had been a capacitor the calculation would be

1/ (2*PI*50*0,35)

Brackets important again

 

[jonnymc11]

still getting 0.009:Blushing

 

[philbas]

Yep I agree, for a capacitor its .009Guinness