Course questions

pprendeville · Sep 14, 2009

Can anyone help me out with the questions in the pictures attached? I'm doing a FAS course (FAS is a government funded organisation in Ireland) in Electrical Control Systems and I'm really stuck on these questions. I'm originally a mechanical design and this aint my field at all. I hope I don't get banned from the forum for this coz it's happened me before on engtips.com

For 15, I believe the current only flows through the first lamp but I'm not sure why it doesn't go through lamp 2 and 3. Is it because current tries to take the shortest path back to the other pole of the battery. I thought current could be compared to a river in that it would break up at the first junction.

Not really sure how to do 26 or 31.

Notsosmart · Sep 14, 2009

Hi,

I will try to help you but that means helping yourself

~ so I will leave some blanks for you to insert & hopefully then, you will come to the correct answer & know how you did it.

Ohms law needs to be applied

(Remember Current remains constant in series within resistor circuits but splits across branches, the opposite is true of Voltage)

lets tackle Q26 ~
The p.d. across the resistor sets is: Insert your answer

The total resistance of the first resistor branch is r1 + r2 = 2 + 3 = 5 ohms,
therefore branch current (V/R) = Insert your answer (i)
Volt drop across r1 (2 ohm) = I(branch 1) x R(1) = Insert your answer (ii)
The total resistance of the second resitor branch is r3 + r4 = 2 + 1 = 3 ohms,
therefore branch current (V/R) = Insert your answer (iii)
Volt drop across r3 (2 ohm) = I(branch 2) x R(3) = Insert your answer (iv)

P.d.. across points A & B = volt drop r3 (answer (iv)) minus volt drop r1 (answer (ii)) = Insert your answer (Should align with one of the multiple choices)

Q31 ~
Volt drop across R1 = (I X R) 0.02 X 200 = Insert your answer (i)
This leaves how much voltage across the other resistors = Insert your answer (ii)
Given that 20mA in total flows in the circuit (shown next to R1) & 19mA flows through R2, therefore how much current flows through R3 & R4? : Insert your answer (iii)
Volt drop across R3 = answer (ii) x answer (iii) = Insert your answer (iv)
Therefore Voltmeter = answer (ii) minus answer (iv) = Insert your answer (Should align with one of the multiple choices)

For Question 15:

Add some resistances to the bulbs (assume they are all identical) then apply ohms law as above (Hint: apply volt drop across the single bulb first)

I hope this is useful & not confused you even more.

1 will try to help again tomorrow if needed (email alerts will nudge me)

Regards,
Gaz.