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Old 11-29-2017, 08:16 AM   #61
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Originally Posted by eddy current View Post
That type of question is common on license tests.

The question we get like that is a 240 volt, 3000w hot water tank.
What would the current draw be if you fed it with only 120 volts?
A more interesting question is how many watts would it produce at 120 V. Most people would assume it would be 1500 watts at 120 V.... but nooooo.
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Old 11-29-2017, 08:26 PM   #62
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These types of questions are used to sort the men from the boys,
Or those whom truly understand electricity
as opposed to those whom have just learn't it !
And there's a world of difference.
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Old 11-29-2017, 08:31 PM   #63
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Originally Posted by dmxtothemax View Post
These types of questions are used to sort the men from the boys,
Or those whom truly understand electricity
as opposed to those whom have just learn't it !
And there's a world of difference.
You're the man now dog.
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Old 11-30-2017, 06:28 AM   #64
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IMO this one is very little electrical theory and more algebra.

Before the math, a very little electrical theory, you need to understand the rating of the bulb, and figure a bulb is a purely resistive load, and the resistance is the same at 120V and 130V.

With that understanding of the bulbs, you're given power and voltage and you can solve for resistance. (The circuit doesn't matter yet.)

R=E*E/P
R1=130*130/75=225.33
R2=120*120/75=192

Now a very little more electrical theory, with resistance and voltage, with the switch open, it's a simple series circuit. A 240V circuit with two resistors in series. You have voltage and two resistance values, you can solve for current.

I = E/R = E/(R1+R2) = 240/(225.33+192) = .575
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Old 11-30-2017, 07:31 AM   #65
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Originally Posted by splatz View Post
IMO this one is very little electrical theory and more algebra.

Before the math, a very little electrical theory, you need to understand the rating of the bulb, and figure a bulb is a purely resistive load, and the resistance is the same at 120V and 130V.

With that understanding of the bulbs, you're given power and voltage and you can solve for resistance. (The circuit doesn't matter yet.)

R=E*E/P
R1=130*130/75=225.33
R2=120*120/75=192

Now a very little more electrical theory, with resistance and voltage, with the switch open, it's a simple series circuit. A 240V circuit with two resistors in series. You have voltage and two resistance values, you can solve for current.

I = E/R = E/(R1+R2) = 240/(225.33+192) = .575
How would you calculate the voltage dropped across each resistor?

Edit: I would just multiply the R value by the circuit current to come up with the voltage across the resistor.
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Last edited by hardworkingstiff; 11-30-2017 at 07:38 AM.
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Old 11-30-2017, 08:04 AM   #66
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Originally Posted by hardworkingstiff View Post
How would you calculate the voltage dropped across each resistor?

Edit: I would just multiply the R value by the circuit current to come up with the voltage across the resistor.
Yep, that's how.

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Old 05-03-2018, 02:44 AM   #67
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Originally Posted by Peter Goldwing View Post
I just did it R=V square/power. therefore first bulb is 225ohm, second 192 ohms
combined 417 ohm. 240/417= 0.575A. too bad i didnt have this amount of time to come up with it
You are right, the 130 volts on the other lamps make the confusion, but its not really a factor, the missing data is the resistance of that rated 130 volts lamp. And as you nail the resistance, its just an ordinary series circuit.
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Old 05-05-2018, 03:13 PM   #68
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Originally Posted by Peter Goldwing View Post
In my test I had this question
The bulbs are both 75W. One 130V one 120V. What is the current flowing thru the circuit when the switch is open?

The 120 volt bulb has a current of 75/120V=0.635A with the switch closed.
If the second bulb would also be 120v then if you open the switch the current would be the same 0.63A. But they are not the same. Anyone knows how to solve it?
Sorry for the miserable sketch.
When I look at the drawing, it looks like the neutral is on a switch that will create an open neutral if you turn off the switch. With the switch open, the current will be 0 because there will be no current flow, Unless I'm missing something? Maybe I should read the comments before I post lol, I'm leaving this here though. Lets see
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Old 05-06-2018, 02:04 AM   #69
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