# Current drain

#### tjjam2003

Joined Apr 5, 2006
22
I have looked and looked in the all about circuits...This newbie is still stuck on 2 problems.

What formulas do I use if they are looking for a CURRENT drain if P=7.5 and E=120?
R is not known and I is not known.

I also am looking for the VOLTAGE required to cause a dissipation of 10W in a resistor when I through the resistor is 150Ma.

If some one can just show me how to solve for these I will be greatful !

#### paultwang

Joined Mar 8, 2006
80
Originally posted by tjjam2003+Apr 9 2006, 08:13 PM--><div class='quotetop'>QUOTE(tjjam2003 @ Apr 9 2006, 08:13 PM)</div><div class='quotemain'>What formulas do I use if they are looking for a CURRENT drain if P=7.5 and E=120?
R is not known and I is not known.
[post=16017]Quoted post[/post]​
[/b]

Again and again...
Use the simplified equation
Power = Voltage x Current

Hint: Current drain is current out of the battery/voltage source.

<!--QuoteBegin-tjjam2003
@Apr 9 2006, 08:13 PM
I also am looking for the VOLTAGE required to cause a dissipation of 10W in a resistor when I through the resistor is 150Ma.
[post=16017]Quoted post[/post]​
[/quote]

150 mega amps?
Hint: Some prefixes are case-sensitive. M = mega. m = milli.

#### Papabravo

Joined Feb 24, 2006
19,841
Originally posted by tjjam2003@Apr 9 2006, 10:13 PM
I have looked and looked in the all about circuits...This newbie is still stuck on 2 problems.

What formulas do I use if they are looking for a CURRENT drain if P=7.5 and E=120?
R is not known and I is not known.

I also am looking for the VOLTAGE required to cause a dissipation of 10W in a resistor when I through the resistor is 150Ma.

If some one can just show me how to solve for these I will be greatful !
[post=16017]Quoted post[/post]​
The problems you keep posting all assume that you can manipulate one equation for power and one equation for ohms law into the forms you need to solve various problems. You keep asking for the solutions to individual problems. Giving you those solutions does absolutely no good because you fail to use the specific solution to get your arms around the big picture.

FIRST RULE: If you have any two things you can find the third.

SECOND RULE: V(Voltage in Volts) = I(Current in Amperes) * R(Resistance in Ohms)

THIRD RULE: P(Power in Watts) = V(Voltage in Volts) * I(current in Amperes)

FIRST REQUEST: Do the Algebra
SECOND REQUEST: Solve the problem

If the algebra is beyond you then you need algebra help and not electronics help.

#### tjjam2003

Joined Apr 5, 2006
22
Maybe I am dense but How do I use Watt's Law when they give P=7.5 and E=120 and I have to solve for I???

Originally posted by paultwang@Apr 9 2006, 09:22 PM
Again and again...
Use the simplified equation
Power = Voltage x Current

Hint: Current drain is current out of the battery/voltage source.
150 mega amps?
Hint: Some prefixes are case-sensitive. M = mega. m = milli.
[post=16018]Quoted post[/post]​

#### paultwang

Joined Mar 8, 2006
80
Originally posted by tjjam2003@Apr 9 2006, 08:29 PM
Maybe I am dense but How do I use Watt's Law when they give P=7.5 and E=120 and I have to solve for I???
[post=16020]Quoted post[/post]​

P is power
E is voltage

Solve for I (current)

#### Papabravo

Joined Feb 24, 2006
19,841
Originally posted by paultwang@Apr 9 2006, 10:32 PM
P is power
E is voltage

Solve for I (current)
[post=16021]Quoted post[/post]​
Rich (BB code):
P = E * I
Divide both sides of the equation by E
P/E = (E * I)/E
Cancel out the E's on the right hand side to get
P/E = I
You really don't know any algebra do you?

#### tjjam2003

Joined Apr 5, 2006
22
The 2 questions read: If a 7.5-watt light bulb were connected to a 120 volt line, what would be the current drain of this bulb. they came up with 62.5 m amps. I'm not sure how to set the formula up.
#2) What voltage is required to cause a dissipation of 10 watts in a resistor when the current through the resistor is 150 mA? they come up with 66.67 volts. How do I set up the formula?

#### paultwang

Joined Mar 8, 2006
80
Originally posted by tjjam2003@Apr 9 2006, 08:45 PM
..... I'm not sure how to set the formula up.

..... How do I set up the formula?
[post=16023]Quoted post[/post]​

#### windoze killa

Joined Feb 23, 2006
605
Originally posted by tjjam2003@Apr 10 2006, 01:45 PM
The 2 questions read: If a 7.5-watt light bulb were connected to a 120 volt line, what would be the current drain of this bulb. they came up with 62.5 m amps. I'm not sure how to set the formula up.
#2) What voltage is required to cause a dissipation of 10 watts in a resistor when the current through the resistor is 150 mA? they come up with 66.67 volts. How do I set up the formula?
[post=16023]Quoted post[/post]​
All the required information has been supplied.

P=E*I

With this formula and the transposition of it you can solve all your questions.

It seems as if you would like us to give you the answers which I for one won't. I am not trying to be derogatory but these are very simple problems and shouldn't take much to work out.

You have the P, I and they give you the E in their answer. It should be easy enough to work it out.

#### JoeJester

Joined Apr 26, 2005
4,390
You said you reviewed the material, but I just looked at Calculating Electric Power, and what did I find ... the formula you are seeking was there in plain view.

#### Papabravo

Joined Feb 24, 2006
19,841
Originally posted by JoeJester@Apr 10 2006, 09:40 AM
You said you reviewed the material, but I just looked at Calculating Electric Power, and what did I find ... the formula you are seeking was there in plain view.
[post=16036]Quoted post[/post]​
I think we're being spoofed. I'm just incredulous at this level of discourse. There seems to be a low order of probability for any alternative.

#### tjjam2003

Joined Apr 5, 2006
22
No, you are not being spoofed. I am very weak in trig and algebra. I am not trying to get the answers the easy way. I am trying to work out the problems myself. I was just asking for HELP. I am starting on a very BASIC level (which is not basic enough because of my math skills).
I realize that you are way beyond my questions but it's all new to me.
Sorry if I rubbed you guys the wrong way. I will try harder
Thank you guys for all your help so far.

#### windoze killa

Joined Feb 23, 2006
605
Originally posted by Papabravo@Apr 10 2006, 01:41 PM
Rich (BB code):
P = E * I
Divide both sides of the equation by E
P/E = (E * I)/E
Cancel out the E's on the right hand side to get
P/E = I
You really don't know any algebra do you?
[post=16022]Quoted post[/post]​
tjjam2003, Pababravo gave you the formula in this reply. He even transposed it to the way it needs to be for you calculations. Power divided by Voltage = Current you just need to put in the figures you have. We are sorry but this is the simplest it can go with out giving you the answers.

#### thingmaker3

Joined May 16, 2005
5,083
Perhaps this will help:

http://www.algebrahelp.com/

Attempting to learn electronics without math is like attempting to cook without a heat source.

• mjb1972