Ohms law

Status
Not open for further replies.
I

Ingenieur

Senior Member
Location
Earth
Rahim84 said:
Can I calculate it without using pf (power factor?). Just off of knowing the voltage and current ?


Sent from my iPhone using Tapatalk
Click to expand...

Well you'll have to
depending on load make an assumption
if resistive use 1
if a motor
1 phase 0.6
3 phase 0.8
guesses, but not accurate
 
R

Rahim84

Member
Location
Charlotte, NC, USA
I'm asking because it was a question on a test I took recently and the only reference given was an ohms law circle.. so I figured I must be able to get an answer based off of that information alone.


Sent from my iPhone using Tapatalk
 
I

Ingenieur

Senior Member
Location
Earth
Rahim84 said:
I'm asking because it was a question on a test I took recently and the only reference given was an ohms law circle.. so I figured I must be able to get an answer based off of that information alone.


Sent from my iPhone using Tapatalk
Click to expand...

if they don't give pf assume it to be 1
 
charlie b

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Semi-Retired Electrical Engineer
Rahim84 said:
Can I calculate it without using pf (power factor?). Just off of knowing the voltage and current ?
Click to expand...
If you remove pf from the equations, you get "apparent power" in units of VA, and not "real power" in units of watts. So I would ask what you are trying to calculation, and what you plan to do with the results. If you are, for example, looking to select a transformer size to handle a collection of loads, you need to be calculating VA, not watts.

 
R

Rahim84

Member
Location
Charlotte, NC, USA
50f0316258fec4c91c673dd55d3a8dee.jpg


It was just like this, with a ohms law circle for help...


Sent from my iPhone using Tapatalk
 
G

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
170706-1336 EDT

Ohms law says nothing about power. Go back and read the history of what Ohm did in his experiments.

Separately read the history of Joule's work on development of power equations.

History means a lot and too many people want to muddy history.

One can mix power equations with Ohm's law to get other equations, but that does make those equations Ohm's law.

Ohm's law is strictly about the relationship of conductivity, voltage, and current. It is interesting to note that, if you go back and read about Ohm's work you will find that he was able to make a continuously variable DC voltage source without any of today's modern electronics.

.
 
I

Ingenieur

Senior Member
Location
Earth
gar said:
170706-1336 EDT

Ohms law says nothing about power. Go back and read the history of what Ohm did in his experiments.

Separately read the history of Joule's work on development of power equations.

History means a lot and too many people want to muddy history.

One can mix power equations with Ohm's law to get other equations, but that does make those equations Ohm's law.

Ohm's law is strictly about the relationship of conductivity, voltage, and current. It is interesting to note that, if you go back and read about Ohm's work you will find that he was able to make a continuously variable DC voltage source without any of today's modern electronics.

.
Click to expand...

Granted
but the op is asking about an elementary question on a basic level test
they only gave him what they call the 'ohms law circle'
a mnemonic of sorts
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
In an "Ohm's Wheel", then Ingineur is correct, albeit the question was poorly worded because the entire concept is not that simple. So without context it can be misleading. These Ohm's Wheel diagrams are basically assuming purely resistive loads, not inductive.

ohms-law-wheel.jpg
This of course is also just single phase, hence the addition of the 1.732 factor for 3 phase equations.

If you want to impress your instructor though, point out the fact that the question, as worded, is making an assumption about the load being purely resistive based on facts not in evidence. :p
 
R

Rahim84

Member
Location
Charlotte, NC, USA
Jraef said:
In an "Ohm's Wheel", then Ingineur is correct, albeit the question was poorly worded because the entire concept is not that simple. So without context it can be misleading. These Ohm's Wheel diagrams are basically assuming purely resistive loads, not inductive.

View attachment 17949
This of course is also just single phase, hence the addition of the 1.732 factor for 3 phase equations.

If you want to impress your instructor though, point out the fact that the question, as worded, is making an assumption about the load being purely resistive based on facts not in evidence. :p
Click to expand...

Gotcha thanks.


Sent from my iPhone using Tapatalk
 
C

Carultch

Senior Member
Location
Massachusetts
Rahim84 said:
Where is the 1.732 from? I remember seeing this before but it's a little blurry


Sent from my iPhone using Tapatalk
Click to expand...

1.732 is the approximate value of the square root of 3. A mathematics purist would always use sqrt(3) in this formula, instead of 1.732, but to prepare you for this calculation when you have only a basic 4-function calculator, 1.732 is good enough for our purposes.

It comes from the geometry of an equilateral triangle, which represents the details of three phase electricity. Suppose you draw three line segments that are 12 inches long from a center point, and 120 degrees apart, which will look like the letter Y. Then you connect the end points of those line segments, with three other line segments, which will each be 20.8 inches long. The ratio between the 20.8" edge length, and the 12" length to the center of this triangle, is the square root of 3.
 
R

Rahim84

Member
Location
Charlotte, NC, USA
Carultch said:
1.732 is the approximate value of the square root of 3. It comes from the geometry of an equilateral triangle, which represents the details of three phase electricity. Suppose you draw three line segments that are 12 inches long from a center point, and 120 degrees apart, which will look like the letter Y. Then you connect the end points of those line segments, with three other line segments, which will each be 20.8 inches long. The ratio between the 20.8" edge length, and the 12" length to the center of this triangle, is the square root of 3.
Click to expand...

Got ya


Sent from my iPhone using Tapatalk
 
I

Ingenieur

Senior Member
Location
Earth
The difference between sqrt3 and 1.732 (sqrt3 to 3 signiicant digits) on an hp calculator is 0.0029%

close enough for government work :)
 
Status
Not open for further replies.
Top