Electrical question

Thread Starter

Charley1616

Joined Jul 1, 2020
3
Anyone able to help with a question?
If the supply voltage is 100v ac with a period of 20ms and the supply current taken at the coil is 6A, calculate
Frequency
Impedance
Inductive reactance
And inductance
 

WBahn

Joined Mar 31, 2012
25,918
You need to provide or describe the circuit in question.

You also need to show your best attempt to answer each of those questions.
 

Thread Starter

Charley1616

Joined Jul 1, 2020
3
Is this Homework?
The resistance is also normally needed.
Max.
It’s a practice question but I’m a bit thrown off as resistance wasn’t mentioned but thought I could get this by dividing voltage by current which gave me 16.6 but that was the same answer I got for inductive reactance when I did 2πfl so I must have gone wrong somewhere
 

MrAl

Joined Jun 17, 2014
7,669
Anyone able to help with a question?
If the supply voltage is 100v ac with a period of 20ms and the supply current taken at the coil is 6A, calculate
Frequency
Impedance
Inductive reactance
And inductance
It sounds to me like this is a single inductor, with an AC supply voltage.
So if you know how to calculate values in a circuit that has one inductor and one AC voltage source you should be able to come up with something.
Analyze the circuit the way you would any circuit like this.

All I know is it’s a coil, no more info is given 100v rms, 6A, and 20ms, which I believe I worked out to be 0.05 mHz
Is inductance really measured in Hz? What unit is inductance measured in?
I think you are close to getting the right result, but you should also give more than one digit of precision. For example, if the answer is 0.012345 you should at the least show 0.012 not just 0.01 ok.
 
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WBahn

Joined Mar 31, 2012
25,918
It sounds to me like this is a single inductor, with an AC supply voltage.
So if you know how to calculate values in a circuit that has one inductor and one AC voltage source you should be able to come up with something.
Analyze the circuit the way you would any circuit like this.



Is inductance really measured in Hz? What unit is inductance measured in?
I think you are close to getting the right result, but you should also give more than one digit of precision. For example, if the answer is 0.012345 you should at the least show 0.012 not just 0.01 ok.
If nine orders of magnitude counts as close....
 

WBahn

Joined Mar 31, 2012
25,918
It depends what unit of measurement you are using.
Huh ?????

When talking about a two values for the same parameter differing by nine orders of magnitude, you are talking about the ratio of two quantities with the same dimension, which is dimensionless. So how can it depend on what unit of measurement you are using.

Though I got a bit careless thinking it through in my head and the difference is actually "only" six orders of magnitude (which, ironically, it what I almost typed until I "corrected" myself). I think it's because I was working some figures in a spreadsheet at the same time that coincidentally has a parameter that was 20 microseconds).

The proposed answer for the frequency of 0.05 mHz differs from the correct answer for the frequency of 50 Hz by a factor of one million. Which is six orders of magnitude. You can use whatever units you want for the two frequencies -- radians/sec, degrees/fortnight, doesn't matter -- the ratio between these two frequencies will still be six orders of magnitude.
 

MrAl

Joined Jun 17, 2014
7,669
Huh ?????

When talking about a two values for the same parameter differing by nine orders of magnitude, you are talking about the ratio of two quantities with the same dimension, which is dimensionless. So how can it depend on what unit of measurement you are using.

Though I got a bit careless thinking it through in my head and the difference is actually "only" six orders of magnitude (which, ironically, it what I almost typed until I "corrected" myself). I think it's because I was working some figures in a spreadsheet at the same time that coincidentally has a parameter that was 20 microseconds).

The proposed answer for the frequency of 0.05 mHz differs from the correct answer for the frequency of 50 Hz by a factor of one million. Which is six orders of magnitude. You can use whatever units you want for the two frequencies -- radians/sec, degrees/fortnight, doesn't matter -- the ratio between these two frequencies will still be six orders of magnitude.
You were measuring in "Henries" i presume.
I was measuring in units of distance graphically, or simply in a unit of a typeset such as we see in a paper newspaper article.

You might note that sometimes we see an article written in a column where the beginning of each line forms an almost straight line down the page, as is usual with most English language writing. But we also sometimes see the END of the line also forming a nearly straight line down the page. How can that be if the number of characters are different in each line as is typical in any language? It is because they vary the kerning of the characters so that they can make the end appear nearly straight down the page and that makes the column appear more rectangular on the page and so self contained and more distinguished from other columns.

I will use resistance as an example.
With a value like 0.01 mOhms if we simply drop the little 'm' we get 0.01 Ohms.
So that is a small difference in the graphical appearance:
0.01 mOhms
vs
0.01 Ohms

Another example:
0.1 kOhms
vs
0.1 Ohms

or the other way around:
0.1 Ohms
vs
0.1 kOhms

Just one letter difference in each case.
 

WBahn

Joined Mar 31, 2012
25,918
You were measuring in "Henries" i presume.
I was measuring in units of distance graphically, or simply in a unit of a typeset such as we see in a paper newspaper article.

You might note that sometimes we see an article written in a column where the beginning of each line forms an almost straight line down the page, as is usual with most English language writing. But we also sometimes see the END of the line also forming a nearly straight line down the page. How can that be if the number of characters are different in each line as is typical in any language? It is because they vary the kerning of the characters so that they can make the end appear nearly straight down the page and that makes the column appear more rectangular on the page and so self contained and more distinguished from other columns.

I will use resistance as an example.
With a value like 0.01 mOhms if we simply drop the little 'm' we get 0.01 Ohms.
So that is a small difference in the graphical appearance:
0.01 mOhms
vs
0.01 Ohms

Another example:
0.1 kOhms
vs
0.1 Ohms

or the other way around:
0.1 Ohms
vs
0.1 kOhms

Just one letter difference in each case.
So, in your world, 0.05 mV is close to 0.05 kV since they only differ by one letter while, in my world, they differ by six orders of magnitude.
 

MrAl

Joined Jun 17, 2014
7,669
So, in your world, 0.05 mV is close to 0.05 kV since they only differ by one letter while, in my world, they differ by six orders of magnitude.
It is very very very clear that we both live in the same world. We may interpret things differently that's about it. I understand why you want to measure in the electrical units. I was measuring in terms of the number of characters. In that view, 0.001pF is close to 0.001F. Electrically, they are much much different.
In computer graphics things are looked at differently and since i end up doing a lot with that in Windows programming i tend to think both ways. Sometimes i have to think about how many characters i can use for the name of a button in order to be able to fit the whole name on the button, so the button width has to be adjusted, and it may have to be adjusted on the fly if the user has control over the font choice and size.
Here is an excerpt from a function call that informs the caller of the various dimensions of a control:

Code:
typedef struct tagMEASUREITEMSTRUCT {   // mis
    UINT  CtlType;      // type of control
    UINT  CtlID;        // combo box, list box, or button identifier
    UINT  itemID;       // menu item, variable-height list box, or combo box identifier
    UINT  itemWidth;    // width of menu item, in pixels
    UINT  itemHeight;   // height of single item in list box menu, in pixels
    DWORD itemData;     // application-defined 32-bit value
} MEASUREITEMSTRUCT;

The MEASUREITEMSTRUCT structure informs Windows of the dimensions of an owner-drawn control or menu item. This allows Windows to process user interaction with the control correctly.
Another one is this:

Code:
BOOL GetTextExtentPoint32A(
  HDC    hdc,
LPCSTR lpString,
int c,
LPSIZE psizl
);

This provides a pointer to a SIZE structure which gives the size of the text string in logical units, the pointer is psizl (BOOL indicates pass non zero or fail zero).
So the the first (associated) function measures controls while the second measures text string length most usually in pixels. String "12345" would return a larger value than "1234" of course. String "123.456" would return a smaller value than "1230456" if the font is proportional but the same value if the text is non proportional (like Courier New). where every character is the same width.
So measurement of these items is very important in the graphical aspect of getting the displayed text to look right.

Is it better to use electrical units for the case at hand? Probably yes, the graphic view is just a different way of looking at it.
If we had a program that read the value strings that could interpret suffixes properly then:
0.01mF and 0.01F
numerically 0.01F would come out larger than 0.01mF and they are quite a bit different, but if it also had to measure the string length 0.01mF would come out larger than 0.01F and they do not vary in length by much, but if we want to display them both on a button face 0.01F could have a smaller button width than 0.01mF and the buttons would be slightly different in size.
Just another way of looking at it.

I hate to give too much away at this stage, but i was hoping that if he looked at his result and thought that it was not too different than the correct result (in some way) he might see the simple mistake right away.
 
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