Any tips ?? Remembering formula

[Edtwozeronine]

We were told to make formula sheets and refer back to them whenever we needed to, mine was riddled with reminders of what the various symbols meant.

In our college moodle was a great place for pdf books full of useful information to be downloaded, it's amazing just how much help is there if you just look. I say that because I was always trying to point the younger students towards it, but if it wasn't an app or some crap they could view on their phone they wouldn't seem to take it on board. Mobile phones should be banned in college time in my book, you seriously need to concentrate to do the work and phones are the antithesis to concentration!

 

[Jay 1987]

transposing a formula is easy if you follow this rule.

whatever you do to one side of the equation, do same to the other. e.g:

P = IR. easy. now suppose you want to find if I as you don't know it's value, but you know the value of P and R.


so.. divide each side by R, thus maintaining the equality.
so you have

P/R =I

ssimplesss.

same principle for more complex eaquations,

Yes the example you gave is very easy to understand

I mean like for example.....


A 5kw unit is connected to a 230 supply and draws a current of 24 Amps

Work out the effienciency of the unit ?????????

 

[Pete999]

Yes the example you gave is very easy to understand

I mean like for example.....


A 5kw unit is connected to a 230 supply and draws a current of 24 Amps

Work out the effienciency of the unit ?????????

Well don't leave us all in suspense Jay, what's the answer?

 

[Jay 1987]

Getting ready for college be back on after

 

[Lucien Nunes]

A 5kw unit is connected to a 230 supply and draws a current of 24 Amps, Work out the effienciency of the unit ?????????

You need to take more care over details in engineering. 5kW what? Input? Output? a '5kW' motor, that would usually mean the mechanical output rating. A 5kW heater, that would be input power. Presumably 230 means 230V single-phase AC but you don't say that. 24A with what power factor or is this DC?

If I rephrase the question as:
A device uses 24A at unity power factor from a 230V single-phase AC supply and outputs 5kW useful power. What it its efficiency?

Now it can be calculated because I've filled in the missing info.

Input power = V.I.cos(phi)
Efficiency = useful output power / input power
=5000 / (230 x 24 x 1)
=91%

 

[MFS Electrical]

Hey paddy I believe we are in the same position !

I started my level 2 in September and have been going over and over the theory as telectrix stated and believe it or not it does start to sink in

Maybe we could have some theory questions based on formula suitable for “level 2 students”
From some of the more experienced guys!

Just a thought.....

So far we have covered powers, ohms law triangle, power triangle, working out energy, work done, but the hardest for me is deffo transposition!

I have looked at some of the electrical design questions on the trainee section! But they are a little advanced for me at the moment!

But I still read it to get a flavour for real life scenarios

Even though you may think they are a bit advanced just have a bash. a wise man once told me you learn best by making an a**e of it. get it wrong and we can show you where you went off the tracks get it right and that’s all good. How will you know if you never even try!

 

[Jay 1987]

Very true can’t argue with that !

 

[Silly Sausage]

Even though you may think they are a bit advanced just have a bash. a wise man once told me you learn best by making an a**e of it. get it wrong and we can show you where you went off the tracks get it right and that’s all good. How will you know if you never even try!

Confucius say, "If it too easy now, it bite you on arse later"

 

[MFS Electrical]

Confucius say, "If it too easy now, it bite you on arse later"

Is it bad that I read that with the accent
That’s what this part of the forum is for no matter how daft your reply is we won’t bite we are only looking to help but we can’t do that if no one is gonna try

 

[Jay 1987]

You need to take more care over details in engineering. 5kW what? Input? Output? a '5kW' motor, that would usually mean the mechanical output rating. A 5kW heater, that would be input power. Presumably 230 means 230V single-phase AC but you don't say that. 24A with what power factor or is this DC?

If I rephrase the question as:
A device uses 24A at unity power factor from a 230V single-phase AC supply and outputs 5kW useful power. What it its efficiency?

Now it can be calculated because I've filled in the missing info.

Input power = V.I.cos(phi)
Efficiency = useful output power / input power
=5000 / (230 x 24 x 1)
=91%

Thanks it was a question I remembered from my exam, as everybody got it wrong! Hence the reason I relayed it incorrectly.

The formula we learnt to work out efficiency
Was

Output power
————————. X 100
Input power


But we never knew how to work out input power from volts and amps

Using power triangle I understand

P
I V

But I don’t understand the p = v2 part


And where did you get 1 from

5000/ (230 x 24 x 1?)

 

[Silly Sausage]

V=IR
I=V/R
P=VI
substitute I with V/R gives you ...
P=V * V/R = (V^2)/R

^2 is to the power of 2, ie V squared.

 

[Edtwozeronine]

Dropbox - Electrical Science.pdf - https://www.dropbox.com/s/69sr9tof37moj6p/Electrical%20Science.pdf?dl=0 Use this for walk throughs on all things electrical science.

 

[Deleted member 72666]

Some of you may find this useful when typing mathematical symbols.
Type mathematical symbols - online keyboard - http://math.typeit.org/

 

[Jay 1987]

V=IR
I=V/R
P=VI
substitute I with V/R gives you ...
P=V * V/R = (V^2)/R

^2 is to the power of 2, ie V squared.

I see what you’ve done, but I don’t understand what you’ve done!

What’s the need to substitute I for V

In what situation would you use V2?

 

[Richard Burns]

I see what you’ve done, but I don’t understand what you’ve done!

What’s the need to substitute I for V

In what situation would you use V2?

The variations in formulae are to account for the known variables.
On a piece of equipment designed for 230V ac if you use an ohmmeter to measure the resistance of the load then you now know two variables, R and V, if you want to know the power then you can use P=V²/R.
It would also be possible to get I from I = V/R and then use this calculated value for I in the P = VI equation but then you have to do two calculations.
Effectively you do the first calculation, I=V/R, in advance by substituting V/R for I in the P=VI equation.
Other versions of the equations can be used where you know different values.

 

[telectrix]

as richard burns said. if you know R but don't know I, then as above.

 

[Silly Sausage]

@stantheman

Cheers Stan

 

[Spunkywads26]

I am teaching electrical science at the minute with my students and have just covered ohms law, series, parallel, series+parallel and power.

I have found that it is easy for them to remember ohms law (V = I x R) and power formula (P = V x I). I then say that they can either remember the other 2 power formulas (P = I^2 x R & P = V^2/R) or if they can transpose and substitute formula then they only need the 1st two formula to work out the rest.

 

[telectrix]

this might be useful.:

 

[satori]

Hey I’m studying 2365 at college and I’m at the science part now .. any tips on remembering formula or is it just a case of going over and over it till it’s in your head .. any help much appreciated

Flash cards. That's what I used. Have a Google, there's software that lets you make your own. It helps having a buddy so you can each test each other. I found it worked very quickly. I can't remember any of them now - well, apart from Ohms Law, and power.