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New to Electronics, need help with formula rearrangement.

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david tipton

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I am in first year Electronics class, in high school, and my teacher assigned us the task of calculating internal resistance. The book has absolutely nothing about it, and whenever I ask him about it, he just says I need to study, or to go rearrange the formula.
 
Hi,

I have exactly the same problem at uni, i can do engineering science - because it has a purpose, like finding a force or a speed - but calculating x for the sake of it sucks.

Anyway I downloaded this a few weeks back cos I am crap at transposing forumlas - mabey it will help.
 

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In order to help you we need more information. You want to calculate the internal resistance of what? Post as much information as possible, but be advised we normally don't answer direct homework or test questions. We will be glad to give you lots of guidance however.
 
If it's the internal impedance of a battery, for example, you can compare the battery voltage under a known load to the open circuit voltage. From this you can calculate the value of the battery impedance.
 
The internal resistance of the battery is in series with the load resistance. You know the open circuit voltage of the battery and you know the loaded voltage of the battery. The loaded current is simply the loaded voltage divided by the load resistance.

Then the internal resistance is simply the voltage dropped divided by the loaded current.

I didn't re-arrange anything. I just used common sense.
 
There's two ways to calculate internal resistance:
For both ways you need to know a few device values.

Rb = (Vs / I) - RL

Or, rearranging for I / RL you get:
Rb = (Vs - V) / I

- Rb is internal resistance of battery
- Vs is the battery voltage without a load (so like 9V)
- V is the battery voltage with a load (use ohms law to find this)
- RL is the load resistance (For example a speaker would be 8ohms)
- I is the total current supplied by the battery (usually written on the battery, a 9V is 625mA)

But you should also know the theory behind internal resistance of a battery. Basically it means the amount of 'resistance (impedance) that has built up within a battery that it can no longer provide a sufficient current to power up a load (like a light bulb).
 
But you should also know the theory behind internal resistance of a battery. Basically it means the amount of 'resistance (impedance) that has built up within a battery that it can no longer provide a sufficient current to power up a load (like a light bulb).
This isn't strictly true. All batteries, including fresh or recently charged ones, have internal resistance.
 
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