# Finding out internal resistance of a lemon when used as a power supply

1. Dec 13, 2007

### Dearster

Hi There,

Right I recently did an experiment in Physics to find out the internal resistance of a lemon when used as a power supply.

We connected the lemon to a voltmeter, ammeter and variable resistor, changed the resistance on the variable resistance and measured the voltage and current against each other.

So i have a table of voltage against current.

I have drawn a graph of voltage against current, voltage on y axis, and current on the x axis. And drawn a line of best fit. Voltage is in milli volts, 10^-3 and current is in micro volts 10^-6

How would i go about finding the EMF and eventually the Internal resistance of the lemon when used as a power supply.

Also how would i go about finding the intercept of the graph if the line of best fit does not go through the y axis on the scale i have used.

Anything to help would be fantastic

2. Dec 13, 2007

### mgb_phys

The intercept is the point at which the line crosses the Y axis.
This would be the voltage with no current (hint what's the definition of the emf of a cell?)
The slope of the graph is voltage/current, ( hint what's ohm's law?)

3. Dec 13, 2007

### Staff: Mentor

A straight line must pass through the y-axis somewhere if it's extended far enough, unless it's vertical (and therefore parallel to the y-axis).

I suspect you need to "zoom out" your graph scale so the intercept is actually visible on the graph.