Required Practical 6: Investigating emf & internal resistance of electric cells & batteries

What are you trying find from this experiment?

• Aiming to find the internal resistance of the cell.

What apparatus is needed for this experiment?

• Cell

• Variable resistor

• Ammeter

• Voltmeter

How are these electronic components connected?

• Cell & variable resistor are in series.

• Ammeter also in series

• Voltmeter position can be either across the cell or variable resistor.

Give a brief description of how this experiment works:

• .. starting from maximum resistance with the switch open record the reading, V, on the voltmeter

• Close the switch and take the readings of pd, V, on the voltmeter and current, I, on the ammeter.

• Adjust the variable resistor

• .. & measuring voltage & current for each interval.

How do you plot the graph for this experiment?

• Plot a graph of V against I

• .. should expect a straight line.

What type of graph do you expect to plot? Why?

• A downwards straight line

• .. starting at x= 0amp & y = max emf

• This is because when resistance is maximum, voltage also is (as R = V/I)

• .. and thus current will be at a minimum (0).

What is the graphical analysis for this experiment?

• Start with the equation E = I(R + r)

• .. which when you expand you end up with: E = V + Ir

• .. rearranged this to make into y = Gx + C

• .. you’ll end up with V = E - Ir.. which =

• V = -rI + E

• .. where gradient = -r

Why do you first measure the voltage, with the switch open?

• When the switch is open no current will flow

• .. this will allow you to measure the maximum emf of the cell,

• without it being limited from lost volts due to internal resistance.

What are you measuring when you close the switch?

• The terminal p.d.

• .. as voltage is being lost (lost volts)

• & also measuring current.