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Internal Resistance of a cell

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Topic: Internal Resistance of a cell

Aim: To measure the internal resistance and emf (the potential differences across a voltage Source    when no current is flowing) and to observe the combination of cells

Hypothesis: The emf  of the old cell is less than the emf of the new cell but the internal resistance of the old cell is much greater than the new cell.


Resistance in electricity, property of an electric circuit or part of a circuit that transforms electric energy into heat energy in opposing electric current. Resistance involves collisions of the current-carrying charged particles with fixed particles that make up the structure of the conductors. Resistance is often considered as localized in such devices as lamps, heaters, and resistors, in which it predominates, although it is characteristic of every part of a circuit, including connecting wires and electric transmission lines. (Britannica.2006)

The dissipation of electric energy in the form of heat, even though small, affects the amount of electromotive force, or driving voltage, required to produce a given current through the circuit. In fact, the electromotive force V (measured in volts) across a circuit divided by the current I (amperes) through that circuit defines quantitatively the amount of electrical resistance R. Precisely, R = V/I. Thus, if a 12-volt battery steadily drives a 2-ampere current through a length of wire, the wire has a resistance of 6 volts per ampere, or 6 ohms. Ohm is the common unit of electrical resistance, equivalent to one volt per ampere and represented by the capital Greek letter omega, Ω. The resistance of a wire is directly proportional to its length and inversely proportional to its cross-sectional area. Resistance also depends on the material of the conductor. Seeresistivity.

The resistance of a conductor, or circuit element, generally increases with increasing temperature. When cooled to extremely low temperatures, some conductors have zero resistance. Currents continue to flow in these substances, called superconductors, after removal of the applied electromotive force.

The reciprocal of the resistance, 1/R, is called the conductance and is expressed in units of reciprocal ohm, called mho. (Encarta.2006)

...read more.


1.4 volts

1.49 volts

This circuit is intended when cells

are in "parallel"

The above table shows the prediction made before the actual measurement started off, so the best

combination we chose was in letter ‘b’ because the value of the emf is increasing compared to a single cell as well as its voltages increasing the most when the two cells combined

Identification for the combination of cells

Series combinationParallel combination


(b)        (C) (C)image17.png


The results of the old and new battery

The old battery

Voltage and the currents

VOLTAGE (volts)


CURRENTS ( amps)
















The New battery

The voltage and the currents































The two tables above resulted after the currents from the rheostat adjusted so the voltage depended upon the currents, it was given us a glue that when the currents from the rheostat increased the voltage getting decreased

Graphing the results of the two cells (old and new)


The above graphs were deriving from the previous table; it showed the emf and the internal resistances of the two batteries together. The upper most graphs were the new cell while the old cell held the lower part of the grid.

Calculation page for the old cell:

Old battery

A) Coordinate of the best fit line:image07.png

= (0.00, 1.31) and (0.60, 0.62)

Slope = internal resistance (-r)


...read more.


 The best combination we found after the practical experiments was in letter ‘b’ connected in series as it doubled the voltage of each cell.

The value of the emf of the new cell was 1.54 volts with it internal resistance of -0.49 and the value of the emf of the old cell was 1.31volts withit internal resistance of -1.15 Ω

So it looks like this; at first the new cell rich in potential differences but after time, the internal resistance getting built up that means it becomes outlive it potential differences (flat) and then becomes as one of the old cell because it emf is less than before and the internal resistance is increasing


  • P.Howison, 1999,Physics year 13, ESA Publications (NZ) Ltd, Singapore
  • Encarta 2006, Internal Resistance, Microsoft Corporation
  • Britannica 2006, Internal Resistances, Britannica Corporation
  • G.Alex, 23 May, Internal resistances, www. battery.com,

Appendix: 1


The set up photo of the practical using the old cell


The above picture shows the components of the cell together with the lighting bulb


The set up appratus for the new cell

...read more.

This student written piece of work is one of many that can be found in our AS and A Level Electrical & Thermal Physics section.

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Here's what a teacher thought of this essay

5 star(s)

This is a 5 star piece of work investigating the emf and the internal resistance of an old and a new cell. It also investigates the effect of different combinations of cells to get the maximum voltage. Great introduction and excellent scientific knowledge and understanding displayed throughout. A good clear plan of work and hypothesis made. Excellent results but no repeats for reliability. Good graphs and analysis of results using suitable accurate equations. Overall an excellent piece of work achieving good results. A logical order which is easy to follow.

Marked by teacher Patricia McHugh 13/05/2013

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