# Design and Carry Out an experiment to determine the EMF and Internal Resistance of a standard laboratory power pack.

FINDING THE INTERNAL RESISTANCE OF A POWER PACK

### AIM

Design and Carry Out an experiment to determine the EMF and Internal Resistance of a standard laboratory power pack.

### THEORY

This information was taken from the Collins advanced science Physics textbook, the Cambridge Advance Science Physics 1 textbook and from notes taken in class.

E.M.F or Electro Motive Force is the opposite of potential difference, in that; it is the situation where a voltage is gaining energy.  This seems unlikely, but it is required in order to allow an electric circuit to function.  An electric current is a flow of electric charge, the charge flows around the circuit, transferring some of its energy to areas of resistance along the way (resistors, filament lamps, buzzers).  At some point along the way the energy must be initially supplied, otherwise the whole process couldn’t function.  At the beginning of the process the power supply provides the charge with energy to pass to the circuit.  This is the electro-motive force.  EMF is a type of voltage along with potential difference and these are defined as:

A Voltage where the charge is losing energy is a potential difference, V.

A Voltage where the charge is gaining energy is an electromotive force, E.

A relationship exists between volts and joules.  A 10v power supply, for example, will give 10J of energy to each coulomb that it pushes round the circuit.  When a single coulomb passes through a component with a p.d. of 5v, it transfers 5J of energy to the p.d.  In it’s simplest form when 1C passes through a p.d. of 1v, it transfers 1v of energy.

My investigation is concerned with the insides of the power supply, commonly a power pack is made up of many wires, and a cell contains chemical electrolytes and electrodes, both however work to the same effect.  When the charge passes through the source of e.m.f it gains lots of energy, but also instantly loses some of that that it gains, this is due to internal resistance. Internal resistance is resistance that is found only in the power source.  Commonly written as r and graphically represented by a resistor enclosed in a circle surrounding both the resistor and the power source.

It is possible to work out the current that flows when a power source is connected to an external resistor, R. R and r are in series and given that the current flows through both, one after the other, their combined resistance can be written:

E=IR + Ir        or        E=I(R + r)

E cannot be directly measured because a voltmeter can only be connected across the entire cell, including the cells internal resistance, r.  When a voltmeter is connected across the cell the result returned is the terminal p.d. V using this formula

V=IR

By examining the previous formula, this is less than the ...