# In this experiment, we will measure the e.m.f. and the internal resistance of a dry cell.

Experiment 2A: Centre of gravity of a body(irregular shape only)

Objectives:

To determine the center of gravity of a body of different shapes

## Experimental Design

Apparatus:

Electronic Diagram

Description of design:

In this experiment, we will measure the e.m.f. and the internal resistance of a dry cell. In order to investigate the objective of the experiment, we should connect the apparatus as the above electric diagram. The voltmeter should be connected in parallel circuit while the ammeter should be connected in series circuit, otherwise, it may cause the inaccurate reading of the meters. Besides, we investigate the terminal potential difference V varies with the current I, hence we find out the internal resistance and the e.m.f by plotting the voltage – current graph. By vary the resistance of the rheostat R, the current I also varies. The terminal potential difference V across the dry cell is given by V =  – Ir.

Theory:

Electromotive Force (e.m.f) of a dry cell is the amount of electrical potential energy gained by a coulomb of charge which passes through the dry cell. Simply, A Voltage where the charge is gaining energy is an electromotive force. It is total opposite the potential difference (p.d), which is the energy released when a unit of charge passes between two points.

Dry cell is very common in daily life and has a wide variety of usages. In an ideal dry cell, there should be no any resistance. However, in the reality, there is no ideal dry cell and it must have an internal resistance. Internal resistance of a dry cell just like that there is a resistor in the dry cell. Internal resistance is a measure of the opposition of flow of charge in the power supply such as the dry cell in the experiment. When the electrons pass through the internal resistance part, they have to release energy.

From the data obtained in the experiment, we can find out the internal resistance and the e.m.f of the dry cell by both mathematical and graphical methods.

For mathematical method, 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 e.m.f by Ir, referred to as lost volts, and by combining the last two equations I get the following:

V = IR – Ir or V = E – Ir

For graphical method, we can plot a graph of Voltage against Current.  The graph should result in a straight line because the graph of V against I is proportional.  The equation for a straight line is Y = MX + C, which can be applied to the e.m.f formula V = E-Ir, given ...