Core Practical 8: Investigating E.M.F. & Internal Resistance
Series resistance modelling treats the power supply as an ideal e.m.f. source in series with an internal resistance , which ensures that all circuit current passes through both elements. This explains the linear relationship between terminal voltage and current.
Ohm’s law across internal resistance shows that the voltage drop inside the cell is , meaning internal resistance directly reduces the voltage available to the external load. As current increases, this internal drop becomes increasingly significant.
Energy distribution principle states that the e.m.f. equals the sum of terminal p.d. and lost volts. This decomposition highlights how energy is split between external work and internal heating.
Linear V–I relationship arises from rearranging the cell equation as , which produces a straight line with negative gradient. This means experimental data can be used to extract and through a simple graphical method.
E.m.f. vs terminal p.d. highlights the difference between ideal and real behavior: e.m.f. is the maximum theoretical energy per charge, whereas terminal p.d. is the energy actually delivered during operation.
Internal vs external resistance distinguishes between unavoidable energy loss due to cell structure and intentional load resistance that performs useful work. Confusing these often leads to incorrect circuit modelling.
Open‑circuit vs closed‑circuit voltage separates the true e.m.f. reading from the voltage reduced by internal effects. Only with no current present does measured voltage equal e.m.f.
Gradient interpretation differentiates the negative slope representing internal resistance from the intercept representing e.m.f., helping avoid misreading of the linear graph.
Check axis placement by ensuring voltage is always on the vertical axis and current on the horizontal axis, because reversing these prevents meaningful interpretation of intercepts and gradients.
Look for a linear trend and avoid forcing a curved fit when drawing the best‑fit line, as small experimental imperfections do not invalidate the theoretical linear relationship.
Use large triangles for gradient to reduce percentage error in calculating internal resistance. Small triangles exaggerate reading uncertainties.
Verify physical realism by checking that e.m.f. exceeds all measured terminal voltages and that internal resistance is positive, which acts as a quick validity check.
Assuming terminal voltage equals e.m.f. under load leads to underestimating internal resistance because students forget that real cells always lose energy internally.
Leaving the switch closed for too long can heat the cell and alter internal resistance, giving misleading data that appears curved instead of linear.
Misinterpreting gradient sign often causes students to report negative resistance values, forgetting that the negative slope must be negated to obtain internal resistance.
Using inconsistent units introduces systematic error; all voltage should be in volts and current in amperes to preserve the correct slope interpretation.
Link to power dissipation because knowledge of internal resistance allows prediction of maximum power transfer, which occurs when load resistance equals internal resistance.
Connection to battery performance as real batteries degrade over time, increasing internal resistance and reducing useful terminal p.d. under load.
Relevance to renewable systems such as solar cells, where internal resistance modelling determines optimal load matching.
Integration with circuit analysis because understanding e.m.f. and internal resistance is foundational for Kirchhoff’s laws and power calculations in complex circuits.