Internal Resistance and Potential Dividers

What Internal Resistance and Potential Dividers covers

Internal Resistance and Potential Dividers is examined in Unit AS 1, Forces, Energy and Electricity, and drawn on again in Unit AS 3, Practical Techniques and Data Analysis. It covers internal resistance and terminal potential difference, Consequences of internal resistance for external circuits, measuring electromotive force and internal resistance, the potential divider and variable output, potential dividers in lighting and heating control circuits and loaded potential dividers and output voltage calculations.

The 6 subtopics

Each one has its own page. CCEA AS Physics students sit this topic in the first year.

• Internal resistance and terminal potential differenceInternal resistance of a source of electromotive force, lost volts of a cell and terminal potential difference of a cell on open circuit.Open →
• Consequences of internal resistance for external circuitsRecall questions on why headlamps dim when the starter turns and why an old battery gives a dim torch, the greatest current a source can deliver, the power delivered to the load against that wasted internally, why efficiency falls as the external resistance falls, and why a school extra-high-tension supply has a deliberately large internal resistance.Open →
• Measuring electromotive force and internal resistanceRecall questions on the apparatus and method, what is plotted against what, the shape of the graph and why it is a straight line, what the intercept and the gradient each represent, a worked determination, the simpler two-reading method and why it is less reliable, and why the voltmeter must have a high resistance.Open →
• The potential divider and variable outputRecall questions on what a potential divider is, the divider equation and its ratio form, why the supply voltage divides in proportion to the resistances, using a single variable resistor as a divider, the advantage over a series variable resistor, worked calculations, and the effect of scaling both resistances together.Open →
• Potential dividers in lighting and heating control circuitsRecall questions on the light-dependent resistor and the thermistor as sensing elements, how the output changes with light or temperature, the effect of swapping the sensor and the fixed resistor, using the output to switch a lamp or a heater, a frost-warning circuit, and worked calculations for both sensors.Open →
• Loaded potential dividers and output voltage calculationsRecall questions on what loading means and why it lowers the output, the procedure of combining the load in parallel with the lower arm first, worked calculations for several load resistances, the condition for the loading effect to be negligible, why low-value resistors reduce it and what that costs, and the loading effect of a voltmeter.Open →

What you need to know

Answer each one before you reveal it.

Take Internal Resistance and Potential Dividers further

The same content, four other ways.

Questions from this topic

Each one with the model answer underneath.

1. 1

   Define the internal resistance of a source of electromotive force.

   Model answer

   The internal resistance of a source is the resistance of the source itself to the current it drives, so it is the resistance of the chemicals, electrodes or windings through which the charge must pass inside the source.

2. 2

   Why does every real source of electromotive force have some internal resistance?

   Model answer

   Charge has to travel through the material of the source itself, which is never a perfect conductor, so energy is dissipated inside the source as well as in the external circuit.

3. 3

   State the equation relating terminal potential difference to electromotive force and define every symbol with its unit.

   Model answer

   The equation is V = E − Ir, where V is the terminal potential difference in volts, E the electromotive force in volts, I the current in amperes and r the internal resistance in ohms.

4. 4

   What is meant by the lost volts of a cell?

   Model answer

   The lost volts are the quantity Ir, the potential difference across the internal resistance, which represents the energy per coulomb converted to internal energy inside the source rather than delivered to the external circuit.

5. 5

   Show how V = E − Ir follows from conservation of energy in a circuit of external resistance R.

   Model answer

   Each coulomb receives energy E from the source and gives up IR in the external circuit and Ir inside the source, so E = IR + Ir; since the terminal potential difference is V = IR, this rearranges to V = E − Ir.

6. 6

   A cell of electromotive force 6.0 V and internal resistance 0.50 Ω drives a current of 2.0 A. Calculate the terminal potential difference.

   Model answer

   The terminal potential difference is V = 6.0 − (2.0 × 0.50) = 5.0 V, and adding the lost volts of 1.0 V back to 5.0 V returns the 6.0 V electromotive force as a check.

What is inside Levo

Six ways to work the same material.

Internal Resistance and Potential Dividers revision questions

Is Internal Resistance and Potential Dividers examined at AS in CCEA Physics?

Yes. It sits in Unit AS 1, which you take in the first year, so it is on the AS paper as well as counting towards the full A level.

What does Internal Resistance and Potential Dividers cover in CCEA AS-Level Physics?

6 subtopics: internal resistance and terminal potential difference, consequences of internal resistance for external circuits, measuring electromotive force and internal resistance, the potential divider and variable output, potential dividers in lighting and heating control circuits and loaded potential dividers and output voltage calculations. Each one has its own page with questions and model answers.

How is Internal Resistance and Potential Dividers organised?

By the CCEA 1210 specification, subtopic by subtopic, so it matches the order you are taught it in.