What is the "lost volt" in a battery circuit, and how is it determined experimentally?

STATION 3: Determination of "lost volt"

The "lost volt" refers to the voltage drop across the internal resistance of a battery when current flows through the external circuit. It is the difference between the electromotive force (e.m.f.) of the battery and its terminal potential difference.

Diagram (2 marks)
A circuit diagram showing a battery, a switch, an external resistor, and a voltmeter connected in parallel across the battery terminals.

\begin{tikzpicture}[scale=0.8] % Battery \draw (0,0) to[battery] (0,2); \node[left] at (0,1) {Battery}; % Switch \draw (0,2) -- (2,2); \draw (2,2) to[switch] (4,2); \node[above] at (3,2) {Switch}; % Resistor \draw (4,2) -- (6,2); \draw (6,2) to[R] (6,0); \node[right] at (6,1) {Resistor}; % Connecting back to battery \draw (6,0) -- (0,0); % Voltmeter \draw (0,2) -- (0.5,2); \draw (0.5,2) |- (0.5,0); \draw (0.5,0) -- (0,0); \draw (0.5,2) to[voltmeter] (0.5,0); \node[right] at (0.5,1) {V}; \end{tikzpicture}

Method/Precaution (4 marks)

• Method:

  1. Set up the circuit as shown in the diagram, connecting the battery, switch, and resistor in series.
  2. Connect the voltmeter in parallel across the terminals of the battery.
  3. With the switch open, record the reading on the voltmeter. This reading represents the electromotive force ($E$) of the battery.
  4. Close the switch, allowing current to flow through the circuit.
  5. Record the new reading on the voltmeter. This reading represents the terminal potential difference ($V$) across the battery when current is flowing.
  6. Calculate the "lost volt" by subtracting the terminal potential difference from the e.m.f.: Lost Volt $= E - V$.

• Precaution:

  • Ensure all connections are tight to minimize contact resistance.
  • Read the voltmeter at eye level to avoid parallax error.
  • Do not leave the switch closed for an extended period to prevent the battery from draining quickly or the resistor from overheating.

Observation (2 marks)

• When the switch is open, the voltmeter shows a higher reading (e.g., $E = 1.5 \text{ V}$).
• When the switch is closed, the voltmeter reading drops to a lower value (e.g., $V = 1.3 \text{ V}$).

Inference (2 marks)

• The decrease in the voltmeter reading when the switch is closed indicates a voltage drop within the battery itself.
• This voltage drop, known as the "lost volt", is caused by the internal resistance of the battery when current flows through it.