Physics
Physics
Problem
A cell shows 1.60 V on a high-resistance voltmeter but its terminal voltage falls to 1.45 V when it drives a current through a lamp. Which statement correctly explains the fall in voltage? a) The emf of the cell itself immediately falls from to as the chemicals are used up when current flows. b) Part of the energy per coulomb is expended in overcoming the cell’s internal resistance when current flows. c) The high–resistance voltmeter draws an appreciable current, so it shows a larger reading than the true emf; once the lamp is connected the reading falls. d) When the lamp filament heats up its resistance rises, increasing the total circuit resistance and thereby reducing the terminal voltage of the cell.Intuitive explanation
The key is understanding internal resistance—every real battery has resistance inside it. When no current flows (high-resistance voltmeter), you measure the cell's true EMF of 1.60 V. But when current actually flows through the lamp, that current also passes through the internal resistance, causing a voltage drop inside the battery itself. This internal drop reduces what appears at the terminals, so the terminal voltage falls to 1.45 V. The difference tells you the battery isn't ideal—it has resistance fighting against its own output.
Intuitive explanation
The key is understanding internal resistance—every real battery has resistance inside it. When no current flows (high-resistance voltmeter), you measure the cell's true EMF of 1.60 V. But when current actually flows through the lamp, that current also passes through the internal resistance, causing a voltage drop inside the battery itself. This internal drop reduces what appears at the terminals, so the terminal voltage falls to 1.45 V. The difference tells you the battery isn't ideal—it has resistance fighting against its own output.