Laboratory work: determining the EMF and internal resistance of a current source
The aim is to measure ℰ and r of a source. Equipment: a low-voltage source (a battery or school supply, not above 6 V), an ammeter, a voltmeter, a switch, a rheostat or 10 Ω and 20 Ω resistors, wires. Procedure: 1) assemble the circuit (voltmeter in parallel with the terminals, ammeter in series with the external resistance) and show it to the teacher; 2) with the switch open the voltmeter reading U₀ ≈ ℰ (the voltmeter resistance is much greater than the internal resistance); 3) close the switch, read the ammeter I and the voltmeter U, and open the switch immediately (so the source does not run down); 4) from Ohm’s law ℰ = U + Ir, so r = (ℰ – U)/I; 5) repeat with another resistance and average the results. Sources of error: instrument accuracy, contact resistance, heating of the source. Never short-circuit the source.
Only under the teacher’s supervision: assemble the circuit, close the switch only while reading, record U₀, U, I, ℰ, r in a table. Never use the mains (220 V) on your own.