Ohm’s law for a part of a circuit
If the resistance of a conductor is constant, the current grows as many times as the voltage across it: I ~ U. If the voltage is constant, the current falls as many times as the resistance grows: I ~ 1/R. In 1827 the German physicist Georg Ohm combined these results: for a part of a circuit the current is I = U/R, that is, the current in a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. From it U = I·R and R = U/I follow. The unit of resistance is the ohm: 1 Ω is the resistance of a conductor through which 1 A flows when the voltage across it is 1 V. The formula R = U/I does not mean that resistance depends on voltage: resistance is fixed by the conductor itself (l, S, substance). The smaller the resistance of an ammeter and the larger that of a voltmeter, the less they disturb the circuit.
Check a table: for a conductor let U = 2 V, 4 V, 6 V give I = 0.1 A, 0.2 A, 0.3 A. Calculate U : I for each column (20 Ω) and draw the graph of I against U — it is a straight line. Pencil and paper only.