For the top pic,my take is that the voltmeter reading will
decrease.
wrong.
total e.m.f. of circuit = total p.d. of the components of the
circuit
This is because as the rheostat resistance increases,
current flowing through circuit deceases. which means that the
voltage over at the rheostat component remains relatively the same
since if R changes, Current will change too.(if R increases, I
decreases, and vice versa. So V should be the same) Looking over at
the fixed resistor we know that the resistance is the same at the
time and will not change regardless of resistance of rheostat, BUT
the resistance of rheostat WILL affect current flowing through
since this is a SERIES circuit. So my take is that the current
decreases but fixed resistor has same resistance. This will
decrease the voltage. And hence voltage of circuit decreases.
If rheostat increases in resistance, the voltage across it will
increase accordingly. And since you kept blabbering abt SERIES
CIRCUIT yet NEVER realised that current is constant
throughout a series circuit (physics law)
Now this is settled, let's look at the other ones.
2nd pic, V decreases.
3rd pic, V increases.
Any further queries?
