Question

How would changing the battery in a circuit from 9 volts to 1.5 volts most likely affect the circuit?

Answer 1

That change would most likely cause the current at every point in the circuit
to decrease by 83-1/3% ( 5/6 ) of its original value. Also, the voltage across
each component of the circuit ... resistor, light bulb, motor, open switch etc. ...
would become 5/6 less.