In the circuit shown in fig. e26.31 the batteries have negligible internal resistance and the meters are both idealized. with the switch s open, the voltmeter reads 15.0 v. (a) find the emf of the battery. (b) what will the ammeter read when the switch is closed

Reading on the ammeter: .

Value of : .

Explanation:

The three resistors are connected to the power supply in parallel. Hence, the voltage across each one of them would be equal to the voltage of the power supply.

Apply Ohm's Law to calculate the voltage across the resistor at the center:

Value of this resistor: .

Current through this resistor:

By Ohm's Law, the voltage across this resistor would be .

Hence, by the reasoning above, the voltage of the power supply and the voltage across the other two resistors would all be .

Apply Ohm's Law (again) to calculate the current through the resistor, given that the voltage across that resistor is :

.

The ammeter is connected to the resistor in a serial configuration. Hence, the reading of the ammeter would be equal to the current through this resistor: .

Also because the three resistors are connected to the power supply in parallel, the current through the power supply would be equal to the sum of the current through each resistor.

Current through the power supply: .

Current through the and the resistor: and , respectively.

.

Hence, the current through the unknown resistor would be:

.

Apply Ohm's Law to find the value of resistor, given that the voltage across it is (same as the power supply) and that the current through it is :

.