Now that we know the current across each resistor, calculating the power dissipated is a
straightforward application of the formula P = I^2 R:
Common Devices in Circuits
In real life (and on SAT II Physics) it is possible to hook devices up to a circuit that will
read off the potential difference or current at a certain point in the circuit. These devices
provide SAT II Physics with a handy means of testing your knowledge of circuits.
Voltmeters and Ammeters
A voltmeter, designated:
measures the voltage across a wire. It is connected in parallel with the stretch of wire
whose voltage is being measured, since an equal voltage crosses both branches of two
wires connected in parallel.
An ammeter, designated:
is connected in series. It measures the current passing through that point on the circuit.
EXAMPLE
In the diagram above, = 9 V, = 5 , = 5 , and = 20. What are the values
measured by the ammeter and the voltmeter?WHAT DOES THE AMMETER READ?
Since the ammeter is not connected in parallel with any other branch in the circuit, the
reading on the ammeter will be the total current in the circuit. We can use Ohm’s Law to
determine the total current in the circuit, but only if we first determine the total
resistance in the circuit.