MCAT Physics and Math Review

Chapter 6: Circuits

6.4 Meters

While we’ve been focused on calculations of hypothetical circuits so far, it’s important for us to spend some time considering real ones. While we will not analyze any complex circuits here, it is important to be familiar with meters, the devices that are used to measure circuit quantities in the real world.

AMMETERS

Ammeters are used to measure the current at some point within a circuit. Using an ammeter requires the circuit to be on, or the current will be 0 A. Ammeters are inserted in series where the current is being measured and use the magnetic properties of a current-carrying wire to cause a visible needle movement or a calibrated display of the current. If there is a particularly high current, this will overwhelm the ammeter, and a special low resistance shunt is used in parallel with the ammeter to allow a reading. Ideally, an ammeter will not change circuit mathematics when it is inserted into the circuit. To do so, it must have an extremely low resistance. Ideal ammeters have zero resistance and no voltage drop across themselves.

VOLTMETERS

voltmeter, like an ammeter, requires a circuit to be active. Voltmeters also use magnetic properties of current-carrying wires. However, voltmeters are used to measure the voltage drop across two points in a circuit. They are wired in parallel to these two points. Because the goal with any meter is to minimize its impact on the rest of the circuit, and voltmeters are wired in parallel, an ideal voltmeter has infinite resistance.

OHMMETERS

Unlike voltmeters and ammeters, an ohmmeter does not require a circuit to be active (in fact, some ohmmeters will give false readings or can be damaged by an active circuit). Ohmmeters will often have their own battery of known voltage and then function as ammeters through another point in the circuit. Because only one circuit element is being analyzed, Ohm’s law can be used to calculate resistance by knowing the ohmmeter’s voltage and the current created through another point in the circuit.

MCAT Concept Check 6.4:

Before you move on, assess your understanding of the material with these questions.

1.    What do each of the following types of meters measure? Where are they placed in a circuit? What is their ideal resistance?

2.    True or False: A voltmeter and an ammeter should not be placed in the same circuit.