To understand how to measure current and voltage in a circuit, you must also have a general understanding of how a circuit works and how its electrical measurements are related.
An electrical circuit is a type of network that has a closed loop, which provides a return path for the current. A simple circuit consists of a voltage source and a resistor and can be schematically represented as in .
A Simple Circuit
A simple electric circuit made up of a voltage source and a resistor
According to Ohm's law, The electrical current I, or movement of charge, that flows through most substances is directly proportional to the voltage V applied to it. The electric property that impedes current (crudely similar to friction and air resistance) is called resistance R. Collisions of moving charges with atoms and molecules in a substance transfer energy to the substance and limit current. Resistance is inversely proportional to current. Ohm's law can therefore be written as follows:
where I is the current through the conductor in amperes, V is the potential difference measured across the conductor in volts, and R is the resistance of the conductor in ohms (Ω). More specifically, Ohm's law states that R in this relation is constant, independent of the current. Using this equation, we can calculate the current, voltage, or resistance in a given circuit.
For example, if we had a 1.5V battery that was connected in a closed circuit to a lightbulb with a resistance of 5Ω, what is the current flowing through the circuit? To solve this problem, we would just substitute the given values into Ohm's law: I = 1.5V/5Ω; I = 0.3 amperes. If we know the current and the resistance, we can rearrange the Ohm's law equation and solve for voltage V: