The volt is named in honor of Alessandro Volta, who invented the first chemical battery in 1800. Volta discovered how to convert chemical energy into electrical energy by alternating plates of two different metals in a dilute acid or salt solution. In practical terms, a volt (V) tells you the amount of electrical power carried by one ampere of current. For example, each ampere of current flowing out of a 12 V battery carries 12 W of electrical power. The same ampere of current flowing out of a 120 V wall socket carries 120 W of electrical power. Voltage in a circuit is measured with a voltmeter.
More on Volta's invention of the battery
By the middle of the 18th century a number of researchers, including Benjamin Franklin, had begun to understand the basics of static electricity and knew how to charge an object—just like you do when you drag your feet across a rug on a dry day. The problem was that objects charged by static electricity would discharge very quickly, preventing 18th-century researchers from studying electric current. You can experience this yourself when you touch a doorknob and create a shock; the shock is instantaneous; there is not enough current to hurt you or allow you to power an electric circuit such as a flashlight. Static electricity was a poor source of electric current because it would not last long enough to study.
Volta discovered that chemical energy can be converted into electricity when two different metals are placed in a dilute acid or salt solution. By connecting more than one of these chemical devices together, Volta created the first electrical battery. We now know that his device created an electrical potential difference between the two metals, similar to the difference in gravitational potential energy between the top and bottom of a dam. We now call that potential difference by the name voltage, and we measure it in units of the volt, in honor of Volta’s discoveries.
How are voltage and current related?
Current is what flows and does work. Voltage, in contrast, is what causes current to flow. The idea of water pressure in pipes makes a good analogy for voltage. A pressure difference causes water to flow in a pipe, and voltage causes electric current to flow in a wire. If there is no voltage, then no current flows. You can connect a loop of wire to a bulb in a closed circuit but current will not flow unless there is a battery or some other voltage source in the circuit. Current only flows in response to a voltage difference.
The term potential difference is another way to describe voltage. A 1.5 V battery has a potential difference of 1.5 V between its positive and negative terminals. Electrical potential difference is analogous to the difference in potential energy for water at two different heights.
What does the sign of voltage mean?
Voltage can only be measured between two points, such as the two terminals of a battery. If we measure the voltage of an ordinary 1.5 V battery from the flat (−) end to the slightly protruding (+) end, we would record a voltage gain of +1.5 V, indicating an increase in electric potential. If we swapped the leads on our voltmeter so that we measured from the + to the − end of the battery, we would measure a voltage drop of −1.5 V. When we say that a battery is “1.5 V” we are implicitly choosing the negative end of the battery to be our reference of 0 V.
What does nominal mean here?
If you measure the actual voltage of a fresh “1.5 V” alkaline battery it will be about 1.62 V. The voltage of a battery drops if the current flowing out of the battery increases and also as the battery ages and the chemical energy in the battery is used up. The term “nominal” means that the battery voltage is designed to stay above 1.5 V for up to a certain amount of current for a certain amount of time.
Which direction does current flow?
By convention, electric current flows from high (more positive) potential to low (more negative) potential. In circuit diagrams, an arrow is often used to indicate the direction of current flow. “Conventional” current flows out of the positive terminal of a battery and returns to the negative terminal. In most conductors it is actually negative electrons that move to carry electric current. Because electrons are negatively charged, the electron flow is opposite from the direction of conventional current.
Test your knowledge
Jermaine connects a resistor across a battery. As the current passes through the battery, is this a voltage gain or a voltage drop? As the current passes through the resistor, is this a voltage gain or a voltage drop?
Passing through the battery is a voltage gain. The battery acts like a pump, moving charges from the low (−) potential end to the high (+) potential end of the battery.
Passing through the resistor is a voltage drop. Charges lose electric potential energy as they flow through the resistor and back to the negative terminal of the battery.