What Is a Voltage Drop in an Electric Circuit? - Technical Articles

In this "Common Engineering Questions", we will briefly explain the voltage drop concept and discuss the polarity of the voltage drop across resistors, capacitors and inductors.

A battery converts chemical energy into electrical energy, thereby generating a voltage (ie, a potential difference) between its two terminals. A resistor is a component that produces a specified amount of resistance to current. When we connect the two terminals of the resistor to the two terminals of the battery, charge carriers move in the circuit, so we call it current.

Voltage conveys the ability to complete the movement of charge from one point to another. For example, a 5 V battery can complete 5 joules of work per coulomb of charge. When current flows through a resistor, we can measure the work (per unit charge) required to keep the current flowing through the resistor.

This is the essence of voltage drop: battery (or voltage source)

The energy required to move the charge. When current flows, components such as resistors

Energy, and the amount of work per unit charge associated with the current flowing through a given component, is the voltage drop of that component.

The voltage dropped by the components accounts for part of the voltage generated by the battery. In other words, the work performed by the battery is distributed among the various components in the circuit.

We can intuitively realize that driving a given amount of current through a larger resistance will require more work. Therefore, if two resistors are connected in series (which means they have the same current), the resistor with the larger resistance will have a greater voltage drop. This is the basis of operation

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The resistor always acts as a load, that is, a component that consumes energy. If we use the traditional current model, where current flows from a higher voltage to a lower voltage, the voltage drop across the resistor is positive when the current enters the resistor, and negative when the current flows out of the resistor:

This polarity is "opposite" to the power supply voltage: if we connect the battery in the same polarity direction, it will drive current in the opposite direction (or it will cancel out the power supply voltage, depending on your thinking).

Capacitors and inductors store energy, so they can be used as loads or power sources. When they act as loads, their step-down polarity is the same as the resistance.

The polarity of the voltage drop of the capacitor does not change when it starts to discharge. Even if it acts as a power source, it still generates a current in the opposite direction to the charging current.

However, when the inductor discharges, it will try to keep current flowing. Therefore, the polarity of the inductor voltage drop will change because it is generating current in the same direction as the charging current generated by the power supply.

Do you have any other questions about voltage drop? Share your query in the comments below.

It is appropriate to emphasize that the driving force or EMF (in volts) produced by a battery or power supply is achieved by a chemical reaction on the battery, which is achieved by effectively accumulating negative charges (electrons) on a board. The other leaves a positive charge. After connecting the load, the natural driving force is for electrons to travel in the circuit and return to the positive plate to rebalance the situation. In fact, this drive is EMF. In the process of flowing through the "load", the electrons are depleted from the negative electrode, but are replaced by the chemical reaction of the electrons from the positive plate, so the current must flow through the internal chemical composition. The battery also has flow resistance. Therefore, the circuit shown above should show a source resistance so that the new terminal voltage on the battery terminals can be estimated, which will now drop depending on the amount of source resistance. As the internal chemical composition gradually degrades, the internal resistance will increase until the terminal voltage drops to the point where we think the battery is flat.

Although the term "voltage drop" is commonly used in the analysis of electronic circuits, it makes people confused about circuit science. It often leads to things like

"Why did the voltage drop?"

"What is the voltage drop at the base of the transistor?"

It is best to avoid using the term "voltage drop" in the entry level of electrical and electronic instructions.

A preferred expression is "potential difference", for example, there is a potential difference between point A and point B. In other words, the voltage will not drop. It will not disappear anywhere in the circuit. It is always there. It only depends on the measurement method and location.

This is a hypothetical example. Connect the 0 ohm load to the 12V battery. What is the "voltage drop" on the load? Did it suddenly disappear to 0V?

Instead, we can measure the potential difference between the battery terminal and the load terminal through experiments. We can then make theoretical assumptions about any surprises in the observed measurement.

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