In this explainer, we will learn what an electric current is and how to determine the direction of an electric current in a circuit.
Electric current is the flow of electric charge. Recall that electric charge comes from the different parts of an atom, as seen below.
Together, the positively charged protons, shown in pink, and neutral neutrons, shown in green, make up the nucleus. The negatively charged electrons, shown in blue, are outside of the nucleus.
Electric current is the the flow of electric charge through an electrical conductor. A typical electrical conductor is a wire made of a metal such as copper, iron, or silver.
When discussing the flow of electric charge, it is the electrons that move through the wire. The protons and neutrons do not move. When we refer to electric current, we mean the flow of electrons traveling in the same direction along the same path. The diagram below shows a wire, with electrons moving through it.
Electrons are always inside of a wire, even when they are not moving. If the electrons are moving, then it means there is electric charge flowing through the wire. Electric current is the flow of electric charge through a wire.
If the electrons are in a wire but not moving, then electric charge is not flowing, as seen in the diagram below.
The electrons must have movement in order for there to be an electric current.
Let’s look at an example.
Example 1: Determining Which Parts of an Atom Flow in a Wire
The picture shows the electrons and atomic nuclei in a section of copper wire. The blue circles represent electrons and the red circles represent atomic nuclei.
- When there is an electric current in the wire, do the electrons move
along the wire?
- No
- Yes
- When there is an electric current in the wire, do the atomic nuclei
move along the wire?
- Yes
- No
Answer
Part 1
When there is an electric current in a wire, the electrons are moving. If the electrons are not moving, there is no electric current.
The answer is B: yes, the electrons move along the wire.
Part 2
When there is an electric current, the atomic nuclei stay still. Only the electrons move when there is an electric current.
The answer to the second part is B: no, the atomic nuclei do not move along the wire.
Electric current is measured in amperes, abbreviated as A. So 10 amperes can be written as 10 A. When there are 0 A of current in a circuit, there is no movement of the electrons at all.
When specifically discussing the movement of electrons in a wire, we can refer to it as electron current, also called electron flow.
Electrons are negative, so they flow toward the positive terminal of a cell and away from the negative terminal, as seen in the diagram below.
The movement of electrons are what create electric current, since no other charged particle is moving.
Let’s look at example question.
Example 2: Finding the Direction of Electron Flow in a Circuit
The diagram shows an electric circuit containing a cell and a bulb.
In what direction do electrons flow around the circuit?
- Clockwise
- Counterclockwise
Answer
The direction of electron flow in a circuit is away from the negative terminal of a cell and toward the positive terminal.
The shorter side of the cell at the top of this circuit is its negative terminal. The longer side of the cell is the positive terminal. The electron flow in this circuit will thus be moving counterclockwise, like in the circuit diagram below.
The correct answer is B: counterclockwise.
Though the names are very similar, electron current and electric current are not the same. Electron current is the flow of electrons in a wire, and electric current is the flow of charge carriers in a wire.
Early scientists did not know that electrons flowed in a circuit when electric current was present. When these scientists wrote about the flow of electric charge, they assumed that the electric charge flow came from positively charged particles.
This means that they measured the direction of the electric charge carriers from the positive terminal to the negative terminal of a cell. Such an electric flow is shown in the diagram below.
The convention these scientists set is still used today as the default direction of electric current. This default current direction is called conventional current.
The direction of conventional current is the opposite of electron current, since it assumes positive charge carriers. In reality, no positive charges flow at all; they are purely fictitious. Only the electrons move in a wire. A diagram comparing conventional current with electron current is shown below.
Electron current, or electron flow, specifically refers to the flow of electrons. Electric current is more general, as it refers to the flow of charge carriers, and, by default, it assumes the direction of conventional current.
Let’s look at some example questions.
Example 3: Finding the Conventional Current Direction from Electron Flow
The picture below shows the electrons and atomic nuclei in a section of copper wire. The blue circles represent electrons and the red circles represent atomic nuclei. There is an electric current in the wire, and the electrons in it are moving to the right.
What is the direction of the conventional current in the wire?
- To the right
- To the left
Answer
The flow of the negatively charged electrons in the diagram is to the right, but conventional current assumes that the charge carriers in the current are positive.
This means that the conventional current direction is in the opposite direction of the electrons. This would be to the left, since the electrons are flowing to the right.
The correct answer is B: to the left.
Example 4: Determining the Conventional Current Direction in a Circuit
The diagram shows an electric circuit containing a cell and a bulb.
What is the direction of the conventional current in the circuit?
- Clockwise
- Counterclockwise
Answer
Conventional current direction in a circuit assumes positive charge carriers. This means the flow of these charges would be from the positive terminal of a cell to the negative terminal.
The positive terminal of the cell, the longer line, is pointed downward. Positive charges would then have to flow counterclockwise, meaning the correct answer is B.
Whether a charge carrier is the fictitious positive charge in conventional current or the electron in electron current, charge must flow in order for there to be electric current.
It does not matter how much charge is present, only that it is moving. The diagram below shows two circuits, one with many more electrons than the other.
Both circuits have a current of 0 A when the electrons do not move, regardless of how many electrons are there.
Electrons are also not used up as current decreases in a circuit. All the electrons are still present; they just move slower. If the current decreases to 0, it means the electrons have stopped moving entirely. The diagram below shows electrons moving through a light bulb, powering it and causing it to light up.
As the electrons move through the light bulb, the light bulb does not use them up or cause them to disappear. The motion of electrons is what powers the bulb, so if the electrons move slower, the bulb will be more dim. If the electrons stop moving entirely, the bulb will not emit any light at all.
Let’s look at some example questions.
Example 5: Describing the Number of Electrons in a Circuit After Operation
The diagram shows an electric circuit containing a cell and a bulb. This circuit is set up on a workbench and left on for 1 hour.
At the end of the hour, are there more electrons, fewer electrons, or the same number of electrons in the wires of the circuit than at the start of the hour?
- There are more electrons in the wires.
- There are fewer electrons in the wires.
- There are the same number of electrons in the wires.
Answer
When the circuit is on, the electrons move around the circuit, powering the light bulb as they move through it.
The electrons power the bulb through their motion. Over the course of 1 hour, some electrons may have slowed down from powering the light bulb, but there are still the same number of electrons total in the circuit.
At the end of the hour, after powering the light bulb, there are the same number of electrons in the wires. The correct answer is C.
Example 6: Determining the Reason for a Dimming Bulb
The diagram shows an electric circuit containing a cell and a bulb. This circuit is set up on a workbench and left on for 1 hour.
Over the course of the hour, the bulb gradually becomes dimmer. Which of the following statements correctly explains why?
- The number of electrons in the cell decreases over time, so there are fewer electrons that can flow around the circuit.
- The amount of energy in the cell decreases over time, so there is less energy that can be converted into light by the bulb.
Answer
When the circuit is on, the electrons move around the circuit, powering the light bulb as they move through it.
When a bulb dims, it is not because there are fewer electrons in the circuit, there are just fewer electrons in motion able to power it. The number of electrons in the circuit remains the same.
As the energy of the cell slowly runs out, it is unable to push as many electrons through the wire, and thus through the bulb. The correct answer is B.
Let’s summarize what we have learned in this explainer.
Key Points
- Electric current is the flow of electric charge and is measured in amperes.
- Conventional current assumes that charge carriers are positive, meaning they flows away from positive terminals and towards negative terminals.
- Electron current is the actual flow of electrons, which flow in the opposite direction of conventional current.
- Electrons are not destroyed or used up in a circuit when there is no current; they just stop moving.
