It is Easy to Produce a Potential Difference
In this explainer, we will learn how to define electric potential difference and how an electric potential difference across a component in a circuit creates a current in that component.
Electric potential difference (or simply "potential difference") is one of the most important concepts that we use when talking about electricity. Understanding potential difference is essential if we want to describe how circuits and electrical devices work.
To understand potential difference, we need to look at how charge can flow in certain materials. Electrical devices rely on the flow of charge through components. This flow of charge is called current. Materials that allow a current through them (e.g., copper) are known as conductors.
Let's take a closer look at how conductors behave. A conductor is made up of positively charged atomic nuclei (shown in red) and negatively charged electrons (shown in blue). The atomic nuclei are fixed in place.
Recall that objects with opposite charges attract each other and objects with the same charge will repel each other. In many materials, the negative electrons are strongly attracted to the positive nuclei, which means each electron stays closely bound to one of the stationary nuclei. But this is not true for conductors! Conductors contain electrons that are able to move freely. This means that when we say there is a current in a conductor, electrons are flowing.
All electrons are negatively charged, which means they repel each other. This means that they tend to spread out and keep as far away from each other as possible.
We know that the flow of electrons is essential for electrical devices. But what causes electrons to flow? Let's imagine that we added some extra electrons on the left end of our conductor.
Now that we have added extra electrons, we can see that they are "bunched up" on the left. These electrons all repel each other, so they will spread out to get away from each other. Though the individual electrons may move by different amounts and in different directions, there is an overall flow of electrons from left to right.
(Here, we can recall that in physics we often talk about "conventional current." Conventional current is defined in the opposite direction to the actual flow of electrons. So, in this example where electrons are flowing to the right, we would say that the direction of the conventional current is to the left.)
So, what does all of this have to do with potential difference? Well, when the electrons are "bunched up," we have seen that they will naturally spread out in order to evenly fill the available space. In this case, this causes the electrons to move from left to right. Whenever electrons are being made to move from one point to another, we can say that there is a potential difference between these points. In other words, potential difference describes how hard electrons are being driven from one point to another.
Potential difference is always measured between two points. When there is a potential difference between two points, this means that the charge will tend to flow from one point to another. In this example, the "bunching up" of electrons that repel each other means that there is a potential difference between the left end and the right end of the conductor.
In this case, once the electrons have moved and spread themselves out, there is no longer anything causing them to flow in any particular direction. So, once this has happened, there is no longer a potential difference between the ends of the conductor.
Example 1: Describing the Effects of a Potential Difference within a Conductor
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.
An electric potential difference is set up between the left-hand end and the right-hand end of the wire. Which of the following statements best describes what will happen in the wire?
- Nothing will happen.
- The electrons will move outside of the wire.
- The electrons will move toward the center of the wire.
- The electrons will begin to move toward one end of the wire.
Answer
First, let's note that wires are conductors. This means that the electrons in the wire are free to move throughout it.
Looking at the diagram, we can see that the electrons are, more or less, evenly distributed throughout the conductor. This means that if we are just going by the diagram, we might assume that nothing will happen!
However, in this question we are told that a potential difference is set up between the left and the right ends of the conductor. We do not need to know what the cause of this potential difference is. In order to answer the question, we need to know how a potential difference will affect the electrons in the conductor.
Let's recall that a potential difference is always measured between two points. The existence of a potential difference between these two points means that the current will flow from one point to the other, if possible.
The fact that we are told there is a potential difference between the two ends is enough for us to conclude that electrons will flow from one end of the wire to the other if they are able to, and there is nothing stopping them. In other words, option D is correct: The electrons will begin to move toward one end of the wire.
Example 2: Identifying the Direction of a Current Caused by a Potential Difference
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.
An electric potential difference is set up between the left-hand end and the right-hand end of the wire. This creates an electric current in it. Which of the arrows best shows the direction of the current in the wire?
Answer
Here, we are told that a potential difference is set up between the left and right ends of the wire shown. The existence of a potential difference between two points means that electrons will tend to flow from one point to the other. In other words, there will be a current from one point to the other.
In this case, since there is a potential difference between the ends of the wire, we know that this will create a current from one end of the wire to the other. However, we do not know which direction this will be: the right or the left.
Looking at the answer options, we can see that we cannot pick "left" as an option. This means we know the correct answer is iii: the current will go to the right.
Potential difference is measured in volts, represented by the symbol V. We also use the symbol to represent potential difference in equations. For this reason, potential difference is often referred to as "voltage."
Certain devices are capable of producing a potential difference. One of the most common in physics is the cell. We can think of a cell as a device that pushes electrons out of one terminal and pulls them in at the other terminal.
If we connect the terminals of a cell together with a conductive wire, as shown in the circuit diagram below, a current will be produced in the wire in the direction shown by the arrows.
The following diagram shows the same circuit with a light bulb connected to it.
If a light bulb is connected to this circuit, then the potential difference between the terminals of the cell will produce a current through the light bulb, causing it to light up.
By supplying a potential difference to a circuit, a cell must do work on electrons in the circuit. This is another way of saying that a cell must give energy to the electrons in order to move them around a circuit. The size of a potential difference, measured in volts, describes the amount of energy that will be exerted on each unit of charge that flows around the circuit.
Volts can also be expressed as joules (the unit of energy) per coulomb (the unit of charge):
This means that, for example, a battery with a potential difference of 9 volts between its terminals will expend 9 joules of energy on every coulomb of charge that it moves around a circuit. A cell with a higher potential difference between its terminals will do more work on each unit of charge: it pushes the electrons harder and generally produces a larger current.
Key Points
- Electric potential difference is always measured between two points. It describes how hard electrons are being driven from one point to another.
- Electrons in conductors are free to flow around. If there is a potential difference between two points and these points are connected by a conductor, charge will flow from one point to the other.
- Potential difference is measured in volts, which can be represented by the symbol V.
- We can also think of potential difference as the amount of energy used to move a volt of charge between two points. 1 volt is equivalent to 1 joule per coulomb.
Source: https://www.nagwa.com/en/explainers/107136192625/