Molly Ayala
The concept of electricity and the role of negative and positive charges within it is a fundamental part of physics. Electric charges produce electric fields, and a moving charge produces a magnetic field. The interaction of these charges with an electromagnetic field results in the electromagnetic force, one of the four fundamental forces in physics. The negative charge is defined as the charge carried by an electron, and the positive charge is carried by a proton. The total electric charge of an isolated system remains constant, regardless of changes within the system. This is known as the conservation of charge. The convention of electricity flowing from positive to negative was determined by Benjamin Franklin, although he did not know which charge was moving. This convention does not align with the movement of electrons, which carry a negative charge.
| Characteristics | Values |
|---|---|
| What does a minus sign in electricity indicate? | A minus sign in electricity indicates a negative charge, which is the charge carried by an electron. |
| Why is there a minus sign in the definition of electric potential? | The minus sign is due to the work done by an external force, not the electric field. The kinetic energy of the charged particle is 0, so the energy it possesses is potential energy. |
| Why is electricity shown to flow from plus to minus when mediated by electrons in the opposite direction? | This convention was determined by Benjamin Franklin, who guessed that protons moved and electrons remained in place. |
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What You'll Learn
- The negative sign in electric potential is due to the work done by an external force, not the electric field
- Benjamin Franklin defined positive and negative charges, but he guessed wrong about which charge was moving
- Conventional current shows electricity flowing from positive to negative, but electrons move in the opposite direction
- The total energy kinetic + potential is constant over time, which is why there is a minus in the definition of electric potential
- The negative charge is defined as the charge carried by an electron, while the positive charge is carried by a proton
The negative sign in electric potential is due to the work done by an external force, not the electric field
The negative sign in electric potential is indeed due to the work done by an external force, not the electric field. This is because, during the process of deriving electric potential, we consider the work done by an external force, not the electric field. When a conservative force does negative work, the system gains potential energy.
In the context of electric potential, the work done by an external force is given by the equation:
$$\co: 7> \Delta V = \int (-q \vec{E}) \cdot d\vec{l} = -q \int \vec{E} \cdot d\vec{l}$$
Here, $-q \vec{E}$ represents the work done by the external force, and the negative sign is included to indicate that the external force is doing work on the charged particle. This is fundamentally different from the electric field, which does work on the charged particle when there is relative motion between the particle and the field.
The presence of the negative sign in the electric potential equation is crucial for maintaining the constancy of the total energy (kinetic + potential) over time. If the potential energy were defined with an opposite sign, it would lead to the conservation of the energy difference (kinetic - potential), which is less natural and less physically meaningful.
Furthermore, the negative sign in electric potential is also related to the direction of the electric field. When the electric field is directed from a higher potential to a lower potential, the direction is considered negative. This is consistent with the convention that electron flow goes from the negative end to the positive end, and it reinforces the understanding of the negative sign in electric potential as being associated with the work done by an external force.
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Benjamin Franklin defined positive and negative charges, but he guessed wrong about which charge was moving
Benjamin Franklin is known for his experiments with static electricity, specifically his experiments involving rubbing glass and hard rubber with silk and fur. He would rub a glass rod with silk and a rubber rod with fur, and then hang the rods from a string. He found that if he rubbed two glass rods with silk and brought them together, or did the same with two rubber rods with fur, they would repel each other. However, if he brought a glass rod rubbed with silk and a rubber rod rubbed with fur together, they would attract each other.
Franklin imagined electricity as a type of invisible "electrical fluid" that could build up or be absent from a material. He believed that when this invisible fluid built up, the object was positively charged, and when there was an absence of this fluid, the material was negatively charged. He decided that in electrostatic experiments, what was in reality the negatively charged body was the body that had a lack of electric fluid. However, he got the nomenclature backward; in reality, a positive charge flow is also a current.
Franklin's "one fluid" theory held that electricity flowed within and between objects – excess fluid made some objects positive and a dearth of fluid made others negative. Charge could be moved around but not created or destroyed. He suggested that an excess of this "electrical fluid" be called "positive electricity" and a deficiency of it be called "negative electricity". This suggestion, with some modifications, forms the model we use today. However, it is important to note that Franklin's guess about which charge was moving was wrong, and he had no way of actually determining the sign of the moving charge.
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Conventional current shows electricity flowing from positive to negative, but electrons move in the opposite direction
The concept of conventional current and the actual movement of electrons in a circuit can be confusing, as they are opposite in direction. Conventional current refers to the flow of positive charges from the positive terminal to the negative terminal in an electrical circuit. On the other hand, electrons, which are negatively charged, move in the opposite direction, from the negative terminal to the positive terminal.
This discrepancy arose from early understandings of electricity. Benjamin Franklin, for instance, assumed that electric charge moved from the smooth wax to the rough wool when these two substances were rubbed together, which is the opposite of the actual direction. By the time the true direction of electron flow was discovered, the nomenclature of "positive" and "negative" was already well-established, and it was decided that changing it would be unnecessary and inconvenient.
The terms "positive" and "negative" are associated with "surplus" and "deficiency," respectively. Since electrons are negatively charged, they move from a negative to a positive terminal, which indicates a movement from a point of excess electrons to a point of deficiency. This movement of electrons is what defines electricity. However, conventional current, which is followed by most electrical engineers, maintains the opposite direction of flow due to historical conventions.
It is important to note that both conventional current and electron flow notations are valid ways of understanding and analyzing electrical circuits. Conventional current is often preferred due to its simplicity and the fact that it aligns with the standard labels of positive and negative. Meanwhile, electron flow notation follows the actual movement of electrons but results in seemingly backward labels. Ultimately, as long as consistent symbols are used, both methods can be equally successful for circuit analysis.
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The total energy kinetic + potential is constant over time, which is why there is a minus in the definition of electric potential
The concept of energy conservation is fundamental in physics, and it states that energy cannot be created or destroyed, only transformed from one form to another. This principle applies to both potential energy (PE) and kinetic energy (KE). The total energy of a system, which is the sum of its kinetic and potential energy, remains constant over time. This conservation of energy is represented by the equation:
> KE + PE = constant
Now, let's delve into the relationship between electric potential and the total energy being constant over time. In the context of electricity, potential energy is associated with stored electrical energy, such as in a battery. When a positive charge is accelerated by an electric field, it gains kinetic energy. This process is similar to an object gaining kinetic energy as it falls down due to gravity. The electric potential energy is converted into kinetic energy, and this relationship is described by the equation:
> ΔV = −q ∫ E ⋅ dl
The negative sign in this equation is significant. It arises because, in the definition of electric potential, we consider the work done by external forces rather than the electric field itself. When an external force acts on a charged particle, the kinetic energy of the particle is momentarily zero, so the particle's energy is purely potential energy. By introducing a negative sign, we account for the fact that the external force might be greater or smaller than the force due to the electric field, resulting in a net force that accelerates the charged particle.
The negative sign in the electric potential equation ensures that the total energy, the sum of kinetic and potential energy, remains constant over time. If the potential energy were defined with a positive sign, it would imply that the energy difference between kinetic and potential energy is conserved, which is less intuitive and natural. Therefore, the negative sign in the electric potential definition aligns with the principle of energy conservation and our understanding of how energy transforms between potential and kinetic forms.
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The negative charge is defined as the charge carried by an electron, while the positive charge is carried by a proton
The concept of negative and positive charges is fundamental to our understanding of electricity and electromagnetism. The negative charge is defined as the charge carried by an electron, while the positive charge is carried by a proton. This definition is central to the study of electric fields and magnetic fields, which are produced by the interaction of electric charges with an electromagnetic field.
The understanding of negative and positive charges has evolved over time. Before the discovery of electrons and protons, in the late 1700s, Benjamin Franklin defined a positive charge as the charge acquired by a glass rod when rubbed with silk cloth. However, Franklin did not know which type of charge was moving and, as later discovered by J.J. Thomson in 1897, it is the electrons that carry a negative charge and move within an electric current. This movement of electrons is from the negative end to the positive end of a voltage source, which results in the creation of an electric current.
The convention of electricity flowing from positive to negative, as established by Franklin, has been maintained despite this correction in our understanding. This is because, from an analytical perspective, it does not significantly impact the understanding of electric currents. The behaviour of negative particles moving to the left is similar to that of positive particles moving to the right. Therefore, the convention is considered a useful simplification for understanding electric currents.
The negative charge of an electron is denoted as -e, and it serves as the fundamental unit of electrical charge. The charge of an isolated system, such as a closed surface, is expressed as a multiple of this elementary charge. This is true even when charge seems to behave as a continuous quantity at larger scales. The SI unit of electric charge is the coulomb (C), named after French physicist Charles-Augustin de Coulomb.
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Frequently asked questions
A minus sign indicates that you have a credit balance and that you do not need to make a payment.
The minus sign is used because the work done by an external force is considered while deriving electric potential and not the work done by the electric field.
Benjamin Franklin, while determining the convention, guessed that protons moved and electrons remained in place. However, it was later discovered that electrons carried a negative charge and moved in the opposite direction. The convention was stuck with and has been in use since.
A negative charge is defined as the charge carried by an electron.
Yes, we could start calling electrons positive and protons negative. However, there is little motivation to change the convention as it does not matter if you have positive charges flowing clockwise or negative charges flowing counter-clockwise.
