Have you ever wondered what causes lightning to strike when there is a storm? Why, in an atom, do electrons revolving around the nucleus not fly away, even though they are zooming at such high speeds?
Both of these are due to the same phenomenon, called an electric field.
Electric fields definition
An electric field is an area in space around an electric charge, in which another electric charge will experience a force. The electric field is measured in units of NC-1 (Newtons per Coulomb).
Electric charges exert forces on each other. Analogous to magnetic poles, like charges push each other away, and opposite charges pull each other together. A positively charged particle will attract a negatively charged particle, while repelling another positively charged particle. This relation of force on charges in an electric field is given by the formula:
Here,
E - Magnitude of electric field,
F - Force experienced by charge q,
q - A point charge.
If you redistribute the equation, you can deduce that a larger amount of force will be experienced by a larger charge. You can also see why the electric field's units are NC-1: force is measured in Newtons, and charge is measured in Coulombs, so F/q ≈ N/C.
Electric fields can be visualised as electric field lines. Electric field lines emerge from a positively charged particle but converge into a negatively charged particle. Electric fields for a positive particle emerge and continue on infinitely, while the fields of negatively charged particles originate from infinity and converge on the particle itself.
Fig. 1 - Electric field around a Point Positive Charge
Fig. 2 - Electric field around a Point Negative Charge
Fig. 3 - Electric Field Between two Oppositely Charged Parallel Plates
It is important to know that an electric field is a vector quantity.
Vector is a quantity having both a magnitude and a direction, while a scalar quantity only has a magnitude.
Therefore, electric fields may also be visualised using vectors. The resultant electric field at any point in space is the sum of all electric field vectors at that point.
Properties of electric fields
Electric fields exhibit the following properties:
Electric field lines never intersect each other. If they did, it would mean that some point in space would have an electric field in two directions simultaneously, which is not possible.
Electric fields are strong where the electric field lines are closer to each other, and vice versa.
Electric fields are always perpendicular to the charged surface.
The number of electric field lines is proportional to the electric charge.
Electric field lines originate from the positive charge and terminate at a negative charge.
If there is only a single charge present, the electric field starts or ends at infinity.
It's easy to compare electric fields with magnetic fields. However, they do have some dissimilarities.
- Electric field lines emerge from positive charge. Magnetic field lines emerge from the north pole.
- Electric field lines converge into negative charge. Magnetic field lines converge into the south pole.
- Electric field lines never intersect, which is a similar property to that of magnetic field lines.
- Both electric field lines and magnetic field lines are vector quantities i.e., they have a magnitude and a direction.
- The density of field lines decreases as they spread out for both electric and magnetic fields.
One stark difference is that magnetic field lines can only exist when there is a magnetic dipole i.e., both north and south poles are present. Since magnets cannot exist without either of their poles, magnetic field lines always form closed loops between the north pole and the south pole.
However, electric charges can exist as isolated positive or negative charges.
Electric field types
There are two types of electric fields.
Uniform electric field
As the name suggests, an electric field is called uniform when it does not change over distance. A charge (q) would experience the same magnitude and direction of force at any point in a uniform electric field.
As an example, consider the electric field between two oppositely charged parallel plates, as shown in the figure below.
Fig. 4 - Uniform electric field lines
Non-uniform electric field
Again, as the name implies, a non-uniform electric field is not constant and can vary from point to point. A charge (q) would experience varying magnitude or direction (or both) of force at different points, in a non-uniform electric field.
Consider the example below, which shows an electric field from a single point charge.
Fig. 5 - Non-uniform electric field lines
Electric fields emanating from subatomic charged particles (electrons and protons) hold an atom together. Without them, the atom would cease to exist, and by extension, everything else as well. Electric fields are also responsible for molecular interaction in chemical reactions.
Coulomb's law
Coulomb's law tells us that like charges repel each other, and unlike charges attract each other. But besides these rudimentary statements, Coulomb's law also provides the formula which quantifies this force of attraction or repulsion.
The force of attraction (or repulsion) between two point charges is:
- Inversely proportional to the square of the distance between them
2. Directly proportional to the product of the two charges
Therefore, the force between the point charges may be given as -
Where the proportionality constant k holds the value -
Where, ε₀ = 8.854 Nm2C-2 , is the permittivity of vacuum (also called the electric constant). If you calculate the value of k, it comes as k = 9*10^9 Nm2C-2 .
Since we know the formula for force acting on charge q2 due to charge q1, we can find the electric field strength due to charge q1 at point r. Just substitute the first equation with the equation that Coulomb's law gave us.
Causes of electric fields
What really causes an electric field to form? How can you create an electric field? Electric fields are formed when there is a difference of electric potential between 2 points in space.
Electric potential is defined as the amount of work needed to move a unit charge from infinity to a point in an electric field.
Consider a charge Q. The electric potential at a point at a distance of r from the charge Q is:
- Directly proportional to Q. The greater the charge, the higher the electric potential at that point.
- Inversely proportional to r. The greater the distance, the lower the electric potential at that point.
In other words,
Substituting the proportionality constant, k = 1/(4πϵ0),
Velec is measured in Joules per coulomb, or Volts.
Create a small electric field yourself!
Take a plastic ruler/comb, and rub it on your hair or a piece of cloth. Bring it close to a small piece of paper. What do you notice? The paper seems to be attracted to the ruler. This is because rubbing the plastic causes it to acquire a static negative charge which has a static electric field around it. There is a potential difference between the ruler and paper; when the paper is in the proximity of this electric field, it experiences a force towards it.
Remember that we established that an electric field is the result of a difference in electric potential between 2 points? Mathematically, this can be expressed in the following way –
If you go ahead and differentiate Velec with respect to r, you will get the formula for the electric field. The negative sign indicates that electric field strength decreases as we go away from the charge (i.e. as r increases). Note that E and r are vector quantities in this equation, while Velec is a scalar quantity.
Electric potential energy
As stated earlier, electric potential energy is the energy required to move a charge through an electric field. Energy is required to move a charge through an electric field as the charge is constantly experiencing force due to the electric field. It can also be defined as the total energy required to hold a system of two charges in a particular configuration, due to the electrostatic forces between the two charges.
Consider 2 point charges q1 and q2, separated by a distance r. The electric potential energy of this system (Uelec) is given by:
When r is ∞ i.e., when the charges are very far apart, Uelec is 0. The work done on the charges to bring them at a distance of r is stored in this system as its potential energy. Conversely, the work done by the electrostatic forces of charges on each other is equal to the negative of potential energy of the system. Just like any other form of energy, Uelec is measured in Joules.
If you look closely at the equation for Uelec, You can deduce that you could get Uelec by multiplying electric potential by charge q. That gives us the potential energy required to hold one charge at a distance of r from another charge.
Electric Fields Chemistry - Key takeaways
- An electric field is a region around a charged particle in which other charged particles will experience a force.
- Electric field can be visualised with electric field lines, much like magnetic field lines.
- When a single positive point charge is present, electric field lines will emerge from the point charge and end at infinity. If a single negative point charge is present, electric field lines will start at infinity, and end at the point charge.
- A uniform electric field has uniform intensity at every point in space. Non-uniform electric field intensity can vary from point to point.
- Coulomb's law quantifies the force of attraction between two charged particles.
- Electric field is the result of electric potential difference between two points in space.
- The work done on a system of charges to assemble them in proximity of each other is stored in the system as its potential energy.
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