On the subject of the electric charge, was learned that like charges repel each other while unlike charges attract each other. If a positively charged object is brought close to a negatively charged object, the two objects pull together so that they move toward each other. Conversely, if a positively charged object is brought close to a positively charged object, then the two objects repel each other so that they move away from each other. As studied on the subject of Coulomb’s law, electrically charged objects can accelerate other electrically charged objects because there is an electrical force acting between these electrically charged objects. The electric force that is exerted by an electrically charged object on other electrically charged objects is one example of a force that can act without contact. Another example of the force that can act at a distance is the force of gravity. The gravitational force is exerted by a mass object on the other mass objects.
Friction force, thrust force, normal force are examples of forces that are easy to understand because these forces work when contact occurs. On the contrary, the electric force is an example of a force that is difficult to understand because this force can work from a certain distance without touch. To understand the electrical force that can act from a certain distance, appear the concept of electric field. The electric field concept was developed by British scientist Michael Faraday (1791-1867).
Definition of the electric field (E)
Suppose a glass was initially electrically neutral. After being rubbed with a cloth, the glass rod becomes electrically charged. When the glass rod becomes electrically charged, at the same time an electric field appears around the glass. If the glass turns neutral, at the same time the electric field disappears. So the existence of an E cannot be separated from the presence of an electric charge. The E is not a kind of substance like air and not a type of wave, like electromagnetic waves. An E is something caused by an electric charge and affects space around an electric charge where other electrical charges only feel the influence of an E.
To test whether there is an E in a room, it is assumed that there is a test charge in that space. The test charge is a minimal charge (Q) so that the charge produces an electric field that is very small so it can be ignored. However, the test charge can feel the influence of the electric field generated by other electrical charges. The test charge is presented only to investigate whether there is an E. If there is an E in the space, the test charge must experience the electrical force, while if there is no E, the test charge does not experience the electrical force.
Look at the figure on the side.
In figure 1, the charge +q1 and the charge +q2 produce an E in the surrounding space. When the test charge +qo is placed at a point in space, the test charge +qo experiences the electrical force. F1 is the electric force that is exerted by the charge +q1 on the test charge +qo, and F2 is the electric force exerted by the charge +q2 on the test charge +qo. In Figure 2, when the test charge is removed, the E remains and does not disappear from that point. E1 is an electric field produced by an electric charge +q1, and E2 is an E produced by an electric charge +q2.
The magnitude of the electric field
The E is a vector quantity, where the electric field has a magnitude and direction. The magnitude of an E is usually referred to as an E strength. The strength of the E at a point is defined as the force of electricity exerted by an electrical charge on a positive test charge placed at that point, divided by the size of the test charge.
E = electric field strength
F = electric force
q = test charge
The unit of the electrical force is Newton, and the unit of the electric charge is Coulomb so that the unit of the E is Newton / Coulomb, abbreviated as N/C.
The direction of the electric field
In Figure 1a, a positive electric charge (+Q) exert the electrical force on the positive test charge (+q), where the direction of the electric force (F) is away from the charge of +Q.
In Figure 1b, when the test charge is removed, at this point there is an electric field (E) which its direction is away from the electric charge Q.
In Figure 2a, a negative electric charge (-Q) exert the electric force in a positive test charge (+q), where the direction of the electric force (F) approaches the charge -Q.
In Figure 2b, when the test charge is removed, at this point there is an electric field (E) whose direction is close to the electric charge -Q.
Based on the figure and explanation above, can be concluded that the direction of the electric field is away from the positive electric charge and approaches the negative electrical charge.
Electric field lines
The E is something that is generated by an electric charge and affects the space around the electric charge, where the influence of the E is only felt by other electrical charges. When another electric charge is in the E generated by an electric charge, another electric charge senses the influence of an electric force.
Understanding the electric field above, as described in the article about the E, can be realized with the mind but can only be imagined in mind. To visualize an E, electrical field lines are presented. Electric field lines are a set of lines drawn around an electric charge to indicate the existence of an electric field. Because it aims to show the presence of an E, there is a connection between these lines with an E. In other words, the magnitude and direction of the E can be explained by drawing electrical field lines.
Following is the relationship between direction and electric field strength with electrical field lines:
First, the direction of the E moves away from the positive charge and approaches the negative charge. Thus, the direction of the electric field lines also away from the positive charge and approaches the negative charge.
If the positive charge is adjacent to the negative charge, the electric field lines are drawn out of the positive charge to the negative charge. Conversely, if a positive charge is adjacent to a positive charge, the electric field lines are drawn out of each positive charge and away from each other.
Second, the electric field strength is represented by the distance between the electrical field lines. The closer the distance between the lines of the electric field, the higher the strength of the electric field and the farther the distance between the lines of the electric field, the smaller the electric field strength.
Why is the closer the distance between the lines, the higher the electric field strength? To understand this, look at the following explanation. Suppose a positive charge is at the center of the sphere and the electric field lines spread out in various directions through the surface of the sphere. If the number of electric field lines is N and the surface area of the sphere is 4πr2, then the number of lines per unit area or line density is N / 4πr2. Based on this formula, for the same N, if r gets smaller, the distance between the lines gets closer and if r gets more significant the distance between the lines gets farther away.
The electric field strength formula E = k q / r2 also shows that the electric field strength is inversely proportional to the square of the distance. If r is smaller, then the electric field strength gets more prominent, and if r gets more prominent, then the electric field strength gets smaller.
Based on the above review it can be concluded that if r is getting smaller (getting closer to the charge) the electric field strength is getting bigger and the distance between the lines is also getting closer. Conversely, if r gets more prominent (the farther away from the charge), the electric field strength gets smaller and the distance between the lines also farther away.
Third, the number of E lines is proportional to the electric field strength. The more electric field lines, the higher the electric field strength. The fewer electrical field lines, the smaller the electric field strength.