Understanding Electricity Generation and Electric Fields
Methods of Generating Electricity
By Rubbing:
Rubbing a glass rod against a wool cloth causes electrons (e-) to transfer from the glass to the wool. The wool cloth becomes negatively charged, and the glass rod becomes positively charged. This is known as static electricity.
Piezoelectric Effect:
When piezoelectric materials, such as quartz, are subjected to pressure (P) between two opposite sides, electric charges appear on these surfaces. This principle is used in knock sensors. The process is reversible.
Chemical Action:
When a metal is immersed in an electrolyte (a liquid composed of water and acid), a potential difference appears between them. Batteries and accumulators utilize this principle, converting chemical energy into electrical energy.
Magnetic Induction:
This method is the most common way to generate electricity. When a conductor moves within a magnetic field, perpendicular to the magnetic field lines (F), an electrical current (AC) is induced in the conductor.
Photoelectric Effect:
Experimentally, it has been shown that when a ray of light strikes a metal body, it causes the release of electrons (e-) from the metal’s surface atoms. This is known as the photoelectric effect. Selenium photocells utilize this effect.
Thermoelectric Effect:
When two metal plates of different materials are joined, a potential difference arises because electrons in one metal have more energy than the other and tend to diffuse towards the latter. If the junction of both metals is heated, the emission of electrons (a thermal property) increases. The generated voltage (dpp) is called the Volta effect.
Electric Fields and Capacitors
An electric field is the region where the effect of an electric charge is felt.
Consider two metal plates separated by a short distance and connected to a generator. When the circuit is closed with a switch, the positive terminal of the generator attracts electrons from the negative plate. This creates a movement of electrons, establishing a potential difference between the plates.
This electron movement is brief, lasting only until the plates reach equilibrium. This device is called a capacitor, and it has the ability to store electrical charge.
If a dielectric material is placed between the plates, the atoms in this material are influenced by the electric field created by the generator.
A capacitor is considered charged when the potential difference between its plates equals the applied voltage.
Once charged, if the generator is disconnected and the plates are joined by a wire, the capacitor discharges. The duration of the charge and discharge currents depends on the capacitor’s capacity and the applied potential difference. The unit of measurement is the Farad (charge/voltage), and microfarads (µF) and picofarads (pF) are commonly used.
If a high voltage is applied, electrons can jump from one plate to another, puncturing the dielectric (dielectric breakdown voltage).
A capacitor’s capacitance depends on the surface area of the plates, the thickness of the dielectric, and the type of dielectric material.