Electric charge and its properties

In the field of physics, electric charge can be described as a fundamental property of matter that causes a force to be exerted on it when placed in an electric field. Understanding electric charge is essential to explaining how electricity works and how it interacts with magnetism.

What is electric charge?

Electric charge is a property that is found in particles of matter. The most well-known charged particles are electrons and protons. Electrons carry a negative charge, while protons carry a positive charge. In atoms, these charges balance out to make the atom neutral, but when electrons move or are removed from atoms, this creates an imbalance, resulting in an electric charge. This charge is responsible for the electric forces that act between charged particles.

Diagram of the atom

This simplified diagram shows an atom with a positive charge on the protons in the nucleus and a negative charge on the electrons.

Units of charge

The unit of electric charge in the International System of Units (SI) is the coulomb (C). Charge can also be expressed in smaller units called elementary charges, usually denoted as e, where one elementary charge is approximately equal to 1.6 × 10 -19 C The charge of a proton is +1e, and the charge of an electron is -1e.

Properties of electric charge

The properties of electric charge can be summarized as follows:

1. Quantization of charge

The total charge Q of a system is always an integer multiple of the elementary charge e. Thus, the charge is said to be quantized. This means that you will never find a fraction of e in a charge. Mathematically, it can be represented as:

Q = n × e

Here n is an integer.

2. Conservation of charge

During any physical process, the total charge remains constant. This is known as the conservation of electric charge. Charges can move around and even change from positive to negative, but the total charge will always be the same before and after the reaction.

3. Additivity of charge

When you combine different charges, the total charge is simply the sum of the individual charges. For example, if you have a charge of +2 C and -1 C, the resulting charge will be +1 C

Interaction between charges

Electric charges interact with each other and can exert forces. The nature of the interaction depends on the type of charges involved:

1. Like charges repel

If two charges are both positive or both negative, they repel each other. This means they push each other away. Imagine it like this:

This diagram shows two positive charges repelling each other.

2. Opposite charges attract

When one charge is positive and the other is negative, they attract each other, that is, they are pulled toward each other like magnets.

This shows that the positive and negative charges are attracting each other.

Coulomb's law

The force of attraction or repulsion between two point charges is given by Coulomb's law, which states that the force F is directly proportional to the product of the charges and inversely proportional to the square of the distance r between them. It can be expressed mathematically as:

F = k × |q₁ × q₂| / r²

Where:

• F is the force between the charges (in newtons, N)
• k is the Coulomb constant, about 8.99 × 10 9 N·m²/C²
• q₁ and q₂ are quantities of charges (in coulombs, C)
• r is the distance between the centers of the two charges (in meters, m)

Example problem

Imagine you have two charges, q₁ = +3 C and q₂ = -2 C, which are 2 m apart.

To find the force between them:

F = 8.99 × 10⁹ × |(+3) × (-2)| / (2)² = 8.99 × 10⁹ × 6 / 4 = 13.485 × 10⁹ / 4 = 3.37125 × 10⁹ N

The force 3.37125 × 10⁹ N is directed such that opposite charges attract each other.

Charging by induction

Electric charge can be transferred from one body to another, and a common way this happens is through induction. This involves bringing a charged object near a neutral object, causing regions of positive and negative charge to develop in the neutral object.

Steps of charging by induction

Let us consider that a negatively charged rod is brought near a neutral metal sphere:

1. The negative charge of the rod repels the electrons present in the sphere. This creates a positive charge on the part closest to the rod and a negative charge on the other side.
2. If the sphere is grounded for some time (connected to a conductor that allows charge to flow), some of the electrons flow out, leaving the sphere positively charged.
3. After removing the ground and then the rod, the sphere remains overall positive charge.

Applications of electric charge

Electric charge has many applications in everyday life and technology. Here are some examples:

1. Static electricity

When you rub the balloon on your hair, electrons are transferred from your hair to the balloon, giving the balloon a negative charge. This negative charge allows the balloon to stick to the wall for some time.

2. Electrical appliances

Charged particles moving through a conductor (usually electrons) create an electric current. This is the basis for operating most of our electrical devices, from light bulbs to computers.

3. Capacitor

A capacitor is a device that stores electrical energy in an electric field, created by a pair of conductive plates separated by an insulator. They play an important role in electronic circuits.

Capacitor symbol

4. Electroscope

The electroscope can detect the presence and magnitude of electric charge. It consists of a metal rod connected to two thin metal plates. When a charged object strikes the metal rod, the plates move apart due to repulsion.

Conclusion

Electric charge is a fundamental concept in our understanding of how electricity and magnetism interact. Its properties of quantization, conservation, and additivity underpin the structure of matter and power our world in countless ways. From everyday static electricity to complex electrical circuits, the flow and interaction of electric charges are vital to the function of modern technology.

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