15. Static Electricity

Static Electricity

When objects rub together, they can develop a kind of electricity called static electricity. This makes them attract small things like bits of paper or dust. There are two types of charge: positive and negative, named by Benjamin Franklin. Things with the same charge push each other away, while opposites attract. When objects rub together, electrons can move from one to the other, causing them to become positively or negatively charged. Here's a list of materials: if you rub any material listed with one that comes after it, the first material will become positively charged and the second will be negatively charged.

The list of materials provided follows a pattern: when any material in the list is rubbed against another material that comes after it, the first material becomes positively charged, and the second material becomes negatively charged. 

Material	Charge
Fur	Positive
Flannel	Negative
Shellac	Positive
Sealing Wax	Negative
Glass	Positive
Paper	Negative
Silk	Positive
Human body	Negative
Wood	Positive
Metals	Negative
India Rubber	Positive
Resin	Negative
Amber	Positive
Sulphur	Negative
Ebonite	Positive
Gutta Percha	Negative

Surface density of charge :

The surface density of charge refers to how much charge is present per unit area on the surface of a conductor. It's influenced by the shape of the conductor and the presence of other conductors or insulators nearby. Generally, the surface density of charge is higher at points that are more pointed or have a smaller radius of curvature. This is because the charge tends to accumulate more at these sharp points compared to flatter areas.

Conductor:

Conductors are materials that allow electricity, or charge, to flow through them easily. Good conductors have electrons that are not tightly bound to their atoms, allowing them to move freely.

Examples of conductors include metals like silver, iron, and copper, as well as the Earth's moist soil, which acts as a large conductor. Among metals, silver is known to be the best conductor.

Insulator or Dielectric :

Insulators, also called dielectrics, are materials that don't let electricity pass through them easily. Examples include wood, paper, mica, glass, and ebonite. They prevent the flow of electrical current and are often used to protect against electric shocks or to insulate electrical wires.

Coulomb's law :

Coulomb's law states that the force of attraction or repulsion between two stationary point charges is directly proportional to the product of their charges and inversely proportional to the square of the distance between them. This force acts along the line connecting the two charges.

Electric Field :

The electric field refers to the area surrounding a charge or charged object where its electrical influence can be detected.

Electric field intensity:

Electric field intensity, at a specific point within an electric field, is the force experienced by a unit positive charge positioned at that point.

Electric Field of hollow conductor

Inside a charged hollow conductor, the electric field intensity is zero. This means that any charge given to the conductor remains on its surface only. Because of this property, a hollow conductor acts as an electrostatic shield. That's why it's safer to be inside a car or bus during lightning, as the metal body of the vehicle shields occupants from the electric field.

Electric Potential :

Electric potential at a point in an electric field is the amount of work needed to bring a unit positive charge from infinity to that point. It's measured in volts and is a scalar quantity, meaning it has magnitude but no direction.

Potential Difference :

Potential difference, on the other hand, is the work required to move a unit positive charge from one point to another within the electric field. It's also measured in volts and is a scalar quantity.

Electric Capacity :

Electric capacity, also known as capacitance, refers to the ability of a conductor to store electrical charge. It's defined as the amount of charge required to increase the potential of the conductor by one unit. If a conductor's potential increases by V when a charge Q is given to it, then its capacity is calculated as Q/V. Capacitance is measured in farads (F).

Electrochemical Cell :

An electrochemical cell is a device that converts chemical energy into electrical energy. There are two main types of cells: primary cells and secondary cells.

1. Primary Cell: In a primary cell, electrical energy is produced from irreversible chemical reactions that occur inside the cell. Once a primary cell is fully discharged, it cannot be recharged and becomes unusable. Examples include Voltaic Cells, Leclanche Cells, Daniel Cells, and Dry Cells.

2. Secondary Cell: A secondary cell is rechargeable. It needs to be initially charged from an external electric source, after which it can be used to draw current. These cells can be recharged multiple times, making them convenient for reuse.

The production of electricity from chemical reactions was first discovered by Alessandro Volta in 1794, leading to the naming of the voltaic cell after him. In a voltaic cell, a zinc rod serves as the cathode and a copper rod serves as the anode, placed in sulfuric acid within a glass vessel.

In a Leclanche cell, a carbon rod acts as the anode and a zinc rod acts as the cathode, submerged in ammonium chloride within a glass vessel. The Leclanche cell has an electromotive force (emf) of 1.5 volts and is commonly used for intermittent tasks, such as electric bells.

A dry cell contains a mixture of manganese dioxide (MnO₂), ammonium chloride (NH₄Cl), and carbon, enclosed in a zinc vessel. A carbon rod serves as the anode within the mixture, while the zinc vessel itself acts as the cathode. The emf of a dry cell is also 1.5 volts