ELECTRIC CHARGES AND FIELDS

Electrostatic charge

• Sparks or crackling sounds when removing synthetic clothes in dry weather.

• Electric discharges during thunderstorms (lightning).

• Sensations of electric shock from metal objects (e.g., car doors, bus bars).

Electrostatics

The branch of physics that studies the forces, fields, and potentials created by static charges.

Discovery of static electricity

• Thales of Miletus, a Greek philosopher.

• He observed that amber, when rubbed with wool or silk, could attract light objects such as straw and small bits of paper.

Electric Charge

• A fundamental property of matter that causes it to experience a force when placed in an electric and magnetic field.

• Exists in two types: positive and negative. termed by Benjamin Franklin.

• Unlike Charges Attract

• Like Charges Repel

Rubbing glass rods with wool or silk

• When two glass rods rubbed with wool or silk are brought close, they repel each other.

• A glass rod and wool cloth attract each other.

• A glass rod repels the fur.

Two plastic rods rubbed with cat’s fur

• repel each other but attract the fur

• A plastic rod attracts a glass rod and repels silk or wool

Polarity of Charge

• The characteristic that distinguishes the two types of electric charge is known as the polarity of charge.

• Materials can become electrified through the process of rubbing, resulting in the accumulation of either positive or negative charges.

Gold-Leaf Electroscope

• The gold-leaf electroscope is a simple device used to detect electric charge.

• The degree of divergence indicates the amount of charge present on the object.

Charging a Neutral Body

• To electrify a neutral body, one must add or remove charge.

• A body is considered charged when it has an excess or deficit of charge.

Charging by Rubbing

• insulators

• charge on the surface

• When a glass rod is rubbed with silk, the rod acquires a positive charge, while the silk acquires a negative charge.

• No new charge is created during the rubbing process; the total charge remains constant.

Charging by Conduction

• conductors

• in contact

• same polarity

Charging by induction

• conductors

• no need of contact

• different polarity

Conductors

• Substances that allow the passage of electricity easily.

• Contain electric charges (electrons) that can move freely within the material.

• When charge is transferred to a conductor, it distributes evenly over the entire surface of the conductor.

• Examples:

  • Metals (e.g., copper, aluminum).

  • Human and animal bodies.

  • Earth.

Insulators

• Substances that resist the flow of electricity.

• Contain charges that are not free to move; they offer high resistance to electrical current.

• When charge is applied to an insulator, the charge remains localized at the point of contact and does not spread.

• Examples:

  • Most non-metals (e.g., glass, porcelain, plastic, nylon, wood).

Point Charges

• When the sizes of charged bodies are very small compared to the distances between them, they can be treated as point charges.

• This means all the charge is assumed to be concentrated at a single point in space.

Charge is additive

• Total charge of a body is algebraic sum of the charges present on the body

• charge is a scalar quantity

• while adding charges polarity must be considered

Charge is conserver

• Charge can neither be created nor destoyed but can change from one form to another.

• this is experimentally proved by beta decay of radioactive element

  • ɳ⁰ → p⁺¹ + e^-1

Charge is quantized

• Total charge of body equal to the integral multiple of fundamental charge (e)

• Q±ne

  • Q - total charge who is +ve or -ve

  • e - -fundamental unit of charge

    • e = 1.6 × 10^-19 C

  • n - integer

• if total charge is 0, then equal number of +ve and -ve charges are present.

• This concept was suggested by Michael Faraday through his laws of electrolysis.

• It was experimentally demonstrated by Robert Millikan in 1912.

Coulomb

• One coulomb is defined as the charge that flows through a wire in 1 second if the current is 1 ampere (A).

• it is the SI unit of charge

• 1 mC=10⁻³ C (milli coulomb)

• 1 µC=10⁻⁶ C (micro coulomb)