Electric Charges (12C1A)

Electric charges – An introduction

1. Polarity of Charge (Nature)
   • Two types of charges: Positive and Negative
   • Like charges repel, while unlike charges attract
2. Significance of Signs on Charges
   • The signs (+/-) do not have real numerical meaning like in mathematics.
   • Their only significance is that equal amounts of opposite charges cancel each other.

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2. Electrostatics

• A branch of physics that studies charges at rest and the associated forces, fields, and potentials.

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3. Electrification and Electric Discharge

Electrification

• The accumulation of positive or negative charge on a body.
• Example: A plastic comb gets electrified when rubbed through dry hair.

Electric Discharge

• The loss of accumulated charge from a body.
• Example: Lightning is an electric discharge.

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4. Fundamental Properties of Charge

• Definition: A fundamental property of matter due to which it experiences a force when placed in an electric field.
• Representation: Denoted as Q or q.
• SI Unit: Coulomb (C)
• Smaller Units: MilliCoulomb (mC) and MicroCoulomb (μC)
• Dimension: [Charge]=[At][Charge] = [At]
• Nature: Scalar Quantity

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5. Origin of Charge

• Charges arise due to the transfer of electrons between atoms of different materials.
• A body that loses electrons becomes positively charged.
• A body that gains electrons becomes negatively charged.

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6. Methods of Charging a Body

(i) Charging by Rubbing (Friction)

• Electrons move from one material to another due to friction.
• Substances with loosely held electrons lose them and become positively charged.
• Substances with tightly held electrons gain them and become negatively charged.

Examples:

Material 1	Material 2	Charge After Rubbing
Plastic Rod	Cat’s Fur	Plastic (-), Fur (+)
Glass Rod	Silk	Glass (+), Silk (-)

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(ii) Charging by Contact (Conduction)

• When a charged body touches an uncharged body, charges transfer between them.
• Before Contact: Only one body is charged.
• After Contact: Both bodies share the charge.

Types of Materials Based on Conductivity

1. Electric Conductors:
   • Allow charge to pass through easily.
   • Examples: Metals, Human Body, Earth
2. Electric Insulators:
   • Do not allow charge to pass through easily.
   • Examples: Plastic, Glass, Wood, Nylon, Porcelain

Cases in Conductivity:

• Case I: If a charged object contacts an insulator, charge accumulates locally and does not spread.
• Case II: If a charged object contacts a conductor, charge spreads evenly across both.

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(iii) Charging by Induction

• A body gets charged without direct contact by bringing it near a charged object.

Process of Charging by Induction

1. A charged object is brought near (without touching) an uncharged object.
2. Like charges are repelled, and unlike charges are attracted within the uncharged body.
3. The uncharged body develops separation of charges.

Example: Metal Spheres in Contact

1. A charged object is brought near the spheres.
2. Charges within the neutral spheres separate.
3. If the spheres are separated, each retains a different charge.

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7. Detection of Charge on a Body

Gold Leaf Electroscope

Principle

• Like charges repel each other.
• More charge = Greater divergence of leaves.

Setup Components

• Metal Rod with a Metal Knob
• Glass Box Container
• Two Thin Gold Leaves

Functioning

1. A charged object is touched to the metal knob.
2. Charge transfers to the gold leaves through the rod.
3. The leaves diverge as they gain same charge.
4. More charge results in greater divergence.

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8. Basic Properties of Electric Charge

(i) Charges are Additive in Nature

• Total charge is the sum of all charges in a system.
• Equal amounts of opposite charges cancel each other.
• Example:
  • +5 C and -5 C together result in 0 charge.

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(ii) Charges are Quantized

• Charge on a body is always an integral multiple of the elementary charge (ee).
• Formula: Q=neQ = n e where:
  • QQ = Total charge
  • nn = Integer
  • ee = Charge of an electron (1.6×10−19C1.6 \times 10^{-19} C)

Example:

• 1 Coulomb of charge contains 6.25×10186.25 \times 10^{18} electrons.

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(iii) Charges are Conserved

• Charge cannot be created or destroyed in an isolated system.

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Origin of Charges

Basic properties of electric charges

Introduction to electric charges