Electrical Quantities

1. Electric Charge

What is Electric Charge?

Charge is a physical property of matter that causes it to experience a force when placed in an electric field. Think of it like an invisible "electrical label" that every particle carries.

There are two types of charge:

• Positive charge (like the charge on a proton)
• Negative charge (like the charge on an electron)

Inside every atom:

• Protons carry positive charge (+)
• Electrons carry negative charge (−)
• Neutrons have no charge (neutral)

Normally, atoms have equal numbers of protons and electrons, so their charges cancel out and the atom has zero overall charge.

Charging by Friction

When you rub certain insulating materials together (like a plastic rod with a cloth), they can become electrically charged. This process is called charging by friction.

What actually happens:

1. Friction between the two materials produces heat energy
2. This energy allows electrons to escape from the outer shells of atoms
3. Electrons transfer from one material to the other
4. The material that loses electrons becomes positively charged
5. The material that gains electrons becomes negatively charged

Important: Only electrons move during charging by friction. Protons stay fixed in the nucleus and don't move. So when we talk about "gaining charge" or "losing charge," we're really talking about gaining or losing electrons.

Example:

• When you rub a polythene rod with a cloth:

  • Electrons move FROM the cloth TO the rod
  • The rod gains electrons → becomes negatively charged
  • The cloth loses electrons → becomes positively charged

• When you rub an acetate rod with a cloth:

  • Electrons move FROM the rod TO the cloth
  • The rod loses electrons → becomes positively charged
  • The cloth gains electrons → becomes negatively charged

Detecting Charge - Experiments

Experiment 1: Charging by Friction and Testing Attraction/Repulsion

Method:

1. Charge a polythene rod by rubbing it with a cloth
2. Suspend the rod from a stand using string so it can rotate freely
3. Charge a second polythene rod the same way
4. Bring the second rod close to the suspended rod

What you observe:

• The suspended rod is repelled (pushed away)
• This shows both rods have the same type of charge (both negative)

Now try with an acetate rod:

• Charge an acetate rod by rubbing with cloth
• Bring it near the suspended polythene rod
• The rods attract each other
• This shows they have opposite charges

Experiment 2: Gold-Leaf Electroscope

A gold-leaf electroscope is a device used to detect electric charge. It has:

• A metal plate at the top
• A metal rod going down through an insulating collar
• A thin sheet of gold leaf attached to the bottom of the rod
• All enclosed in a glass case (to prevent drafts)

How it works:

When you bring a charged rod near the plate (without touching):

• The rod repels electrons in the metal
• Electrons move away from the plate, down to the rod and leaf
• Both the rod and leaf now have the same charge
• Like charges repel, so the leaf rises away from the rod

When you move the rod away:

• Electrons redistribute evenly
• The leaf falls back down

When you touch the plate with a charged rod:

• Charge transfers from the rod to the electroscope
• The leaf stays risen even when you remove the rod
• The electroscope is now charged

Conductors and Insulators

Electrical conductors are materials that allow electric charge (electrons) to flow through them easily.

Examples: metals like copper, silver, aluminum, iron

Why metals conduct:

• In metals, the outermost electrons are not tightly bound to atoms
• These free electrons (or delocalized electrons) can move freely through the metal
• When a voltage is applied, these electrons flow, creating a current

Electrical insulators are materials that do NOT allow charge to flow easily.

Examples: plastic, rubber, glass, wood

Why insulators don't conduct:

• In insulators, all electrons are tightly bound to their atoms
• There are no free electrons to carry charge
• Charge cannot flow through the material

Experiment to Distinguish Conductors from Insulators:

Method:

1. Charge a gold-leaf electroscope by touching it with a charged rod (leaf should stay risen)
2. Touch the plate with the material you want to test
3. Observe what happens to the leaf

Results:

• Good conductor (e.g., metal): Leaf falls quickly because charge flows easily through the material to your hand and away to earth
• Poor conductor/insulator (e.g., plastic, glass): Leaf falls slowly or not at all because charge cannot flow easily

Electric Fields (Supplement)

An electric field is a region of space around a charged object where another charged object would feel a force.

Direction of an electric field: The direction of the force that would act on a positive charge placed at that point.

We draw electric fields using field lines (lines with arrows). These show:

• The direction of the force on a positive charge
• The strength of the field (closer lines = stronger field)

Key rules for field lines:

• Field lines point away from positive charges
• Field lines point towards negative charges
• Field lines never cross each other
• Closer spacing between lines means a stronger field

Electric Field Patterns:

1. Around a point charge:

• Field lines radiate outward (if positive) or inward (if negative)
• Lines spread out as you move away (field gets weaker)

2. Around a charged sphere:

• Similar to a point charge
• Field lines are perpendicular to the surface
• Field lines are evenly spaced around the sphere

3. Between two parallel plates (opposite charges):

• Field lines are straight, parallel, and equally spaced
• This is called a uniform electric field (same strength everywhere between the plates)
• Lines go from positive plate to negative plate