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Lecture 1

Notes

  • Lightning
  • GPS system
  • Aurora
  • Smart phones can send/receive electromagnetism waves

Physical Quantity

1 numerical value + 1 physical unit Unit is very important!

SI-based Units

  • Time: Second
  • Length: Meter
  • Mass: Kilogram
  • Current: Ampere

What is Charge?

Charge is an attribute of materials, like how mass is an attribute of a pen. Charge cannot be produced, it can only be transferred between objects.

3 Methods to Make Electric Charge

  1. Friction between insulator (絕緣) materials
  2. Conduction
  3. Induction

Friction

When no friction is happening, the objects stay neutral. When plastic and fur are rubbed with each other, some negative charge from plastic goes to fur, and some positive charge from fur goes to plastic.

Conduction

If an object with positive charge touches another object with neutral charge, the positive charge transfers to the neutral object, and now both objects are in positive charge. If an object with positive charge touches another object with negative charge, the positive and negative charge on each object transfers to each other. Ultimately, both objects are now neutrally charged.

Induction

…

Elementary Charge

A minimum charge (e), it cannot be divided anymore into a smaller charge. The value of elementary charge = the charge of an electron. Any charged object's charge can be written as ne, where n is an integer. Millikan did an experiment to measure the elementary charge.

Charge Conservation

The total electric charge in any closed system never changes.

A "closed system" means no energy is being transferred to the surroundings (like air.) Charge cannot be created or destroyed, only transferring of charge from one object to another happens.

Coulomb's Law

Attractive Force: Positive and negative charges attract. Repulsive Force: Same charges repel. F is a vector (direction + magnitude). k is a constant. q is charge. r is the distance from the source.

Direction of Electric Field

Positive Charges: Outward (positive people like to express themselves with others.) Negative Charges: Inward.

What Does it Mean When we say "Find the Electric Field?"

It means find the strength of the electric field.

Charge Density

  • 1 dimension: Linear charge density = Total charge / Length
  • 2 dimension: Area charge density = Total charge / Surface area
  • 3 dimension: Volume charge density = Total charge / Volume