電磁學 (Electromagnetism)
The combination of electrical and magnetic phenomena
By 吳文欽 The ultimate goal of learning electromagnetism is to understand Maxwell’s equations that govern the motions of charged media, effect and origins of static and dynamic electric and magnetic fields.
I. Electric Charge (電荷)
1. What is “electric charge”?
Electric charge is an intrinsic characteristic of the fundamental particles making up of objects. For example, electron has negative charge, while proton has positive charge.
Charges with same (opposite) electrical sign repel (attract) each other.
2. Application of charged particles
Inject printer, air cleaner, copy-machine, … …
3. Various conducting materials: solid-state physics
Conductor (良導體，金屬) and insulator (不良導體，絕緣體) Semiconductor (半導體), superconductor (超導體)
4. Coulomb’s law
The direction of F (attractive or repulsive) depends on the relative sign of both charges.
Principle of Superposition
5. Charged is Quantized
6. Charged is Conserved
II. Electric Field (電場)
Alternative way to view the charge and electric force
1. What is “electric field”?
Examples:
2. Electric lines: a way to view the magnitude and direction of local electric field
3. Important examples
a) Electric Field due to a point charge
b) Electric Field due to an electric dipole
with P = qd the dipole moment.
c) Electric Field due to a line of charge
The parallel (to z axis) component of dE is
When ,
d) Electric Field due to a charged disk
When ,
When , (q = πR2σ, like a point charge)
4. A point charge in an electric field
Millikan oil-drop experiment
(measuring the elementary charge)
5. A electric dipole in an electric field
H2O
