EP SCI 2: Lec 2: Electromagnetism

Basic atom structure

Protons + Neutrons are found in the nucleus and have roughly same mass. Electrons are around nucleus and are MUCH smaller than protons.

Proton, Neutron, and Electron charge

• Proton: positive

• Neutron: neutral charge

• Electron: negative charge

Outer electrons participate in what?

chemical bonds

Electric charge

is a physical property of matter that causes it to experience a force when placed in an electromagnetic field. § Unit of charge: Coulomb (C)

• Charge can be positive (protons) or negative (electrons). § Electric charge is a conserved property: the net charge of an isolated system remains constant.

Electromagnetic force 

EM force can be attractive or repulsive: § Attraction between opposite charges (+ and -)

§ Repulsion between like charges (++ or --) • Inverse square law: strength depends on 1/d2

Our universe has four fundamental forces….

Strong, Electromagnetic, weak, gravitational

Strong force stats

In Nucleus: 100

Beyond nucleus: 0

Electromagnetic force stats

Within nucleus: 1

Beyond: 1

Weak force stats

within nucleus: 10^-9

beyond: 0

gravity force stats

within nucleus: 10^-43

beyond: 10^-49

Electric field

The altered space surrounding the initial charge is a force field known as an electric field.

Electric fields have both…

magnitiude and direction

Electric field equation for force per unit charge

E = F/q

• Two common ways to represent an electric field:

§ In a vector diagram, the direction of the field is shown by vectors that indicate the direction in which a small positive test charge would be pushed. § In a line-of-force diagram, a small number of lines indicate the direction of the field. The distance between lines denotes the strength of the field.

Electric potential and its unit

is electric potential energy per unit charge.

§ Unit of electric potential: volt (V = Joule / Coulomb = J/C)

§ Electric potential energy is analogous to gravitational potential energy.

Electric Current and its unit

§ Electric current is the flow of electric charge

Unit of current: Ampere (A = Coulomb per second = C s-1)

If there is a potential difference between two ends of a conductor…

charge will flow

conductor

is a material in which it is easy to establish a flow of electrons (a current). Metals are good conductors because the electrons in the outer shells of these atoms are not bound to a specific atom but are free to wander.

Insulator

r is a material in which the electrons are tightly bound to specific atoms and not free to wander. Rubber and glass are good insulators.

Electrical resistance and its unit

is a measure of an object’s opposition to the flow of electric current. § Unit of resistance: ohm (Ω = V / A)

Ohm’s law

The current in a circuit is directly proportional to the voltage applied across the circuit and is inversely proportional to the resistance of the circuit.

Current = voltage/resistance → Amperes = volts/ohms

direct current (dc) vs alternating current (ac)

• In direct current (dc), charges move in one direction. • In alternating current (ac), the direction of motion of charges alternates, typically at a frequency of tens of cycles per second (Hz).

• ac is more easily transformed from low to high voltage for efficient transmission

Electric Power and formula

the rate at which electric energy is converted into another form of energy (e.g., motion, heat).

• Power = current x voltage

  • watts = amperes x volts

Magnetism

Magnetic poles attract or repel each other in a way that is similar to the electric force between charges, including the inverse-square-distance dependence.

Ørsted’s experiment

found that an electric current changes the direction of a compass needle → Magnetism is due to the motion of electric charges.

Magnetic Field and its unit

• The altered space surrounding a magnet (or current) is a force field

• A magnetic field has both magnitude and direction. § Unit of magnetic field: tesla (T = N A-1 m-1) or gauss (1 tesla = 104 gauss). Earth’s magnetic field is ~0.5 gauss

Field induction

An electric field is induced in any region of space in which a magnetic field is changing with time. • A magnetic field is induced in any region of space in which an electric field is changing with time.

Generator

A generator converts mechanical energy into electrical energy. • Electricity can be generated with hydraulic or aeolian power generators.