Introduction to electricity and electric circuits

electric charges, currents and voltage

Example: He-Atom

• 2 protons: + 2e

2 neutrons: no charge

2 electrons: - 2e• elementary charge, e = 1.6 x 10-19 C • Atoms are neutral, unless electrons are removed

(or added)

• Protons and neutrons consists of 3 quarks each.

Properties of Electric Charge• Charges are due to elementary particles: Protons carry (+ e), electrons

carry (- e).

• Charges are quantized: Q = n · e with n = ± 1, ± 2, ± 3, … and e = 1.602 x 10-19 C

• Charges are conserved.

• Charged objects exert a force onto each other: like charges repel each other, unlike charges attract each other.

Insulators versus conductors

• Insulators (plastic, glass, air): All electrons are tightly bound or localized and cannot move.

• No conduction electrons (or only very few): no transport of charge, i.e. electric currents.

Conductors (Metals)• Transport electric charges well.

• There are freely moving conduction electrons and bound electrons that remain bound to the nucleus of each atom.

• ions (nucleus + bound electrons) remain in place and form a crystal lattice (chemical bonds).

• Electric current: Net motion of charges (free electrons in a metal).

• Positive charges (ions) can only move in a liquid or a gas.

conduction electrons in a conductor

Electric current=Q/t

Conservation of current

Q1.

Another example on Page 214, STT 8.2

1) 3A in;

2) 2A out;

3) 1A in;

4) 1A out.

?

How to create an electric current

What is the time-dependence of current in the wire?

A Battery needed to keep currents flowing !!

q

WV chem

Voltage of a battery

Electric potential

Gravitational Potential Energy mgh

Gravitational Potential gh

Electrical potential Energy qV

Electrical potential V

The work done by the charge escalator or

chemical forces W (chem) defines the voltage of a battery:

W(chem) /q =terminal voltage of a battery

Some Typical Voltages

Voltage Source (approx.)

Thundercloud to ground 108 V

High-voltage power line 106 V

Power supply for TV tube 104 V

Automobile ignition 104 V

Household outlet 120 V

Automobile battery 12 V

Flashlight battery 1.5 V

Resting potential across

nerve membrane 10-1 V

Potential changes on skin

10-4 V

Resistance/conductance• Valid for “ohmic” devices mainly metallic

conductors at constant temperatures.

Ohm’s Law

Current I = V / R, 1/R= σA V/d =G

A is a cross section area of a wire, d is length. Conductivity σ.

V is the voltage across the wire.

*** Inverse of σ is called resistivity ρ, ρ = 1/σ

R= ρ d/A

Resistivity of materials Material Resistivity

Copper 1.7 X 10^{-8}Iron 9.7 X 10^{-8}Seawater 0.22Blood 1.6Fat 25Muscle 13

Pure water 2.5 X 10^5

Q1

Two copper conductors, A and B, are of same lengths and are connected to two identical batteries. A has a bigger cross section than B. Which is the right I versus V graph?

Q2

A and B conductors have same cross sections. But A is longer than B. Which is the correct graph?

Electrical Hazards

Electrical Hazards

Feel: 1mApain: few mAdeadly: over 70mA

Estimate the resistance of a human body !!

Grounded High Voltage Lines, Lightning Strikes1 000 000 V

50 000 V 0 V

Even if you are not directly hit by a lightning strike or a hot power line, there is danger: The potential decreases with distance from the location of the impact (potential gradient). If you take a step there may be a large potential difference between your feet.

Electrical Power

• P = VQ/t = VI

• Unit: Watt (W) = VA

• kW, MW, GW

Electrical power on the ohmic device

• P = VI

• V = RI (Ohm’s Law)

• P = V2/R = I2R