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 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 = x 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.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 !! 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 10 8 V High-voltage power line 10 6 V Power supply for TV tube 10 4 V Automobile ignition10 4 V Household outlet120 V Automobile battery12 V Flashlight battery1.5 V Resting potential across nerve membrane10 -1 V Potential changes on skin 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 0.22 Blood 1.6 Fat 25 Muscle 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 Feel: 1mA pain: few mA deadly: over 70mA Estimate the resistance of a human body !!

Grounded High Voltage Lines, Lightning Strikes V 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 = V 2 /R = I 2 R