Introduction to electricity and electric circuits electric charges

Introduction to electricity and electric circuits electric charges, currents and voltage

Example: He-Atom • 2 protons: + 2 e 2 neutrons: no charge 2 electrons: - 2 e • 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

Q 1. 1) 3 A in; 2) 2 A out; 3) 1 A in; 4) 4) 1 A out. ? Another example on Page 214, STT 8. 2

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 Electrical potential Energy q. V Gravitational Potential gh 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 Thundercloud to ground High-voltage power line Power supply for TV tube Automobile ignition Household outlet Automobile battery Flashlight battery Resting potential across nerve membrane Potential changes on skin (approx. ) 108 V 106 V 104 V 120 V 12 V 1. 5 V 10 -1 V 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 Copper Iron Seawater Blood Fat Muscle Pure water Resistivity 1. 7 X 10^{-8} 9. 7 X 10^{-8} 0. 22 1. 6 25 13 2. 5 X 10^5

Q 1 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?

Q 2 A and B conductors have same cross sections. But A is longer than B. Which is the correct graph?

Electrical Hazards Feel: 1 m. A pain: few m. A deadly: over 70 m. A Estimate the resistance of a human body !!

Grounded High Voltage Lines, 1 000 V Lightning Strikes 50 000 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. 0 V

Electrical Power • P = VQ/t = VI • Unit: Watt (W) = VA • k. W, MW, GW

Electrical power on the ohmic device • P = VI • V = RI (Ohm’s Law) • P = V 2/R = I 2 R