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

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Presentation on theme: "Introduction to electricity and electric circuits electric charges, currents and voltage."— Presentation transcript:

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

2 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.

3 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.

4 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.

5 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.

6 conduction electrons in a conductor

7 Electric current=Q/t

8 Conservation of current

9 Q1.Q1. Another example on Page 214, STT 8.2 1)3A in; 2)2A out; 3)1A in; 4) 1A out. ?

10 How to create an electric current What is the time-dependence of current in the wire?

11 A Battery needed to keep currents flowing !! Voltage of a battery

12 Electric potential Gravitational Potential Energy mgh Gravitational Potential gh Electrical potential Energy qV Electrical potential V

13 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

14 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 10 -4 V

15 Resistance/conductance Valid for “ohmic” devices mainly metallic conductors at constant temperatures.

16 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

17 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

18 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?

19 Q2 A and B conductors have same cross sections. But A is longer than B. Which is the correct graph?

20 Electrical Hazards Feel: 1mA pain: few mA deadly: over 70mA Estimate the resistance of a human body !!

21 Grounded High Voltage Lines, Lightning Strikes 1 000 000 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.

22 Electrical Power P = VQ/t = VI Unit: Watt (W) = VA kW, MW, GW

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

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