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Introduction to electricity and electric circuits

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)

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Introduction to electricity and electric circuits

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  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). • Chargesarequantized: Q = n · e with n = ± 1, ± 2, ± 3, … and e = 1.602 x 10-19 C • Charges areconserved. • 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 localizedand 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. • 3A in; • 2A out; • 1A in; • 4) 1A out. ? Another example on Page 214, STT 8.2

  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 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

  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 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 0 V 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.

  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 = V2/R = I2R

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