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Electric current
 

Electric current

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    Electric current Electric current Presentation Transcript

    • Chapter 34 – electric current
    • 34.1 – flow of charge
      • Charge moves when a conductor has a potential difference
      • Charge flows until no difference in potential
      • To sustain flow of charge, something must keep one end at a higher potential
      • Compare this to water flowing from a reservoir
      • Something must continually pump water to maintain a difference in height
    • 34.2 – electric current
      • The flowing of electric charge
      • Only electrons
      • Variable: I, I = q/t
      • SI unit: ampere (A), the “amp”, 1 A = 1 C / sec
      • The same number of e enter conductor as leave
      • The net charge is always zero
    • 34.3 – voltage sources
      • The “pump” for the charges,
      • causes a potential difference
      • Must have capacity to maintain
      • constant flow
      • Batteries  chemical reaction
      • Generators  convert mech. work to electrical energy
      • The voltage (potent. diff) is what forces charges to move – “electric pressure”
      • 120 V give 120 J to each coulomb of charge
      • Current is the flowing of charge through a circuit, voltage causes the flowing
    • 34.4 – Electric resistance
      • Current (charge flow) depends on:
      • Voltage & resistance (R) – the tendency to slow movement of charges
      • We can ↑ current by either:
      • ↑ voltage (electric pressure) or
      • ↓ resistance (or both)
      • Resistance of conductor depends:
      • Conductivity (how well it conducts)
      • Thickness – thicker = < resistance
      • Length – shorter = < resistance
      • Temperature – cooler = < resistance
    • 34.5 – ohm’s law
      • At very low temperatures, some materials loose all resistance & become superconductors
      • High temp superconductor > 100 K
      • Ohm’s “law” states: current is directly related to voltage and resistance  I = V/R
      • 2 x V = 2 x I, 2 x R = I/2
      • Therefore: small R = large current
      • current produces heat, ↑ current = ↑ heat (like toaster)
    • 34.6 – Ohm’s law & electric shock
      • Damage is caused by current – not voltage
      • Depends upon voltage & body resistance
      • Rwith salt water ~ 100 Ω, Rdry ~ 500,000 Ω
      • Voltage drives current: ↑ voltage  ↑ current
      • Touching outlet while dry (120 V)  small current
      • Wet while grounded ↑ current dramatically, poss. Fatal
      • Distilled water – good
      insulator
      • Adding salt ↓ resistance
    • High voltage wires
      • Parts of body at the same elect. potential – no shock
      • Why birds sit on high voltage wires
      • Charges move down path of least resist. – the wires, not bird
      • Safe to hold onto wire – as long at you do not touch anything else
    • Grounding wires
      • If surfaces of appliances are at different potential, touching them creates a path for current to flow (a shock)
      • To prevent this, a third wire of plug is grounded and connected to appliance
      • Any “short” will be “grounded”
      Health Effects
      • Shock causes: overheating of tissue & disrupt nerve functions
    • 34.7 – direct vs. alternating current
      • Direct (DC) – charges flow in only one direction
      • Alternating (AC) – charges move back and forth
      • In US, occurs 60 times/sec (60 Hz) at 120 V
      • The wires constantly change polarity
      • Positive ↔ Negative
      • AC used because: voltage easily changed, produced naturally as AC
    • 34.9 – speed of electrons
      • Electrons move in random directions w/i conductor until an E - field is created by a potential difference
      • The e experience a force, moving them along E – field
      • Constant collisions (w/ rigid particles of conductor) cause heating & slow the motion of e – drift velocity
      • AC the e oscillate back and forth (60 times/sec) from one location, delivering energy
    • 34.10 – source of electrons
      • The e that power circuits come from the conductors that make up the circuit
      • e do not come from power companies, they are already in the conductors (wires)
      • Power companies provide the energy (via an E-field) that causes the charges to move
      • This energy is converted to heat, light, sound, etc.
    • 34.11 – electric power
      • The rate at which electric energy is converted to mechanical energy
      • P = I V
      • Provides a relationship between power & current
      • A kilowatt • hour = energy
      • Energy companies charge some amount per kilowatt • hour