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Electric current
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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