Chapter 34 electric current
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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

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Chapter 34 – electric current

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Chapter 34 electric current

Chapter 34 – electric current


34 1 flow of charge

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

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

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

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

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

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

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

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

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

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

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

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


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