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# Chapter 34 – electric current - PowerPoint PPT Presentation

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

• 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

• 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

• 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

• 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

• 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)

• 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

• 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

• 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

• 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

• 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

• 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.

• 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