Chapter 34 – electric current

1. Chapter 34 – electric current

2. 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

3. 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

4. 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

5. 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

6. 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)

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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.

13. 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