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 lose 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