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
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