Download 34.1 Flow of Charge

34.1 Flow of Charge
• Potential difference – difference in potential
voltage until both ends reach common
equilibrium
• Sustain a difference in potential
34.2 Electric Current
• Charge = through = Amps
• Flow of electric charge through wire = amperes
• Solids – electrons carry charge because they are free to
move
• Protons are fixed inside nucleus
• Measured in amperes – SI unit = I
• One coulomb of charge per second
• One coulomb = 6.24 billion electrons
• Current carrying wire normally has a charge of zero. # of
electrons and protons coming and going are equal
34.3 Voltage Sources
• Voltage = across = Volts = pressure differential
• Whatever provides the potential difference is known as the
voltage source
• Dry cells & wet cells (chemical energy converted to
electrical energy), generators (batteries)
• Generators (like alternators in cars) convert mechanical
energy to electrical energy
• Voltage provides electric pressure to move electrons
between terminals in a circuit (emf)
• *Voltage causes current
• *Charges/current flows through a circuit because of applied
voltage across a ciruit.
34.4 Electric Resistance
• How well a material conducts charge (current)
or how much resistance it offers
– Conductivity – silver less resistant
– Thickness – thicker is less resistant
– Length – shorter is less resistant
– Temperature – lower temp is less resistant
• Superconductors – Ch 32.
• SI unit for resistance is ohms • Georg Simon Ohm – German physicist
34.5 Ohm’s Law
• Amp = I for current – “Intensite de courant”
“current intensity”
• Resistance = R for resistance
• Voltage = V for voltage
34.6 Ohm’s Law and Electric Shock
• Human body – current passing through it
causes damage, depends on amount of
voltage
• Dry skin - 24V barely a tingle/ .001A
• Moist – 24V painful/ .005A
• 120V – cause death – especially if bare feet in
a wet bathtub, reducing resistance
• To receive shock there has to be a difference
in potential
34.7 DC & AC
• DC – direct current
– Flows in one direction only
– Batteries produce this
• AC – alternating current
– Flows in alternating directions at rate of 60
cycles/second (in USA)
– Voltage in USA is 120V (in Europe is 220 -240V)
• Power transmission more efficient at higher V
• Transfer of energy quietly from 1 place to another
34.8 Converting AC to DC
• To convert AC to DC must use transformer,
diode and capacitor
• Lower voltage with transformer
• Diode acts a one way valve to allow electron
flow in only one direction
• Only ½ the cycle passes through the diode
• To maintain smooth current, use a capacitor.
Also used for storage of energy.
34.9 Speed of Electrons in a Circuit
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Signal is moving, not the electrons
Electrons vibrate at rate of 1 million km/hr
The electrons are provided by the metal wire
Drift speed – speed at which an electron
moves due to the electric field
34.10 The Source of Electrons in a
Circuit
• Source of electrons is the wire itself. We
provide this.
• Power companies sell energy, NOT electrons
• The energy causes vibrations, free electrons to
vibrate in unison.
• Large vibrations can be fatal.
34.11 Electric Power
• Power = current x voltage
• Watt = amps x volts
• Power = rate at which electrical energy is
converted into another form; such as
mechanical energy, heat, or light.