17'1 Electric Current

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17.1 Electric Current

• Electric current
• Drift speed
• Current sourcesBatteries

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Electric Current, I
rate of charge flow through a surface
I
q
Units Coulombs/sec Ampere (A)
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Electric Current
I
-q
This is also the rate of q in the opposite
direction.
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A flashlight bulb carries a current of 0.1 A. (a)
Find the charge passed in 10 s. (b) How many
electrons does this correspond to.
(a)
(b)
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Drift velocity
vd
Vd Drift velocity n no. of charge
carriers/volume q- charge per charge carrier
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17.2 Find the drift velocity of electrons in Cu.
For I10 A, A3x10-6 m2. Use density of Cu,
?8.95 g/cm3(each atom of Cu contributes 1
carrier electron) MA63.5g/mole
I
vd
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Charge flow slowly in a wire Carry kinetic energy
like water in a pipe The energy can be released
eg. Lightbulb
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Current flow in conductors
E
e-
Collisions of electron slow it down.
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Voltage sources Batteries
V
Voltmeter
I
e
e
Chemical Energy-gtElectrical Energy Cu has a
higher affinity for electrons than Zn, produces ?V
Zn
ions
Cu
Cu
Zn
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Some Chemical Batteries

• Zinc-carbon battery - standard carbon battery,
  zinc-carbon chemistry is used in all inexpensive
  AA, C and D dry-cell batteries..
• Alkaline battery - Used in common Duracell and
  Energizer batteries, the electrodes are zinc and
  manganese-oxide, with an alkaline electrolyte.
• Lithium photo battery - Lithium, lithium-iodide
  and lead-iodide
• Lead-acid battery - Used in automobiles,
• Nickel-cadmium battery - The electrodes are
  nickel-hydroxide and cadmium, with
  potassium-hydroxide as the electrolyte
  (rechargeable).
• Nickel-metal hydride battery - This battery is
  rapidly replacing nickel-cadmium
• Lithium-ion battery - With a very good
  power-to-weight ratio,

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A 5 g Zn electrode is used in a battery . How
long would the battery last before all of the Zn
is converted to Zn if the battery drew a
current of 0.1A? MA65.4g/mole
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Voltages in series
V2
VV1V2
V
V1
Voltmeter
V1?V
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Voltages in series
Voltage sources with opposite polarity
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Voltages in parallel
V
VV1
V1
V
1
Maximum current x2
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Voltages in parallel
I
V
V0
V
V1
1
Short circuit