PH104 w/Dr G Lec 25 (filled)
ELECTRIC CURRENT
(Ch. 23)
Can you imagine life
without electricity?
PH 104 w/ dr. g
Lec 25
Charged objects: Spark = an electric current
 Spark jumping air gap with charged object:
 Electrons attracted across the gap, but air is insulator
 When charge high enough, air becomes temporary conductor
 Temporary CHARGE FLOW = “CURRENT” through air
 Electrons hop from one atom to next, bucket-brigade style
 When (-) ion gives up excess electron to next atom: energy released!
 After spark, less charge imbalance
1
2
How to sustain current through a conductor:
 Current in a spark: electrons “pulled”
-through the air
- - -  Away from negative, and/or towards
High e/c
High p.e.
positive, OR:
 From higher to lower electric potential
energy
 Field: each point has “potential” =
energy/charge (voltage)
 Electric current flows from higher
Low e/c
Low p.e.
voltage to lower voltage
+
+
+
+
+
+ +
 OR: charges “fall down” a voltage
difference
 Voltage difference across a conductor:
 Electrons relatively free to move around
 Charges will flow from high voltage to low
voltage:
conductor)
current  From (+) voltage to (-) voltage (+ end to – end of
Voltage drop
‐
+
Charged objects: Spark = an electric current
–
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4
How to sustain current through a conductor:
 Sustained current (direct current or DC):
 Voltage difference must be maintained :
 Battery: “re-cycles” charges through conductor outside
 Voltage difference = J/C or lost by charges, to conductor
 1 Volt = energy equal to 1 Joule per Coulomb
 12-V: maintains voltage difference of 12 V across the conductor
 Inside battery: Each coulomb of charge given 12 J of energy
+
How to sustain current through a conductor:
Charges: after losing potential energy, must be “re-energized” in
cycle
 Performs WORK on the lower-energy charges
current  Connect conductor across a BATTERY !!
+
Voltage drop
 Need: charge “pump” or “voltage source”
High V
Low V
–
–
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1
PH104 w/Dr G Lec 25 (filled)
D.C. through a conductor: Details
D.C. through a conductor: Details
 Current = amount of charge flow per second
 Flowing past any point: Coulombs/sec = Amperes (A)
 In conductor (outside battery): from (+) terminal to (-) terminal
 Current ~ Voltage difference: I ~ V
V
 Conductor details:
+
–
I  Materials: different RESISTANCE (R) to electric current
 SAME voltage across different R: get different current
 I = V / R (Definition of R)
(or: V = I R)
Units: I – amperes, A; V – volts, V; R – V/A = ohms (Ω)
 Power rating of circuit element: energy needed per sec
 J/sec = J/C x C/s = V x I (Volt-amps = WATTS)
V
+
–
I R
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Electrical circuits: I pumped by V in loop thru R
Electrical circuits: I pumped by V in loop thru R
 R: details:
 Usually: higher-R “resistor(s)” connected by low-R “leads”
 Series combination of R’s: connected in “train” from (+) to (-)
 Parallel combination of R’s: each R connect across terminals
 V is divided among R’s ; same I through each
 I is divided among R’s ; connected across same V
 Energy delivered to circuit: same across each R
 Adding R decreases resistance of combination (more choices)
 Energy delivered to circuit: shared among the R’s
I1
 Adding R increases resistance of combination
I
I
–
+
V1
V2
+
I2
–
I1 + I2 + I3 = I
I3
V3
V1 = V2 = V3
V = V1 + V2 + V3
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