Physics ch 281
Current, Resistance, and Electromotive Force
Sections 1-5, 7
Chapter 28
Physics ch 282
Current
Movement of charges Scalar quantity Amount of charge transferred per unit time
Measured in amperes or amps 1A = 1 C/s
t
QI
Physics ch 283
Traditional current direction
Current flows in the direction that positive charge would flow.
It is impossible to experimentally determine which charges are moving.
We know that the electrons are actually moving in the opposite direction of current flow, but we keep with tradition.
Physics ch 284
Drift velocity
Speed of the moving particles. n particles per unit volume are moving in a
conductor with speed v. In a time Dt, each particle moves vDt. If the conductor has a cross sectional area A,
then the number of particles in a given section during a given time interval is nAvDt.
Physics ch 285
Drift velocity
If each particle has a charge q, the total charge moving through the volume in a given time interval is DQ = nqvADt.
nqvAt
QI
Physics ch 286
Current density
Current per unit cross-sectional area
nqvA
IJ
Physics ch 287
Resistivity
Ratio of electric field to current density
Big resistivity means that a big E field is needed to cause a given current density,
Or that a small E field will cause a small current density
J
E
Physics ch 288
Resistivity
For metals, increases with temperature.
For semiconductors, it decreases with temperature.
For superconductors it drops suddenly as temperature decreases.
00 1 TTT
Physics ch 289
Resistance
Related to resistivity. Often more useful, because it uses total
current, not current density. It also uses potential difference instead of E field.
Measured in ohms 1 W = 1 V/A
A
lR
Physics ch 2810
Ohm’s Law
Very important
IRV
Physics ch 2811
Temperature dependence
00 1 TTRRT
Physics ch 2812
Resistors
Circuit elements designed to have a specific resistance.
Used to control current.
Physics ch 2813
Circuits
We need a complete circuit to have a steady current.
Charge moves in the direction of decreasing potential energy – like rolling downhill.
We need to have a device to move the charge back uphill – like the pump in a fountain.
Physics ch 2814
Electromotive force
What makes the charge move from lower to higher potential.
Abbreviated emf – say each letter. Batteries Solar cells Generators
Physics ch 2815
Circuits
Contain resistors, sources of emf, and possibly other circuit elements.
The algebraic sum of the potential differences around the path is zero.
In a simple loop, the current is the same everywhere.
Physics ch 2816
Sources of emf
Maintain a constant potential difference, or voltage, regardless of the current flowing through them.
Have positive and negative terminals.– The potential is higher at the positive terminal.
VE
Physics ch 2817
Internal resistance
Real sources of emf have some internal resistance to current flow.
So, IrV EIRIr E
rRI
E
Physics ch 2818
Example on page 632
Look at schematic drawings of circuits.
Physics ch 2819
Kirchoff’s loop rule
The sum of the voltages around a loop is zero.
Choose a current direction and draw it on the diagram. It’s OK if you’re wrong.– If at the end we get negative current, then the
direction was wrong. No big deal.
Physics ch 2820
Kirchoff’s loop rule
If we go across a battery from – to +, we add the voltage. + to – we subtract it.
Always subtract IR from a resistor See example on page 635
Physics ch 2821
Power
Work done per unit time, measured in watts. 1 W is 1 J/s 1 A is 1 C/s 1 V is 1 J/C (1A)(1V) = 1 J/s
IVP
Physics ch 2822
Power
Using Ohm’s law, V = IR
IRIP
RIP 2
VR
VP
R
VP
2
Physics ch 2823
Power output of emf source
rIIIVP 2 E
Physics ch 2824
Power input of emf source
rIIIVP 2 E
Physics ch 2825
Physiological effects of currents
Nervous system is electrical Currents as low as 1 mA can disrupt the
nervous system enough to cause death by fibrillation.
Larger currents through the heart may actually be safer – the heart is temporarily paralyzed and has a better chance of restarting with a normal heartbeat
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