19 current, resistance, & dc circuits. electric current current: net charge passing through a...
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19 Current, Resistance, & DC Circuits
Electric Current
Current: net charge passing through a cross-section per unit time.Units: Ampere (A), 1 A = 1C/s (mA, A, nA, pA)Scalar , Positive current=flowing direction of positive charges
t
QI
Resistance and Ohm’s Law
R: resistance, L: length, A:cross section, : resistivityUnit(R): Ohm(), 1= 1V/A; Unit(): ohm.meter (.m)
Temperature dependence: RT = R0[1+(T-T0)]T in Celsius, R0 = R(T=T0=20oC): temperature coefficient (0.003~0.005 oC-1)
A
LRor
A
LR
IRVI
VR
VIVI
/1
L
A
Resistivities of Different Materials
Superconductivity & Non-ohmic Conductors
Electromotive Force & Circuits
A pump is needed to keep the water circulatingpump~electromotive force (emf)
Circulating without pumpNo emf~imposiple
emf Source ()
Terminal voltage:Vab = Vab = = IR (ideal emf)
Vab = r (with internal resistance, r)
Voltage and Current Measurements
Potential drops and rises in circuit
Power: the energy consumed per unit time.
Power Output of a Source
Ir2 is the energy dissipated inside the sourceheat
Energy and Power in Electric Circuits
R
VRIIV
t
QV
t
WP
22
rIIIrIIVP 2)(
Resistors in Series & Parallel
(in series)
(in parallel)...1111
...
321
321
RRRR
RRRR
eq
eq
Combinations
Kirchhoff’s Rules
Junction Rule
Loop Rule
0 I
0V
Kirchhoff’s Rules
Electrical Measuring Instruments
AmmetersGalvanometer
Voltmeters
Resistance-Capacitance Circuits and Transient Effects
RC
t
RC
t
eQqeIiC
qiR 100
Summary: Current
Summary: Resistance & Ohm’s Law
Summary: emf
Summary: Energy & Power
R
VRIIV
t
QV
t
WP
22
rIIIrIIVP 2)(
Summary: Resistors in Parallel & Series
(in series)
(in parallel)...1111
...
321
321
RRRR
RRRR
eq
eq
Summary: Kirchhoff’s Rules
Summary: Charging & Discharging
HomeworkCh19: 1, 5, 7, 12, 24, 31, 33, 42, 48, 52, 57, 61, 63
Problem 57
Problem 61
Problem 52 & 53