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