Physics 102: Introduction to Physics - Princeton...

7 Feb 2002 Physics 102 Lecture 1 1

Physics 102:Introduction to Physics

n Same course procedures as Physics 101

n Labs start next week

n Lectures by Ed Groth

n Electromagnetism-Waves-Optics-Quantum-Nuclear

n Course web page ishttp://physics.princeton.edu/~groth/phys102s02/


Electric Charges

n Electric charge is a conserved property of matteru Charge can be moved from one body to anotheru The smallest unit of charge appears to be

the proton charge. The electron charge is -e

n Opposite charges attract each other, and likecharges repel each other.

n Coulomb’s Law:

note similarity to Newton’s law of Gravity.

Fkq1q2

r2

with k 8.99x109 Nm2 / C2

e = 1.6 ×10−19 C


A

E

C D

B

F

G none of the above


Charging

n Charging by contact

u a charged rod is brought intocontact with a conductor, and

u charges are transferred to theconductor

n Charging by induction

u a charged rod is brought neara conductor

u the conductor is grounded

u the charged rod is removed.

Charges move freely through conductors (e.g., copper, gold, …) butstay where they are placed on insulators (e.g., wood, rubber, …).

Electroscope

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- -

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Kelvin Generator

electroscopes

mildly conducting water

water drops

wire loops

How doelectroscopesget charged ?

Hint: imagine that one drop startsout with a small charge on it.


The Electrophorus

n First the rubber (insulator)is negatively charged byfriction.

n When the metal plate(conductor) is broughtnear, a charge separationis induced, then

n negative charge flows toground from top of plate.

n When the plate is removed,it has a net positive charge


Conductors

If we place charge onto a conductingsurface, where does it reside?

Why?


The Electric Field

n The electric field is the force per unit charge on atest charge:

n It is a vector, pointing in the direction a positivecharge would be accelerated.

n The field at a point is the (vector) sum of the fieldsdue to each charge.

•Field lines begin at positive charges andend at negative charges.•The electric field is tangent to the fieldlines at all points.•The density of field lines is proportionalto the strength of the field.

E = F / q


The electric fieldinduces charges onthe grains which inturn produce torquesthat align the grainswith the field.


From http://web.mit.edu/jbelcher/www/

A point chargeattracted to aconducting coppersphere. The chargeon each is the same.

The point charge isinitially movingaway from thesphere but does nothave enough energyto escape.

The green lines show the electric field


An electric field points to the right as shown.

Initial velocity

If the charge isnegative, which of thepaths is possible?

A) 1, 2

B) 3

C) 4

D) 2

E) 6

1

23

45

6


Gravitational and Electric Potential Energy

n Recall that the work doneagainst gravity is thechange in gravitationalpotential energy

n WAB=mghA-mghB=GPEA-GPEB

n We also do work moving acharge in an electric field(force is )

n WAB=EPEA-EPEB

(change in electric potentialenergy)

F = qE


Voltage

n The voltage of an object tells you its

n Just as for gravitational potential energy, we cansense only differences in electrical potentialenergy --OR VOLTAGES.

n Zero is arbitrary: far away (infinite separation) or inlab, “ground”.

Electrical potential energy per unit charge

∆V∆EPE

q0

Units are J/C


The Van de Graaff Generator

brush

+

+

+

++

++

+


The Human Electroscope

Physics 102: Introduction to Physics - Princeton...

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