Chapter 20 - ElectricityChapter 20 - Electricity

Physics 104 – Winter 2005Physics 104 – Winter 2005

Electric ChargeElectric Charge Electric charge – a fundamental quantity of Electric charge – a fundamental quantity of

our universeour universe Charge is QUANTIZED – the smallest charge Charge is QUANTIZED – the smallest charge

that one sees in nature is the charge of the that one sees in nature is the charge of the electron – 1.602 x 10electron – 1.602 x 10-19 -19 CC

Even in our most modern quantum field Even in our most modern quantum field theories, nobody can calculate this number!!!theories, nobody can calculate this number!!!

The charge of the proton is equal in The charge of the proton is equal in magnitude, but opposite in sign, to that of magnitude, but opposite in sign, to that of the electron – THERE ARE TWO TYPES OF the electron – THERE ARE TWO TYPES OF CHARGECHARGE

Insulators and ConductorsInsulators and Conductors Most objects are composed of NEUTRALLY CHARGED atoms. Most objects are composed of NEUTRALLY CHARGED atoms.

In an ordinary atom, the number of electrons = the In an ordinary atom, the number of electrons = the number of protons. The neutrons are uncharged.number of protons. The neutrons are uncharged.

In most substances, the nuclei are rigidly positioned, and In most substances, the nuclei are rigidly positioned, and the electrons of a particular atoms orbit that atom’s the electrons of a particular atoms orbit that atom’s nucleus. They do not move around from atom to atom. nucleus. They do not move around from atom to atom. Because the electric charges do not move around easily, we Because the electric charges do not move around easily, we say that they do not CONDUCT electricity. These are known say that they do not CONDUCT electricity. These are known as INSULATORSas INSULATORS

In some substances, like metals, and water, the atoms In some substances, like metals, and water, the atoms “overlap”, and the electrons in fact become almost free to “overlap”, and the electrons in fact become almost free to move around from atom to atom. When this happens, we move around from atom to atom. When this happens, we have a CONDUCTOR.have a CONDUCTOR.

In some very special substances, like SILICON, for example, In some very special substances, like SILICON, for example, the atoms do not quite overlap, but they nearly do. By just the atoms do not quite overlap, but they nearly do. By just giving the electrons a tiny amount of extra energy, we can giving the electrons a tiny amount of extra energy, we can turn the substance from an insulator into a conductor. turn the substance from an insulator into a conductor. These substances are known as SEMICONDUCTORS.These substances are known as SEMICONDUCTORS.

Massive Charged ObjectsMassive Charged Objects Even in insulators, it is possible to “transfer” Even in insulators, it is possible to “transfer”

charge from one object to another. We can take charge from one object to another. We can take some electrons away from one object, and put some electrons away from one object, and put them onto another objectthem onto another object

We usually accomplish this by “rubbing” the We usually accomplish this by “rubbing” the object – e.g. balloon on your hairobject – e.g. balloon on your hair

This accumulated extra charge just sits there on This accumulated extra charge just sits there on the object – since it is stationary, we call it STATIC the object – since it is stationary, we call it STATIC ELECTRICITYELECTRICITY

For every electron we put onto one object, we For every electron we put onto one object, we take an electron away from another object – take an electron away from another object – CHARGE IS CONSERVEDCHARGE IS CONSERVED

Electric ForcesElectric Forces Like charges repel … opposites attract …Like charges repel … opposites attract …

Opposites don't attract happinessLeigh Dayton, Science writer14feb05

WHEN it comes to love, forget the notion that opposites attract.

University of Iowa psychologists have studied 291 newly married couplesand discovered the opposite to be true. People select romantic partnerswho are like them in interests, values and background, say the authorsof the study, Eva Klohnen and graduate student Shanhong Luo.

Hollywood’s ViewpointHollywood’s Viewpoint

Hollywood’s Viewpoint …Hollywood’s Viewpoint …

How to tell if you are kissing correctly …How to tell if you are kissing correctly …

Coulomb’s LawCoulomb’s Law

Notes: We ALWAYS put in the charges as POSITIVE, Notes: We ALWAYS put in the charges as POSITIVE, even if they are negative!!!! Then, we figure out the even if they are negative!!!! Then, we figure out the DIRECTION of the force from the (likes DIRECTION of the force from the (likes repel/opposites attract) rule.repel/opposites attract) rule.

Hey, it goes like 1/rHey, it goes like 1/r2 2 !!!!!!!!!!! Just like !!!!!!!!!!! Just like gravity !!!!!!!!!gravity !!!!!!!!!

229

221

/1099.8 CNmxkrqqkF

Q1 Q2

Initially, Q1 = Q2 –> Equal in Magnitude and Sign ….

Then, we reduce Q1 to HALF of it’s original value ….

What happens to the ANGLE ???

Newton’s 3rd Law – Equal and Opposite Forces – Angle will be smaller for BOTH

Electric Force vs. GravityElectric Force vs. Gravity Which is inherently a stronger force, Which is inherently a stronger force,

the electric force, or gravity?the electric force, or gravity? Let’s answer that question by Let’s answer that question by

considering a hydrogen atom …considering a hydrogen atom …

Electric Force vs. GravityElectric Force vs. Gravity

Electric Force vs. GravityElectric Force vs. Gravity

Electric Force vs. GravityElectric Force vs. Gravity

Electric Force vs. GravityElectric Force vs. Gravity

Electric Force vs. GravityElectric Force vs. Gravity