Electrostatics

The basic unit of positive charge is the proton The basic unit of positive charge is the proton. (Although protons are ultimately made up of quarks) The basic unit of negative charge is the electron. It is almost always electrons that are moving when charge “flows” The SI unit of charge is the Coulomb ( C). Charge of 1e- = 1 proton = 1.6x10-19 Coulombs

Q

Jeff rubs a piece of fur on glass rod, giving the rod a negative charge. What is the most likely thing that happens? Protons are removed from the rod Electrons are added to the rod The fur is also charged negatively The fur is left neutral

Jeff rubs a piece of fur on glass rod, giving the rod a negative charge. What is the most likely thing that happens? Protons are removed from the rod Electrons are added to the rod The fur is also charged negatively The fur is left neutral

Conductors & Insulators Conductors are materials in which charges are free to move. Metals are a good example. Insulators are materials in which charges can not move. Glass, plastics, and wood, are examples. Q

Which of the following best characterizes electrical conductors? Low mass density 2) High tensile strength 3) Poor heat conductors 4) charges move freely 5) All of the above

Which of the following best characterizes electrical conductors? Low mass density 2) High tensile strength 3) Poor heat conductors 4) charges move freely 5) All of the above

Consider the “pith” ball… - + - - - + + + - + + - Charges are balanced, so the ball is neutral

Why is the neutral pith ball attracted to the negative rod? Consider the “pith” ball… Why is the neutral pith ball attracted to the negative rod? - + - + - - - - - - - - - + + - - + + -

Why does it then bounce away? Consider the “pith” ball… + - - - + - -

Consider the electroscope

Consider the electroscope + + + + + + + + + + + +

Charging by conduction—A physical transfer of charge + Explain what happens in pictures & words + Consider the electroscope

Polarized but Still neutral + - + Polarized but Still neutral

Electrons transfer from the scope to the rod + - +

+ Scope is left positive. I can tell it is charged because the leaves repel each other. Scope is left positive. +

Consider the electroscope Charging by Induction - A transfer of charge, but only two neutral objects touch + + + + + + + Be ready to explain what happens in pictures & words + + + + + Consider the electroscope

Charging by Induction + + + + + + + + + + + + - + Still neutral

Charging by Induction + + + + + + + + + + + + - - - - - - - - - + + + + + + + +

These charges remain held in place - - Charging by Induction + + + + + + + + + + + + - - - - - - - - - These charges remain held in place - - These charges flow from the ground to the electroscope. + + + - - + - + + +

These charges now spread out. Explain what happens in pictures & words Charging by Induction http://regentsprep.org/Regents/physics/phys03/aeleclab/induct.htm + + + + + + + + + + + + - - - - - - - - - These charges now spread out. The electroscope is now charged. - - - - - - -

Q

An uncharged conductor is supported by an insulating stand An uncharged conductor is supported by an insulating stand. I pass a positively charged rod near the left end of the conductor, but do not touch it. The right end of the conductor will be… Negative 2) Positive 3) Neutral 4) Attracted 5) Depends on the materials.

An uncharged conductor is supported by an insulating stand An uncharged conductor is supported by an insulating stand. I pass a positively charged rod near the left end of the conductor, but do not touch it. The right end of the conductor will be… 1) Negative 2) Positive 3) Neutral 4) Attracted 5) Depends on the materials.

An uncharged conductor is supported by an insulating stand An uncharged conductor is supported by an insulating stand. I pass a positively charged rod near the left end of the conductor, but do not touch it. The right end of the conductor will be… 1) Negative 2) Positive 3) Neutral 4) Attracted 5) Depends on the materials. + + - + + - + + + - + - + + - + +

Two charges, +Q and –Q, are located two meters apart as shown Two charges, +Q and –Q, are located two meters apart as shown. Which vector best represents the direction of the electric field at the point above them? 2 1 3 4 + -

Two charges, +Q and –Q, are located two meters apart as shown Two charges, +Q and –Q, are located two meters apart as shown. Which vector best represents the direction of the electric field at the point above them? 2 1 3 4 + -

Two charges, +Q and –Q, are located two meters apart as shown Two charges, +Q and –Q, are located two meters apart as shown. Which vector best represents the direction of the electric field at the point above them? + -

Two point charges, separated by 1. 5cm, have charges of +2 and -4C Two point charges, separated by 1.5cm, have charges of +2 and -4C. Suppose we determine that 10 field lines radiate out from the +2C charge. If so, what might be inferred about the -4C charge with respect to field lines? 1) 20 radiate out 2) 5 radiate out 3) 20 radiate in 4) 10 radiate in 5) 5 radiate in

Two point charges, separated by 1. 5cm, have charges of +2 and -4C Two point charges, separated by 1.5cm, have charges of +2 and -4C. Suppose we determine that 10 field lines radiate out from the +2C charge. If so, what might be inferred about the -4C charge with respect to field lines? 1) 20 radiate out 2) 5 radiate out 3) 20 radiate in 4) 10 radiate in 5) 5 radiate in

Shocking Fingers and Lightning Rods On a regularly shaped object, charges are evenly spread. On an irregularly shaped object, charge tends to accumulate at areas of largest curvature/smallest radii. Small radius, large curvature Big radius, small curvature

In other words, charge accumulates at sharp points

Ben Franklin invented the lightning rod. Q

At what point is the charge per unit area greatest on the surface of an irregularly shaped conducting object? Where the surface curves inward Where the surface is flat Where the curvature is greatest (smallest radius) Where the curvature is least (largest radius)

At what point is the charge per unit area greatest on the surface of an irregularly shaped conducting object? Where the surface curves inward Where the surface is flat Where the curvature is greatest Where the curvature is least

Faraday Cage http://www.youtube.com/watch?v=Zi4kXgDBFhw

The Electric Field inside a conducting surface is zero. Conducting Cup Insulating Stand

+ Pict. 1 The Electric Field inside a conducting surface is zero. Charged Ball + Pict. 1

+ Pict. 2 The Electric Field inside a conducting surface is zero. + - Polarized Charged + - + + - - + + - - - - - - + + Pict. 2 + + +

Charge remains on the outside only The Electric Field inside a conducting surface is zero. The negative charges from the polarized inside get neutralized as the positive ball comes in contact with them. The charge from the positive ball is now left on the outside of the cup. Charge remains on the outside only + + + + + + + + + Pict. 3 + + + +

A negative charge comes close to the conductor and the conductor polarizes. Pict. 1 But not all the negative charge can accumulate on the far side because then the far side would be “too” negative so some stays in the center leaving the center neutral. - + -

The rod touches, the electrons transfer, and the outside is left negative while the inside is still neutral. Pict. 2 - + -

Pict. 3 - - - + + + - - - - - + + + - - + - - + - + + - + + - - - + -

A faraday Cage is a metal enclosure in which charge will always flow to the outside, thus leaving the inside neutral.

A faraday Cage is a metal enclosure in which charge will always flow to the outside, thus leaving the inside neutral. Not only do Faraday cages block charge, they more importantly block electromagnetic radiation. Applications of a Faraday Cage Microwave Oven Electronic shielding Lightning protection

During a lightning storm you are relatively safe in… A metal framed building. A car. (not a convertible however)