Electric Charge,

Energy and

CapacitanceChapter 17 and 18

Essential Questions

What is electricity?

How can you shock someone by scuffing your feet on the carpet and then creating contact?

Objective(s): Students will be able to…

Understand the basic properties of electric charge.

Differentiate between conductors and insulators.

Distinguish between charging by contact, charging by induction, and charging by polarization.

Agenda:

Welcome back!

Discuss the rest of the year.

Term 3 Grades.

Intro to electricity.

Notes: What is electric charge? Electrical Conductors and Insulators Charging Objects

Chapter 15 Tests back Tomorrow

Electricity at an Atomic Level

All matter has electric charge because it contains: protons (positive charge),

and electrons (negative

charge)

Usually these are stuck together, but in electrical conductors (like copper), electrons are free to move around.

Electric Charge

Like magnets: like charges repel each other and unlike charges attract.

Electric Charge

Electric charge is conserved. If an object transfers electrons

to another object, it becomes more positively charged while

the other object becomes negatively charged.

Losing electrons = more positive

Gaining electrons = more negative

In the picture above, the balloon gains electrons from the woman’s hair.

Quantifying Electric Charge

Because charge comes from the transfer of electrons, all amounts of charge are multiples of the charge of an electron (e).

Charge (q) is measured in Coulombs (C). electron: -1.60 x 10-19 C proton: 1.60 x 10-19 C neutron: 0 C

A charge of -1.0 C (1/e) contains 6.2 x 1018 electrons. Therefore, 1 Coulomb is a HUGE amount of charge.

Conductors and Insulators

Conductors allow electrons to flow freely through them. Charge is easily distributed

through the material evenly. Most metals.

Insulators inhibit the flow of electrons. Excess charge has nowhere to go,

and remains on the surface. Glass, rubber, silk, plastic…

Conductors and Insulators

Semiconductors are used to control the flow of electrons under specific conditions. Shared properties of conductors and insulators. Silicon and germanium.

Superconductors allow the flow of electrons perfectly (no “friction”). But they only work at very low temperatures (~ -

250°C)

Charging by Contact

When two materials are rubbed together, like balloons and hair, electrons are actually knocked off the hair onto the balloon.

This is charging by contact.

Conductors that are charged quickly become neutral unless protected by an insulator. Think copper rod with a rubber

handle.

Charging by Induction

If an object is grounded, it can be charged by induction.

Excess electrons flee to the ground when a charged object is brought nearby, leaving a positive charge.

This is induction. The charge is induced on the object.

Polarization

A surface charge can be induced in insulators.

The electrons can’t flow, but they can turn away. This is polarization.

This is why a charged balloon can stick to a wall (but not a metal doorknob).

Recap

Electric charge is created when there is an unequal number of electrons and protons.

Conductors allow the flow of charge easily, while insulators inhibit the flow of charge.

Objects can be charged by contact, induction, and polarization.

Homework

Due Wed:

p633 #1, 3-5

Electric Force and Electric Fields17.2 and 17.3pp 634 - 652

Essential Questions

How do electric charges exert forces on each other?

What do the forces from charges or acting on charges look like?

Objective(s): We will be able to…

Calculate electric force using Coulomb’s Law.

Calculate electric field strength.

Draw and interpret electric field lines.

Agenda:

New Seats!

Review Chapter 15 Test

Review 17.1 Homework

Notes Electric Force and Coulomb’s Law Electric Field Strength Electric Field Lines

Start Homework

Electric Force

Because electric charges (electrons and protons) attract and repel each other, they exert forces on each other.

These forces are equal in magnitude, and opposite in direction.

These forces are field forces; no direct contact. Like gravity.

The forces between charges can be measured using Coulomb’s Law.

Coulomb’s Law

k is a constant relating force to the strength of the charges and the distance between them.

k ≈ 8.99 x 109 Nm2/C2 (“Newton-meter squared per Coulomb squared”)

Coulomb’s Law Qualitatively

What happens to the force if one of the charges doubles?

What happens to the force if the distance between charges doubles?

Practice Problem

Calculate the force between two 0.001 C charges that are 3.00 meters apart.

~1000 N

Stuff to watch out for: metric conversions! expect forces to be large and charges to be

small.

What we are skipping.

Theory of superposition.

i.e. more than two charges.

at angles.

yuck.

The Electric Field

A charged object sets up an electric field in the space around it. Kind of like the pull of gravity around a planet.

The strength of the field can be defined as the ratio of the force it applies to charged object and the charge of the object

Or…

The Electric Field

The units of E, the electric field strength, are [N/C] (Newtons of force per Coulomb of charge)

In other words, how much force acts on a charge this big?

Electric Field Strength If we assume the charge is very very small (a

point charge) we can calculate the general electric field strength:

Here are some common electric field strengths.

Electric Field Lines We can visualize what electric fields

look like by drawing electric field lines.

These are drawn by following the simple mantra: WWPCD? What would a positive charge do?

The answer? Run away from positive and go toward negative.

Convention: The number of field lines is proportional to the size of the charge.

Field Lines of Equal ChargesSame Charge Opposite Charges

Field Lines of Unequal Charges

Note that the positive charge is twice that of the negative, so it has twice the field lines.

It overpowers the negative charge.

Recap

Electric charges create forces that act on each other and can be measured with Coulomb’s Law.

Electric force is a field force, and the strength of that force acting on other charges can be measured.

Electric field lines help to visualize the electric force field from charges.

Homework

p 642 #1, 5

p 647 #3

p652 # 2