Electrical Charges and the Electrical Interaction

What do you know about electrical charges and their behavior?

What do you know about electrical charges at the molecular, atomic, and subatomic level?

Electrical Charges and the Electrical Interaction

Charges in nature

nucleon level and larger

sub-nucleon level

questions raised by electrical understanding of atoms

They are part of atoms.

The nucleus contains protons (positively charged).

Electrons (negatively charged) orbit the nucleus andsometimes can hop off and move around freely.

Most materials are neutral.

To give a material net charge, electrons must beadded or removed.

Charge is a conserved quantity.

What are charged particles?

Electrical Charges and the Electrical Interaction

Charges as the bearers of the electrical interaction

one kind of charge, but positive and negative

Like charges repel each other and opposite charges attract each other.

Scotch tape lab

X-rays from Scotch tape

http://www.exo.net/~pauld/summer_institute/summer_day14electrostatic/tape_electroscope.html

Four pieces of tape

• Pull two pieces off of table – bring near each other

• Pull two pieces off of other tape – bring near each other

• Pull one piece off of table, another off of a piece of tape, bring near each other

Demonstrate likes repel, opposites attract?

What are some of the differences between gravitational forces andelectrical forces?

1. Gravitational forces attract, while electrical forces can attract orrepel.

2. Gravitational forces depend on the masses of the interactingobjects, while electrical forces depend on the charges of theinteracting objects.

Similarity: Both decrease as the interacting objects move furtherapart.

What are some of the differences between gravitational forces andelectrical forces?

1. Gravitational forces attract, while electrical forces can attract orrepel.

2. Gravitational forces depend on the masses of the interactingobjects, while electrical forces depend on the charges of theinteracting objects.

Similarity: Both decrease as the interacting objects move furtherapart.

Gravitational ForceElectrical Force(Coulombs Law)

Do assignment 5 worksheet on Coulomb’s law

Van de Graff demo

The dielectric breakdown strength of dry air, at Standard Temperature and Pressure (STP), between spherical electrodes is

approximately 33 kV/cm

The Ability of Materials to Transport Charge

Insulators and conductors

great range of conductivity

Superconductors

Semiconductors

Iron man

Processes of Charging

By frictionBy contactBy induction

Processes of Charging

By friction

By contactBy induction

http://phet.colorado.edu/en/simulation/travoltage

Processes of Charging

By friction

By contactBy induction

Electroscope lab

Processes of Charging

By frictionBy contact

By induction

If the metal sphere is grounded while the wand induces a charge, the electrons will flow away to ground and leave the sphere with a net positive charge.

An electric charge creates an electric field in the space surrounding it. Asecond charge placed in this field will experience an electrical force.

Electric field = force per unit charge

The direction of the field lines represents the direction of the force on apositive test charge. They never cross.

Electric Field

The Electric Field

• The concept of a field compared to action at a distance• Representation of the electric field by lines of force

http://web.ncf.ca/ch865/englishdescr/3DEFldIdentCharges.html

http://phet.colorado.edu/en/simulation/electric-hockey

What would the electric field lines look like for a charged conducting sphere?

What would the electric field lines look like for a charged conducting plate?

Electric Potential

Definition

distinction from electric potential energy

analogy to gravitational potential

E is force per unit charge

V is Electric potential energy per unit charge

E is force per unit charge

V is Electric potential energy per unit charge

Parallel plate capacitor

Charge separation

An atom can become electrically polarized if a nearby charge causes its electron cloud to

become distorted.

Charge separation in atoms can be a widescale effect.

Review of terms

The electric potential is defined as the electric potential energy perunit charge.

charge

energy potentialelectric potentialelectric

charge

forceelectric fieldelectric

The electric field is defined as the electric force per unit charge.

Conceptual questions1) Why do clothes often cling together after tumbling in a clothes dryer?

2) At some automobile toll-collecting stations, a thin metal wire sticks up from the road and makes contact with cars before they reach the toll collector. What is the purpose of this wire?

3) An electroscope is a simple device consisting of a metal ball that is attached by a conductor to two thin leaves of metal foil protected from air disturbances in a jar, as shown. When the ball is touched by a charged body, the leaves that normally hang straight down spread apart. Why? (Electroscopes are useful not only as charge detectors, but also for measuring the quantity of charge: the more charge transferred to the ball, the more the leaves diverge.)

4) Is it necessary for a charged body to actually touch the ball of the electroscope for the leaves to diverge?

Conceptual questions5) The five thousand billion, billion freely moving electrons in a penny repel one

another. Why don't they fly out of the penny?

6) How does the magnitude of electric force compare between a pair of charged particles when they are brought to half their original distance of separation? To one-quarter their original distance? To four times their original distance? (What law guides your answers?)

7) The proportionality constant k in Coulomb's Law is huge in ordinary units, whereas the proportionality constant G in Newton's law of gravitation is tiny. What does this indicate about the relative strengths of these two forces?

8) If you rub an inflated balloon against your hair and place it against a door, by what mechanism does it stick? Explain.

Conceptual questions9) How can a charged atom (an ion) attract a neutral atom?

10) If you place a free electron and a free proton in the same electric field, how will the forces acting on them compare? Their accelerations? Their directions of travel?

11) Suppose that a metal file cabinet is charged. How will the charge concentration at the corners of the cabinet compare with the charge concentration on the flat parts of the cabinet?