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11/22/2011

FOUNDATION PHYSICS II
ELECTRICITY & MAGNETISM GEOMETRICAL OPTICS & PHYSICAL OPTICS MODERN PHYSICS

ELECTRICITY & MAGNETISM
• Electric Charges, Forces & Fields • Electric Potential & Electric Potential Energy • Electric Current & Direct Current Circuits • Magnetism • Magnetism Flux & Faraday’s Law of Induction • Electromagnetic Waves

Electric Charges, Forces, and Fields
• Electric Charge • Insulators and Conductors

19-1 Electric Charge
The effects of electric charge were first observed as static electricity:

• Coulomb’s Law
• The Electric Field • Electric Field Lines • Shielding and Charging by Induction • Electric Flux and Gauss’s Law After being rubbed on a piece of fur, an amber rod acquires a charge and can attract small objects.

Charged Objects and the Electric Force

19-1 Electric Charge
Like charges repel and unlike charges attract each other. Quiz 1 All electrons have exactly the same charge; the charge on the proton (in the atomic nucleus) has the same magnitude but the opposite sign:

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19-1 Electric Charge
The electrons in an atom are in a cloud surrounding the nucleus, and can be separated from the atom with relative ease.

19-1 Electric Charge

When an amber rod is rubbed with fur, some of the electrons on the atoms in the fur are transferred to the amber:

19-1 Electric Charge
We find that the total electric charge of the universe is a constant: Electric charge is conserved. Also, electric charge is quantized in units of e. The atom that has lost an electron is now positively charged – it is a positive ion The atom that has gained an electron is now negatively charged – it is a negative ion

19-1 Electric Charge

Some materials can become polarized – this means that their atoms rotate in response to an external charge. This is how a charged object can attract a neutral one.

19-2 Insulators and Conductors
Conductor:

Insulators and Conductors
Insulator: Almost no charge flows Most other materials Charge flows freely Metals

Not only can electric charge exist on an object, but it can also move through an object. Substances that readily conduct electric charge are called electrical conductors. Conductors are material whose conduction electrons are free to move throughout. Most metals are conductors. Materials that conduct electric charge poorly are called electrical insulators. Insulators are material whose electrons seldom move from atom to atom. Most insulators are non-metals.

Some materials are semiconductors.

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19-2 Insulators and Conductors

19-2 Insulators and Conductors

If a conductor carries excess charge, the excess is distributed over the surface of the conductor.

Semiconductors have properties intermediate between conductors and insulators; their properties change with their chemical composition. Photoconductive materials become conductors when light shines on them.

19-3 Coulomb’s Law
Coulomb’s law gives the force between two point charges:

19-3 Coulomb’s Law
The forces on the two charges are action-reaction forces.

The force is along the line connecting the charges, and is attractive if the charges are opposite, and repulsive if the charges are like.

19-3 Coulomb’s Law
If there are multiple point charges, the forces add by superposition.

19-3 Coulomb’s Law

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19-3 Coulomb’s Law

19-3 Coulomb’s Law

19-3 Coulomb’s Law

19-4 The Electric Field
Definition of the electric field:

Coulomb’s law is stated in terms of point charges, but it is also valid for spherically symmetric charge distributions, as long as the distance is measured from the center of the sphere. Here, q0 is a “test charge” – it serves to allow the electric force to be measured, but is not large enough to create a significant force on any other charges.

The Electric Field

19-4 The Electric Field
E qq 1 F  k 2o qo r qo
If we know the electric field, we can calculate the force...