Static Electricity Test on Electricity and Magnetism on

Static Electricity Test on Electricity and Magnetism on Feb 20 and Feb 21

Electric Charges • Negative charge comes from extra electrons • Positive charge comes from lack of electrons • Law of Conservation of Electric Charge – Net amount of charge produced is zero. • Like charges repel • Opposite charges attract • Charge can be transferred from one object to another • e = charge of one electron = 1. 602 x 10 -19 C

Insulators and Conductors • Conductors allow electrons to pass through them • Insulators prevent electrons from passing through them • Semi-conductors have characteristics of both • Free Electrons

Conduction • When a charged object touches a conductor, some of the charge will be transferred to the conductor. Now both objects will have the same charge and the objects will be repelled. • This charging is permanent. • When the charged object is removed, both objects will retain their charge.

Induction • A charged object is brought close to a conductor without touching it. The charges inside the conductor separate so that likes repel and opposites attract. The objects will be attracted to each other. • This charging is only temporary. • When the charged object is removed, the conductor returns to normal.

The Electroscope • Homework – research and build an electroscope. Make sure you know how it works and what is happening inside of it. Bring it to class in on Friday

Electric Force • Every electric charge produces a field force that attracts or repels other charges. • The electric force obeys the inverse square law. • The force of attraction/repulsion between charges is called Coulumb’s Law F = k. Q 1 Q 2/r 2 • k = 9 x 109 Nm 2/C 2

Electric Force • This equation applies for point charges only, • And only for objects at rest. • If there are two charges, the force on charge A is equal to the force on charge B. • If there are more than two charges, then the forces add together.

How do you feel about some… Examples? !

Electric Field • The electric force is a field force (force that can act without touching) therefore it generates a field around it. Anything that comes into the field will feel its force. • Every charged particle creates its own electric field and multiple fields may exist. • The field obeys the inverse square law. E = k. Q/r 2 = F/Q 2

How do you feel about some more… Examples? !

Electric Field • The electric field can be shown by drawing lines that show the direction a positively charged particle would travel. • The electric field and electric force are vectors so they can be added together. Pay attention to sign.

Electric Fields • Dipoles

Electric Field Lines • The field lines indicate the direction of the electric field • The lines are drawn so that the magnitude of the field is proportional to the number of lines drawn. Closer lines = stronger field • Electric field lines start on positive charges and end on negative ones. The # starting or ending is pro. to the mag. of the charge

Electric Fields • Strength of Field

Conductors in a Field • The electric field inside a conductor is zero. • The electric field is always perpendicular to the surface outside of a conductor.

Parallel Plates • This setup creates a scenario similar to gravity. • A charged particle moving through this setup would feel a pull (+ downward, - upward) • The result is a projectile motion problem with electric force instead of gravity. • F = ma • q. E = ma a = q. E/m

Electric Potential • Conservation of energy • Electric potential – Potential energy per unit charge • Parallel plates • Work-Energy Theorem – The amount of work required to move a charge in an electric field is equal to a change in the kinetic (or potential) energy

Electric Potential • The work done to move that charge depends on the magnitude of that charge. • Positive work is in the direction of motion. Negative work is done in the direction opposite motion. • Electric Potential (V) • V = PE/q • PEelectric = k. Q 1 Q 2/r • V = kq/r

Electric Potential Difference • The difference in electric potential as you move from point A to point B • Measured in volts • V= - Work/q = ΔPE/q • Electric Potential Difference is like Gravitational Potential Energy in the sense that you may choose a place where the electric potential is zero to make calculations easier.

Electric Potential and Electric Field • Vba = Ed • Places where the electric potential are the same are shown by equipotential lines • Drawn perpendicular to the electric field lines

Capacitors • Device that can store electric charge • Consists of two conducting surfaces not in contact • One surface acquires a positive charge, the other surface acquires a negative charge • Eventually the surfaces will become “full” and charge will leak. • A dielectric separates the two surfaces

Capacitors • Parallel Plate Capacitors • C = capacitance C = ε 0 A/d • ε 0= 1/4πk