Electrostatics and Charge

Creating Electric Fields

Electric Charges

Recall that all matter is made of atoms. Neutral atoms can acquire a “charge” in several different ways, all of which require movement of electrons (-).

If an object gains electrons, it is negatively charged.

If an object loses electrons, it is positively charged.

The transfer of electric charges

occurs in 3 ways:

1. Friction – electrons

transferred by rubbing.

2. Contact – electrons

are transferred by direct

contact.

3. Induction – the

charging of an object without

direct contact.

Electric Fields

Once an object has a charge, it can influence the behavior of “stuff” around it. This influence is due to an electric field around a charged object.

Behavior of Charges

The rules that govern charged particles say, Like

charges REPEL and opposite charges ATTRACT.

Movement of Charge

A charge traditionally moves away from positive (+) toward something negatively charged (-).

The electric field lines around a charged particle indicate strength in that area.

These lines are vectors.

Static Electricity Static electricity is a

buildup of charge on an object. This charge will not MOVE from one place to another in a constant, sustainable stream so it is “stationary” for all practical purposes.

2

21

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qqkF

Van de Graaff Generator

In 1929 Robert Jemison Van de Graaff invented the Van de Graaff generator, which was a way to produce high voltages and accelerate particles.

Demos:

1 person hair

Pie plates & punches

Class conductivity

Your turn to Practice

Please do Ch 20 Rev p 477 #s 1,4,7

Pg 478 #s 14,17,18,20,21,25, & 26

From Static to Electric current

You cannot rub a balloon on your head and then plug in your radio to run off the charge on your hair…darn!!!!

Electric current is the flow of electric charge through a conductor either in one direction (DC) or alternating directions (AC).

Pushing charge along

Just as water will not flow in a pipe unless one end is at a different gravitational potential than the other, electric charge will not “flow” unless there is a difference in Electric potential across a conducting path.

Electric potential= electrical potential energy / charge.

1volt = 1 joule / 1 Coulomb

A coulomb is the unit of charge.

Moving Charge

Charged objects seek out the lowest possible electric potential.

Electric current is the flow of electric charge.

The unit of current is the ampere or amp (A). One amp means one coulomb of charge flows per second.

Making Charge move

Charges only move when there is a potential difference between two points.

Anything that provides this potential difference is a voltage source.

Voltage provides the electric pressure that moves the charges. Voltage may also be called electromotive force (emf).

Obstacles

The flow of electric current can be interrupted by the object it passes through.

This is called resistance. Resistance is measured in ohms ( ).

Length & thickness of a wire can effect its resistance. The material the wire is made of, and it’s temperature will also determine the resistance.

Resistors

Resistors dissipate energy by heat. The material of the resistor is conductive, but not as conductive as the wire used to connect circuit components. The symbol for a resistor is:

15

Capacitors

Capacitors store energy as an electric field bound between the plates of the capacitor. The charge polarity of the capacitor will match the polarity of the circuit power source. Once a capacitor is charged, no additional current will flow through. The symbol for a capacitor is:

50 F

Look inside a Resistor

Resistor in a Circuit

i

E

Capacitor in a Circuit

EEE

Ohm’s Law

ohm

voltamp

1

11

voltagecurrent

resistance

V

RI

Series Circuit

When circuit elements are one after another, it is called a series circuit.

In a series circuit, the same current flows through every element.

Series Circuit Math

Resistance in series adds.

8 412

R1 R2

Req

Req = R1 + R2

Series Circuit Math II

The voltage drop across each element in a series circuit adds to up to the voltage of the source.

Vdrop = I * R (V8Ω) = 1A * 8Ω = 8V(V4Ω) = 1A * 4Ω = 4V

12V = 8V + 4V

12V

8 4

When circuit elements are side-by-side, it is called a parallel circuit.

The current splits in a parallel circuit.

Parallel Circuit

Parallel Circuit Math

Resistance in parallel adds by inverses.

Req

Req

=R1 + R2

1 1 1

55

2.5

Parallel Circuit Math II

The voltage drop across parallel branches is the same.

The current is greatest in the branch with the least resistance.

Your turn to Practice

Please do CD worksheet 35-1 & 35-2