Physics Chapter C Final Review

Conductivity Spectrum

Materials are classified based upon how easily electrons move throughout the material.

Conductor

Materials in which charges can flow easily. For example, in metals, negative charges flow. Atoms in a metal conductor contain loosely bound negative charges that are able to move freely throughout the entire material. These electrons may move throughout the material and are easily transferred from one conductor to another.

Insulator

Materials that may build up charges on the surface but do not allow motion of negative charges within the material. Material in which the negative charges (electrons) are tightly bound within the atom. When two different insulators are rubbed vigorously together, some electrons can transfer from the surface of one insulator to the other. Even then, the charges will only move from the surfaceof one insulator to the surfaceof the other insulator.

Mechanisms

Mechanistic models explain how something happened. In the case with static electricity, they explain how the tapes become charge

During rubbing and ripping

Negative charges move from one object to the other object. This leaves the object that gave away some negatives with more positives than negatives (net positive charge). The object that received the negatives ends up with more negatives than positives (net negative charge).

Net Charge

The overall charge in an object is determined by the balance of positives and negatives. When an object contains an equal number of positives and negatives, the object has no overall charge (net charge = 0) or "neutral"

Model for Static Electricity

(1) In nature there are two types of charge. Scientists have named these charges positive(+) and negative(-). (2) Like charges repel (+ and +) and (-and -); unlike charges attract (+ and -). (3) Within most materials, only negative charges move. (4) Objects are said to be negatively charged when they have more negatives than positives or positively charged when they have more positives than negatives. (5) Objects that have an equal number of positive and negative charges have zero net charge and are said to be uncharged (neutral). (6) Materials contain a large number of charges that cannot be created or destroyed. (7) Insulators build up excess charge on the surface. Negative charges do not move through the insulator.

Observation of Repulsion

- Both balloons that were rubbed by wool repelled each other - Both T tapes repelled each other - Both B tapes repelled each The Model forStatic Electricity can explain why objects repel: (1) when both objects have negative charges added to them (both net negatively charged) (2) when both objects have negative charges removed from them (both net positively charged)

Observation of charged and neutral objects

- Both negatively and positively charged tapes attracted neutral objects - In the simulator, the negatively charged balloon was attracted to the neutral wall Both positive charges and negative charges exist in the neutral object. When a negatively charged object is brought near the neutral object, the negative charges in the neutral object are repelled and move awayfrom the surface. This leaves a higher concentration of positives near the surface of the object, resulting in attraction to the negatively charged object. The opposite happens when a positively charged object is brought near the neutral object: the negative charges in the neutral object are attracted and move towardthe surface. This leaves a higher concentration of the negatives near the surface of the object, resulting in attraction to the positively charged object.

Observation of Attraction

- The balloon and the wool attracted each other - The T and B tapes attracted each : After two different objects are rubbed together, they attract one another. We can infer that one object became positively charged and the other became negatively charged. According to the model, negative charges are transferred from one object to the other during the rubbing/ripping process. This leaves one object with an excess of negatives and the other with an excess of positives.

C.1 Questions

1. Summarize the process of making and revising models in science. 2. What two things can a scientific model do? 3. When does a scientific model need to be revised? 4. What does it mean for an object to be neutral? 5. A student makes the claim that neutral objects do not have any charges. How would you respond to this student? 6.Before being rubbed with wool, a balloon is neutral. Draw a picture of the charges in a neutral balloon. 7. Scientists use the term net chargeto refer to the balance of charges in an object. What is meant by the term balance? 8. If you observe attraction between two objects, what inference can you make about the net charge on each of the objects?

Charged vs Uncharged

Charged: - Positively or Negatively Charged - Nonzero Net Charge - Unbalanced Charges - Rubbed Uncharged: - Neutral - Zero Net Charge - Balanced Charges - Un-rubbed

Conductivity

Easily negative charges (electrons) move through a material. Materials with high conductivity are called conductors and materials with low conductivity are called insulators.

Charging by transfer

Insulators become charged when they are rubbed together or ripped apart and negative charges move from one object to the other.

Charge on the surface

Net charge is built up on the surface of insulators in the location where the rubbing/ripping happened. Rubbing insulators (like rubber, Styrofoam, wool, fur, glass, etc.) only causes a net charge on the specific location where the objects were rubbed together. Charges do not move throughout the insulator or across the surface.

Repulsion and Attraction

Objects with like charges repel and objects with opposite charges attract. Charged objects are attracted to neutral objects.

Gain or lose negative charges

Some insulators tend to become positively charged and others tend to become negatively charged.The direction of charge transfer depends on what materials are rubbed together

Gain Negatives

Styrofoam, rubber, plastic

Lose Negatives

Rabbit fur, Glass, human hair

Insulators

Rubber, Plastic, Glass

Conductors

Silver, Copper, Aluminum

Discharge of a Material

When a conductor comes into contact with a material that has excess charge, the electrons will move through the conductor so the charged object becomes neutral.

Overall

When charges build up on the surface of an insulator from rubbing or ripping, some negative charges can move to a conductor when the insulator touches the conductor. Some of the negative charges from the surface of the insulator will then move through the conductor. You observed the effects of charge movement in conductors. When the negatively charged insulator was brought near the soda can apparatus, the negative charges in the soda can (conductor) were pushed toward the opposite end of the can and into the conducting foil tinsel. The negatively charged tinsel then repelled each other and the can, causing the tinselto stand up.

Electrical Discharge

happens when electrons suddenly flow through a material from a location more negatively charged to a location less negatively charged. The greater the difference in overall charge between the two locations, the greater the discharge.