Static%20electricity%20hw.docx

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Static Electricity HW Directions: -

Read and highlight important information. Answer the questions on your paper Then log into Schoology to submit your answers for a grade so take your time and make sure you reread if needed.

Neutral vs. Charged Objects Atoms are the building blocks of matter. There are different types of atoms, known as elements. Atoms of each element are distinguished from each other by the number of protons that are present in their nucleus. The number of electrons that surround the nucleus will determine whether or not an atom is electrically charged or electrically neutral. The amount of charge on a single proton is equal to the amount of charge possessed by a single electron. A proton and an electron have an equal amount but an opposite type of charge. Thus, if an atom contains equal numbers of protons and electrons, the atom is described as being electrically neutral. On the other hand, if an atom has an unequal number of protons and electrons, then the atom is electrically charged. Any particle, whether an atom, molecule or ion, that contains less electrons than protons is said to be positively charged. Conversely, any particle that contains more electrons than protons is said to be negatively charged. Charged versus Uncharged Particles Positively Charged Negatively Charged Uncharged Possesses more Possesses more Equal numbers of protons than electrons electrons than protons protons and electrons

Charged Objects as an Imbalance of Protons and Electrons An atom was described as being a small and dense core of positively charged protons and neutral neutrons surrounded by shells of negatively charged electrons. The protons are tightly bound within the nucleus and not removable by ordinary measures. While the electrons are attracted to the protons of the nucleus, the addition of energy to an atom can persuade the electrons to leave an atom. Similarly, electrons within atoms of other materials can be persuaded to leave their own electron shells and become members of the electrons shells of other atoms of different materials. In short, electrons are migrants - constantly on the move and always ready to try out a new atomic environment. All objects are composed of these atoms. The electrons contained within the objects are prone to move or migrate to other objects. The process of an electron leaving one material object to reside (perhaps only temporarily) in another object is a common everyday occurrence. Even as you read the words of this web page, some electrons are likely moving 1

through the monitor and adhering to your clothing (assuming that you are using this resource online) (and wearing clothes). If you were to walk across the carpeting towards the door of the room, electrons would likely be scuffed off the atoms of your shoes and moved onto the atoms of the carpet. And as clothes tumble in the dryer, it is highly likely that electrons on one piece of clothing will move from the atoms of the clothing onto the atoms of another piece of clothing. In general, for electrons to make a move from the atoms of one material to the atoms of another material, there must be an energy source, a motive, and a low-resistance pathway. For now, it is sufficient to say that objects that are charged contain unequal numbers of protons and electrons. Charged objects have an imbalance of charge - either more negative electrons than positive protons or vice versa. And neutral objects have a balance of charge - equal numbers of protons and electrons. The principle stated earlier for atoms can be applied to objects. Objects with more electrons than protons are charged negatively; objects with fewer electrons than protons are charged positively.

Check Your Understanding (remember to submit your answers in Schoology) 1. TRUE or FALSE: An object that is positively charged contains all protons and no electrons.

2. TRUE or FALSE: An object that is negatively charged could contain only electrons with no accompanying protons.

3. TRUE or FALSE: An object that is electrically neutral contains only neutrons.

Charge Interactions Suppose that you rubbed a balloon with a sample of animal fur such as a wool sweater or even your own hair. The balloon would likely become charged and its charge would exert a strange influence upon other objects in its vicinity. If some small bits of paper were placed upon a table and the balloon were brought near and held above the paper bits, then the presence of the charged balloon might create a sufficient attraction for the paper bits to raise them off the table. This influence - known as an electric force occurs even when the charged balloon is held some distance away from the paper bits. The electric force is a non-contact force. Any charged object can exert this force upon other objects - both charged and uncharged objects.

Opposites attract. And likes repel. 2

These two fundamental principles of charge interactions will be used throughout the unit to explain the vast array of static electricity phenomena. There are two types of electrically charged objects - those that contain more protons than electrons and are said to be positively charged and those that contain less protons than electrons and are said to be negatively charged. These two types of electrical charges - positive and negative - are said to be opposite types of charge. And consistent with our fundamental principle of charge interaction, a positively charged object will attract a negatively charged object. Oppositely charged objects will exert an attractive influence upon each other. In contrast to the attractive force between two objects with opposite charges, two objects that are of like charge will repel each other. That is, a positively charged object will exert a repulsive force upon a second positively charged object. This repulsive force will push the two objects apart. Similarly, a negatively charged object will exert a repulsive force upon a second negatively charged object. Objects with like charge repel each other.

The Electric Force and Newton's Third Law This electric force exerted between two oppositely charged objects or two like charged objects is a force in the same sense that friction, tension, gravity and air resistance are forces. And being a force, the same laws and principles that describe any force describe the electrical force. One of those laws was Newton's law of action-reaction. According to Newton's third law, a force is simply a mutual interaction between two objects that results in an equal and opposite push or pull upon those objects. Let's apply Newton's third law to describe the interaction between Object A and Object B, both having positive charge. Object A exerts a rightward push upon Object B. Object B exerts a leftward push upon Object A. See diagram at right. These two pushing forces have equal magnitudes and are exerted in opposite directions of each other. Each object does its own pushing upon the other. The push upon Object B (by Object A) is directed away from Object A; and the push upon Object A (by Object B) is directed away from Object B. Because of the away from nature of the mutual interaction, the force is said to be repulsive.

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Now let's apply the same action-reaction principle to two oppositely charged objects - Object C (positive) and Object D (negative). See diagram at right. Object C exerts a leftward pull upon object D. Object D exerts a rightward pull upon Object C. Again, each object does its own pulling of the other. Just as before, these two forces have equal magnitudes and are exerted in opposite directions of each other. However in this instance, the direction of the force on Object D is towards Object C and the direction of the force on Object C is towards object D. Because of the towards each other nature of the mutual interaction, the force is described as being attractive.

Interaction Between Charged and Neutral Objects The interaction between two like-charged objects is repulsive. The interaction between two oppositely charged objects is attractive. What type of interaction is observed between a charged object and a neutral object? The answer is quite surprising to many students of physics. Any charged object - whether positively charged or negatively charged - will have an attractive interaction with a neutral object. Positively charged objects and neutral objects attract each other; and negatively charged objects and neutral objects attract each other. This third interaction between charged and neutral objects is often demonstrated by physics teachers or experienced by students in physics lab activities. For instance, if a charged balloon is held above neutral bits of paper, the force of attraction for the paper bits will be strong enough to overwhelm the downward force of gravity and raise the bits of paper off the table. If a charged plastic tube is held above some bits of paper, the tube will exert an attractive influence upon the paper to raise it off the table. And to the bewilderment of many, a charged rubber balloon can be attracted to a wooden cabinet with enough force that it sticks to the cabinet. Any charged object - plastic, rubber, or aluminum - will exert an attractive force upon a neutral object. And in accordance with Newton's law of action-reaction, the neutral object attracts the charged object.

Repulsion versus Attraction Because charged objects interact with their surroundings, an observed interaction provides possible evidence that an object is charged. Suppose that you enter the physics classroom and observe 4

two balloons suspended from the ceiling. Rather than hanging straight down vertically, the balloons are hanging at an angle, exhibiting a repulsive interaction as shown at the right. The only way that two objects can repel each other is if they are both charged with the same type of charge. Thus, the repulsion of the balloons provides conclusive evidence that both balloons are charged and charged with the same type of charge. One could not conclude that the balloons are both positively charged or both negatively charged. Additional information or further testing would be required to make a conclusion about the type of excess charge present upon the balloons. Nonetheless, one can be convinced that both balloons possess an excess charge - either positive or negative. Now let's contrast the observation of repulsion with that of attraction. Suppose that you now enter the physics classroom and observe two balloons suspended from the ceiling and exhibiting an attractive interaction as shown at the right. There are two underlying reasons for two objects attracting each other. One balloon could be neutral and the other balloon charged or both balloons could be charged with the opposite type of charge. Thus, your only conclusion could be that at least one of the balloons is charged. The other balloon is either neutral or charged with the opposite type of charge. You cannot draw a conclusion about which one of the balloons is charged or what type of charge (positive or negative) the charged balloon possesses. Additional information or further testing would be required to make these conclusions. For example, if you could take each balloon and individually bring them near some neutral bits of paper, you could test to see if each individual balloon is charged or neutral. If a balloon were charged, then it would exhibit an attractive interaction with the neutral paper bits. On the other hand, an uncharged balloon would not interact at all with neutral paper bits. The above thought experiments illustrate the conclusive nature of a repulsive interaction. When objects repel each other, one can be certain that both objects are charged. On the other had, the observation of an attractive interaction leads to limited conclusions. At best, one can conclude that at least one of the objects is charged.

We'll conclude this part of Lesson 1 by asking the question "How can a charged object and a neutral object attract?" As you've read this page, you might have been thinking something like "But I've only heard of two fundamental charge interactions - opposites attract and likes repel. Where did this third charge interaction come from?" In all likelihood, most of us have only heard of two types of charge interactions (opposites attract and likes repel); and both of these charge interactions are fundamental interactions. The third statement - any charged object and a neutral object will attract each other - is simply an observable fact that can be explained by the two fundamental charge interactions 5

Check Your Understanding (remember to submit your answers in Schoology) 1. Electrical forces ____. a. can cause objects to only attract each other b. can cause objects to only repel each other c. can cause objects to attract or repel each other d. have no effect on objects

2. On two occasions, the following charge interactions between balloons A, B and C are observed. In each case, it is known that balloon B is charged negatively. Based on these observations, what can you conclusively confirm about the charge on balloon A and C for each situation.

3. Upon entering the room, you observe two balloons suspended from the ceiling. You notice that instead of hanging straight down vertically, the balloons seems to be repelling each other. You can conclusively say ... a. both balloons have a negative charge. b. both balloons have a positive charge. c. one balloon is charge positively and the other negatively. d. both balloons are charged with the same type of charge. 6

4. Jean Yuss is investigating the charge on several objects and makes the following findings. Object C

Object D

Object E

Object F

attracts B

repels C

attracts D

attracts A

repels F Jean knows that object A is negatively charged and object B is electrically neutral. What can Jean Yuss definitively conclude about the charge on objects C, D, E, and F? Explain.

5. Two objects are charged as shown at the right. Object X will ____ object Y. a. attract

b. repel

c. not affect

6. Two objects are shown at the right. One is neutral and the other is negative. Object X will ____ object Y. a. attract

b. repel

c. not affect

7. Balloons X , Y and Z are suspended from strings as shown at the right. Negatively charged balloon X attracts balloon Y and balloon Y attracts balloon Z. Balloon Z ____. List all that apply. a. may be positively charged b. may be negatively charged c. may be neutral d. must be positively charged e. must be negatively charged f. must be neutral

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