Static Electricity
Ck12 Science
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AUTHOR Ck12 Science
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Chapter 1. Static Electricity
C HAPTER
1
Static Electricity
C HAPTER O UTLINE 1.1
Forces on Charged Objects
1.2
Coulomb’s Law
1.3
References
Electricity is very rarely visible, but it can be seen in the form of lightning. Lightning is a large spark that occurs when many electrons move very quickly to even out an unequal distribution of electrons. Other, smaller sparks can be seen when you scuff your feet across carpet and then touch a metal doorknob. Both these sparks represent the same form of electricity: static electricity. Static electricity is also responsible for allowing a balloon that you rubbed on your sweater to cling to a wall and for causing your clothes to come out of the dryer stuck together.
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1.1. Forces on Charged Objects
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1.1 Forces on Charged Objects • • • • •
Describe the changes that occur in the sub-atomic arrangement in matter when charged. Describe how to charge an object. Define conductors and insulators. Understand the difference between conduction and induction. Summarize the forces between charged objects.
Lightning is the discharge of static electricity that has built up on clouds. Every year, the earth experiences an average of 25 million lightning strikes. Lightning bolts travel at speeds up to 60,000 miles per second, and can reach temperatures of 50,000°F, which is five times the temperature of the surface of the sun. The energy contained in a single lightning strike could light a 100 Watt light bulb 24 hours per day for 90 days. Forces on Charged Objects
Electric charges exist within the atom. At the turn of the 20th century, J. J. Thomson and Ernest Rutherford determined that atoms contain very light-weight negatively charged particles called electrons and more massive, positively charged particles called protons. The protons are lodged in the nucleus of the atoms, along with the neutrally charged particles called neutrons, while the electrons surround the nucleus. When the number of electrons in the electron cloud and the number of protons in the nucleus are equal, the object is said to be neutral. Changes to the nucleus of an atom require tremendous amounts of energy, so protons are not easily gained or lost by atoms. Electrons, on the other hand, are held fairly loosely and can often be removed quite easily. When an object loses some electrons, the remaining object is now positively charged because it has an excess of protons. The electrons may either remain free or may attach to another object. In that case, the extra electrons cause that object to become negatively charged. Atoms that have lost electrons and become positively charged are called positive ions, and atoms that have gained electrons and become negatively charged are called negative ions. Electrons can be removed from some objects using friction, simply by rubbing one substance against another substance. There are many examples of objects becoming charged by friction, including a rubber comb through 2
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Chapter 1. Static Electricity
hair, and a balloon on a sweater. In both these instances, the electrons move from the second object to the first, causing the first object to become negatively charged and the second one positively charged. Friction between the tires on a moving car and the road cause the tires to become charged, and wind causes friction between clouds and air which causes clouds to become charged and can result in tremendous bolts of lightning.
A common method of producing charge in the lab is to rub cat or rabbit fur against stiff rubber, producing a negative charge on the rubber rod. If you hold a rubber rod on one end and rub only the tip of the other end with a fur, you will find that only the tip becomes charged. The electrons you add to the tip of the rod remain where you put them instead of moving around on the rod. Rubber is an insulator. Insulators are substances that do not allow electrons to move through them. Glass, dry wood, most plastics, cloth, and dry air are common insulators. Materials that allow electrons to flow freely are called conductors. Metals have at least one electron that can move around freely, and all metals are conductors. Forces are exerted on charged objects by other charged objects. You’ve probably heard the saying "opposites attract," which is true in regards to charged particles. Opposite charges attract each other, while like charges repulse each other. This can be seen in the image below. When two negatively charged objects are brought near each other, a repulsive force is produced. When two positively charged objects are brought near each other, a similar repulsive force is produced. When a negatively charged object is brought near a positively charged object, an attractive force is produced. Neutral objects have no influence on each other.
A laboratory instrument used to analyze and test for static charge is called an electroscope. Seen below, an electroscope consists of a metal knob connected by a metal stem to two very lightweight pieces of metal called leaves, shown in yellow. The leaves are enclosed in a box to eliminate stray air currents. 3
1.1. Forces on Charged Objects
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When a negatively charged object is brought near the knob of a neutral electroscope, the negative charge repels the electrons in the knob, and those electrons move down the stem into the leaves. Excess electrons flow from the rod into the ball, and then downwards making both leaves negatively charged. Since both leaves are negatively charged, they repel each other. When the rod is removed, the electroscope will remain charged because of the extra electrons added to it.
Conversely, if the rod is brought near the knob but doesn’t touch it, the electroscope will appear the same while the rod is near. That is, the negative charge in the rod repels the electrons in the ball, causing them to travel down to the leaves. The leaves will separate while the rod is nearby. No extra electrons were added to the electroscope, meaning that the electrons in the electroscope will redistribute when the negatively charged rod is taken away. The leaves return to neutral, and they stop repelling each other. If the rod touches the knob, the electroscope leaves are permanently charged but if the rod is brought near but does not touch the knob, the electroscope leaves are only temporarily charged. If the leaves are permanently charged and the rod removed, the electroscope can then be used to determine the type of unknown charge on an object. If the electroscope has been permanently negatively charged, and a negatively charge object is brought near the knob, the leaves will separate even further, showing the new object has the same charge as the leaves. If a positively charged object is brought near a negatively charged electroscope, it will attract some of the excess electrons up the stem and out of the leaves, causing the leaves to come slightly together. Similar to the results of a negatively charged rod, if a positively charged rod is brought near the knob of a neutral electroscope, it will attract some electrons up from the leaves onto the knob. That process causes both of the leaves to 4
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Chapter 1. Static Electricity
be positively charged (excess protons), and the leaves will diverge. If the positively charged rob is actually touched to the knob, the rob will remove some electrons and then when the rob is removed, the electroscope will remain positively charged. This is a permanent positive charge.
Charging an object by touching it with another charged object is called charging by conduction. By bringing a charged object into contact with an uncharged object, some electrons will migrate to even out the charge on both objects. Charging by conduction gives the previously uncharged object a permanent charge. An uncharged object can also be charged using a method called charging by induction. This process allows a change in charge without actually touching the charged and uncharged objects to each other. Imagine a negatively charged rod held near the knob, but not touching. If we place a finger on the knob, some of the electrons will escape into our body, instead of down the stem and into the leaves. When both our finger and the negatively charged rod are removed, the previously uncharged electroscope now has a slight positive charge. It was charged by induction. Notice that charging by induction causes the newly charged object to have the opposite charge as the originally charged object, while charging by conduction gives them both the same charge.
Summary
• Electric charges exist with the atom. • Atoms contain light-weight, loosely held, negatively charged particles called electrons and heavier, tightlyheld, positvely charged particles called protons. • When the number of electrons and the number of protons are equal, the object is neutral. • The loss of electrons gives an ion a positive charge, while the gain of electrons gives it a negative charge. • Materials that allow electrons to flow freely are called conductors, while those that do not are called insulators. • Opposite charges attract, and like charges repel. • Charging an object by touching it with another charged object is called charging by conduction.
Practice
Questions The following video shows a young woman placing her hands on a Van de Graf generator which then gives her a static charge. Use this resource to answer the two questions that follow. http://www.youtube.com/watch?v=87DqbdqBx8U
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1.1. Forces on Charged Objects
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MEDIA Click image to the left or use the URL below. URL: http://www.ck12.org/flx/render/embeddedobject/63057
1. What happens to her hair when she touches a ground? 2. What happens to her hair when she steps off the platform? This video shows the static charge from the Van de Graf generator. http://www.youtube.com/watch?v=prgu6AvauuI
MEDIA Click image to the left or use the URL below. URL: http://www.ck12.org/flx/render/embeddedobject/63059
This video demonstrates superconductivity that occurs at extremely low temperatures. http://www.youtube.com/watch?v=nWTSzBWEsms
MEDIA Click image to the left or use the URL below. URL: http://www.ck12.org/flx/render/embeddedobject/63061
Additional Practice Questions: Questions 1. When a glass rod is rubbed with a silk cloth and the rod becomes positively charged, 1. 2. 3. 4.
electrons are removed from the rod. protons are added to the silk. protons are removed from the silk. the silk remains neutral.
2. Electric charge is 1. 2. 3. 4.
found only in a conductor. found only in insulators. conserved. not conserved.
3. When two objects are rubbed together and they become oppositely charged, they are said to be charge by 1. conduction. 6
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Chapter 1. Static Electricity
2. induction. 3. friction. 4. grounding. 4. Two objects each carry a charge and they attract. What do you know about the charge of each object? 1. 2. 3. 4.
They are both charged positively. They have opposite charged from each other. They are both charged negatively. Any of the above are possible.
5. A material that easily allows the flow of electric charge through it is called a(n) 1. 2. 3. 4.
insulator. conductor. semiconductor. heat sink.
6. What is the most common way of acquiring a positive static electrical charge? 1. 2. 3. 4. 5.
by losing electrons by gain protons by losing protons by gaining electrons by switching positions of electrons and protons in the atom
Review
Questions 1. How does friction generate static electricity? 1. Friction heats the materials, thus causing electricity. 2. Rubbing materials together displaces atoms, causing sparks to fly. 3. Rubbing materials together can strip electrons off atoms, causing one material to become positive and the other to become negative. 4. Rubbing materials together causes neutrons and electrons to trade places. 5. None of the above. 2. What electrical charge does an electron have? 1. 2. 3. 4. 5.
A negative charge. A positive charge. A neutral charge. May be any of the above. None of the above.
3. What happens when opposite charges get close to each other? 1. 2. 3. 4. 5.
They repel each other. They attract each other. Nothing happens. They attract surrounding objects. They repel surrounding objects.
4. What is an electrical conductor? 1. A material that allows electrons to travel through it freely. 2. A material that doesn’t allow electrons to travel through it freely. 7
1.1. Forces on Charged Objects
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3. A material that melts at low temperature. 4. A material that creates free electrons. 5. None of the above. 5. Which of the following is a good insulator of electricity? 1. 2. 3. 4. 5.
Copper Iron Rubber Salt water None of these.
• electrons: A fundamental sub-atomic particle, meaning it cannot be broken into smaller particles. Electrons are found in the “electron cloud” surrounding an atomic nucleus, or they may break free and exist as a free electron. • protons: A stable, positively charged, sub-atomic particle, found in atomic nuclei in numbers equal to the atomic number of the element. • neutral: A neutral particle, object, or system is one that has a net electric charge of zero. • conductors: Materials through which electric charge can pass. • insulator: Substances that block or retard the flow of electrical current or charge. • positive ions: An atom or a group of atoms that has acquired a net positive charge by losing one or more electrons. • negative ions: An atom or a group of atoms that has acquired a net negative charge by gaining one or more electrons. • ions: An atom or a group of atoms that has acquired a net electric charge by gaining or losing one or more electrons. • electroscope: An instrument used to detect the presence and sign of an electric charge by the mutual attraction or repulsion of metal foils. • charging by conduction: Involves the contact of a charged object to a neutral object. • charging by induction: A method used to charge an object without actually touching the object to any other charged object.
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Chapter 1. Static Electricity
1.2 Coulomb’s Law • • • •
State Coulomb’s Law. Describe how electric force varies with charge and separation of charge. State the SI unit of charge. Solve problems using Coulomb’s Law.
Electric cars are becoming more popular. One large advantage for electric cars is the low cost of operation, which may become an ever bigger advantage as gas prices climb. Energy costs for electric cars average about one-third of the cost for gasoline engine cars, but they can only travel about 200 miles per charge at this point. These cars run using the science of electrical charges and forces. Coulomb’s Law
The questions regarding the relationship between the electrical force, the size of the charge, and the separation between the charges were solved by Charles Coulomb in 1785. He determined that electrical force between two charges is directly related to the size of the charges and inversely proportional to the distance between the charges. This is known as Coulomb’s Law. Fe =
Kq1 q2 d2
In this equation, q1 and q2 are the two charges, d is the distance between the two charges, and K is a constant of proportionality. Fe is the electric force, which occurs as a result of interactions between two charged particles. For the purpose of calculating electric forces, we assume all charge is a point charge, in which the entire charge of the particle is located in a massless point. The SI unit of charge is the coulomb, C, which is the charge of 6.25 × 1018 electrons. The charge on a single electron is 1.60 × 10−19 C. The charge on a single electron is known as the elementary charge. The charge on a proton is the same magnitude but opposite in sign. When the charges are measured in coulombs, the distance in meters, and the force in Newtons, the constant K is 9.0 × 109 N · m2 /C2 . The electrical force, like all forces, is a vector quantity. If the two charges being considered are both positive or both 9
1.2. Coulomb’s Law
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negative, the sign of the electrical force is positive and this force is repulsive. If the two charges are opposite in sign, the force will have a negative sign and the force is attractive. Example Problem: Object A has a positive charge of 6.0 × 10−6 C. Object B has a positive charge of 3.0 × 10−6 C. If the distance between A and B is 0.030 m, what is the force on A? Solution: Fe =
Kq1 q2 d2
=
(9.0×109 N·m2 /C2 )(6.0×10−6 C)(3.0×10−6 C) (0.030 m)2
= 180 N
The positive sign of the force indicates the force is repulsive. This makes sense, because both objects have a positive charge. Example Problem: In the sketch below, the charges are q1 = 10.0 × 10−6 C, q2 = 2.0 × 10−6 C, and q3 = −6.0 × 10−6 C. Calculate the total force on q2 .
Solution: Fe = Fe =
Kq2 q3 d2
=
Kq1 q2 d2
=
(9.0×109 N·m2 /C2 )(10.0×10−6 C)(2.0×10−6 C) (2.0 m)2
(9.0×109 N·m2 /C2 )(2.0×10−6 C)(−6.0×10−6 C) (4.0 m)2
= 0.045 N (towards q3 )
= −0.007 N (towards q3 )
Since the two forces act in the same direction, their absolute values can be added together; the total force on q2 is 0.052 N towards q3 . Summary
• Coulomb determined that electrical force between two charges is directly related to the size of the charges and inversely proportional to the distance between the charges: Fe = Kqd12q2 • The SI unit of charge is the coulomb, C, which is the charge of 6.25 × 1018 electrons. • The charge on a single electron is 1.60 × 10−19 C and is known as the elementary charge. • The electrical force is a vector quantity that is positive in repulsion and negative in attraction. Practice
Questions The following video covers Coulomb’s Law. Use this resource to answer the questions that follow. http://www.youtube.com/watch?v=rYjo774UpHI
MEDIA Click image to the left or use the URL below. URL: http://www.ck12.org/flx/render/embeddedobject/63063
1. What happens when like charges are placed near each other? 2. What happens when opposite charged are placed near each other? 3. What happens to the force of attraction if the charges are placed closer together? 10
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Chapter 1. Static Electricity
Practice problems on Coulomb’s Law: http://physics.info/coulomb/problems.shtml Review
Questions 1. Suppose that two point charges, each with a charge of +1.00 C, are separated by a distance of 1.0 m: 1. Will the charges attract or repel? 2. What is the magnitude of the force between them? 3. If the distance between them is doubled, what does the force become? 2. What is the electrical force between two balloons, each having 5.00 C of charge, that are 0.300 m apart? 3. Two spheres are charged with the same charge of -0.0025 C and are separated by a distance of 8.00 m. What is the electrical force between them? 4. A red foam ball and a blue foam ball are 4.00 m apart. The blue ball has a charge of 0.000337 C and is attracting the red ball with a force of 626 N. What is the charge on the red ball? • Coulomb’s Law: States the force of attraction or repulsion acting along a straight line between two electric charges is directly proportional to the product of the charges and inversely to the square of the distance between them. Matter is made up of atoms, which consist of small, charged particles. The tightly held, heavier, positively charged particles are called protons, while the loosely held, lighter, negatively charged particles are called electrons. When there are excess electrons in an object, it is considered to be negatively charged; when there is a deficit of electrons, the object is considered positively charged. Charged objects exert a force on each other, which can be calculated based on the objects’ charges and distance from each other using Coulomb’s Law. Some materials transmit electrons easily and are called conductors, while other materials, called insulators, restrict the transmission of electrons.
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1.3. References
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1.3 References 1. Courtesy of National Oceanic and Atmospheric Administration (NOAA). http://commons.wikimedia.org/wik i/File:Lightning_hits_tree.jpg . 2. Courtesy of NOAA Photo Library, NOAA Central Library; OAR/ERL/National Severe Storms Laboratory (NSSL). http://www.photolib.noaa.gov/htmls/nssl0016.htm . 3. Pixabay:OpenClips. Pullover: http://pixabay.com/en/jumper-pullover-sweater-jersey-150532/; Balloon: htt p://pixabay.com/en/balloon-green-circus-floating-150128/ . 4. Christopher Auyeung. CK-12 Foundation . 5. Samantha Bacic. CK-12 Foundation . 6. Samantha Bacic. CK-12 Foundation . 7. Samantha Bacic. CK-12 Foundation . 8. Flickr:ellenm1. http://www.flickr.com/photos/ellenm1/6698266317/ . 9. Samantha Bacic. CK-12 Foundation .
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