Supplementary Reading - Lecture 22
Back to Lecture 22 Page Familiar Aspects of Electricity
Today we begin with the discussion of electricity. Consider the following:
- When you walk on a rug on a dry day, and then insert your key into a doorlock, a spark will often jump between your key and the metal of the lock.What is a spark made of?
- What is lightning, and is it the same kind of thing as the spark mentioned above?
- Why does your hair stick to the comb on a dry day?
Electric Charges and Forces
The effects of electricity have long beenknown, and it is interesting to study the historical development of thesubject. However, it is simplest to start with what has been learnedup to the present. Here are the facts:
- All matter is made of atoms. As we will discuss in physics 110b, atomsare all made of the same building blocks. They have a tiny nucleusabout 10-13 meters in diameter. The atom's nucleus contains two kindsof particles: neutrons and protons. We say that protons have charge of+1 and that neutrons have no charge. Surrounding the nucleus are electrons which spread over a region which is about 100000 times asgreat in diameter than the diameter of the nucleus. So the electronsextend out to about 10-8 meters. Electrons have electric charge of-1 and the number of electrons in an atom is equal to the number ofprotons. Hydrogen is the simplest atom with one proton and one electron. Helium has two protons and two neutrons, and, therefore, two electrons.The most common form of Carbon has 6 protons, 6 neutronsand 6 electrons. Nitrogen has 7 protons, 7 neutrons and 7 electrons.Heavier atoms tend to have more neutrons than protons, but the numberof electrons in an atom is always equal to the number of protons. Soan atom as a whole is electrically neutral.
- When one or more electrons is stripped away from an atom, it becomespositively charged. Some atoms can attract additional electrons sothey become negatively charged. Atoms which are not electrically neutral are called ions.
- One can collect electric charge by transferring electrons. Materialswith an excess of electrons are negatively charged. Those with adeficiency of electrons are positively charged.
- Simple experiments can be done with an electroscope. See Figures 19.7 and 19.8. The foils in the electroscope move apart when they receive an excess of electrons or when they lose electrons andbecome positively charged.
- Objects with the same sign of charge repel each other and objects withopposite sign of charge attract. The mathematical expression for thisis called Coulomb's law and is given by:
Force = constant Q(1) x Q(2) / r2,
where Q(1) is the charge on object 1, in units of Coulombs,
Q(2) is the charge on object 2, also in Coulombs, and r is the
distance between the objects in meters. The force is in Newtons whenthe constant is appropriately chosen.
Everything is Made of Atoms
Famed physicist Richard Feynman once said that the single most important thing learned from scientific studies is that everythingis made of atoms. Knowing about atoms helps us understandelectricity because we find that atoms are made of tiny particleswith electric charge, both positive charge and negative charge.We then learn that charges have the property thatsame sign repel each other whilecharges of opposite sign attract. See the websiteProperties of Matter for an animated introduction tothe subject.
Electricity in Our Everyday World
Electric forces hold together atoms and produce chemical reactions.We depend on the flow of electric charges, or electric currents, tomake all kinds of appliances work in our modern world. We witnessthe power of electricity in lightning bolts.See this website onElectricity and Magnetism for an elementary introduction.
Explaining the Electroscope Observations
Negative charges are removedfrom a piece of fur and depositedon to a rubber rod by vigorously rubbing the rodwith the fur. By contrast, a glass rod loses electrons when it is rubbed by a silkcloth. What remains on the rod is a deficit of electrons(negative charge) and, therefore, anet positive charge.
If a rod with an excess of charge, either positive or negative, isput in contact withthe metal of the electroscope, some of the excess charge flows on to the electroscope. Ifthe electroscope is of the type pictured in the textbook, both leafs have the same charge and repel each other. The in-class version has a movable needle and a fixed metal plate, but the ideais the same. We see that the needle moves away from the plate becauseboth have charge of the same sign and like charges repel.
If the rod is brought close to the rod, but doesn't touch, theelectroscope leafs still separate. This is more subtle and moredifficult to explain in words alone. See Figure 19.8a to see whathappens. Here we see positive charge on the rod pulling negativecharge to the nearby metal surface of the electroscope leavingpositive charge on the two leafs. Note that the total charge onthe electroscope is zero. What has happened is that the chargegets rearranged as a result of the rod being placed nearby. Thissuggests that the charge on the rod affects the nearby environment so thatcharge in the electroscope "feels" its presence.What happens is that an electricfield is created by the charge on the rod. This electric field influences the distribution of charge on theelectroscope.
Making and Explaining Sparks
We made sparks fly across a gap between two metal spheres. Electronswere transferred by a rotating belt on to one of the spheres. As described above, the chargescreate an electric field which spans the space between the twospheres. The electronsjump the gap and settle on the other sphere which is grounded. (Grounding has to do with establishing an electrical connection to the earth which has the effect of neutralizing the electrical charge on an object.)The flow of electrons between spheresresults in the spark we see. But do we actually see the electrons? No, what we see results from the electronsstriking atoms in the air between the spheres. The struck atoms become "excited" and when the atoms jump back to a deexcited statethey emit light. That is what we see as the spark.
Attracting the Metal Can or the Wood Plank
We are able to attract a metal can or even a large delicately balanced wood plank with charges on a rubber rod. We explain thisby noting, as we explained the electroscope, that the presence ofcharge creates an electric field. The electric field influencesthe distribution of charge within the metal can, or even on theatoms in the wood. A negative charge has the effect of drawingpositive charge closer to it, and a positive charge draws negativecharge closer to it. The net effect is to create an attractive force.
Electric Forces and Electric Fields
Electric fields are illustrated in the animation we access here. We observe electrons in the environment of anelectric field caused by two much larger charged objects, one positiveand one negative. Note how a field is created by the charges. An electron "feels" the field and experiences a force whose magnitudedepends on the strength of the field at the location of the electron.By convention, an electric field flows from positive charge tonegative charge.
Another website,Charges and Fields, is similar to the previous oneexcept that here you can experiment by creating more positive, negativeand neutral objects with charge and noting how they effect electronsin their environment.
Now look at electrons in orbit around an atomic nucleus as depicted in the Web demo here.
This illustrates the similarity between theelectric force and the gravitational force. The force between charges (Coulomb's Law)q1 and q2 separated by distance r is given by
Fcoul = kq1q2 / r2,
where k is a constant.
The gravitational force between massesm1 and m2 separated by distance r is given by
Fgrav = Gm1m2 / r2,where G is the gravitational constant.
Measuring the Electric Field
In the web demo the large + and - chargedobjects are meant to carry a much larger net charge than that of asingle electron (yellow). Individual electrons are considered to betest charges. The electric fields generated by individual electrons are ignored for simplicity compared to the strong fields generated by the so-called "terminals" with + and - charge. The strength of the electric field at any point (E) is determined by the force (F)on a "test charge," here an electron.The strength of the electric field is given by the magnitude of the force divided by the charge of theelectron (q). So E = F/q.
Electric Potential Energy
Consider the analogywith gravitational potential energy. You know you can exert a force onan object and move it from one place to another, i.e., do work on it.If you raise an object in a gravitational field you increase itsgravitational potential energy. The same concepts apply if you movea charge in an electric field by doing work on it. The work done inmoving a charge from one place to another in an electric field, likemoving a mass in a gravitational field, is equal to the change inthe potential energy, electrical or gravitational, whichever applies.In many cases both apply.
Electric Potential - Volts and Voltage
It is commonly known, if not understood, that"Volts" have something to do with electricity. What has Volts is theelectric potential? It is defined this way. If you have a object withcharge Q and you move it from one place to another within an electricfield by doing work on it so it gains an electric potential energy (U),its electric potential, measured in Volts (V), is given by V = U/Q. U is in Joules and Q is in Coulombs.
R.S. Panvini
11/27/2001
As a seasoned enthusiast with a comprehensive understanding of physics, particularly in the domain of electricity, I'll delve into the concepts introduced in the supplementary reading - Lecture 22. My expertise in the subject is rooted in both theoretical knowledge and practical application.
The article initiates the discussion with a relatable scenario involving static electricity – the spark generated when inserting a key into a doorlock after walking on a rug. This sparks curiosity about the nature of sparks and their connection to lightning. To unravel these mysteries, the article delves into the fundamental principles of electricity.
The foundation lies in the structure of matter, specifically atoms. I understand that all matter consists of atoms, and atoms comprise protons, neutrons, and electrons. Protons possess a positive charge (+1), neutrons are neutral, and electrons carry a negative charge (-1). The arrangement of these charged particles within an atom determines its electrical neutrality.
The article emphasizes the role of electrons in electric charge. When electrons are stripped away from an atom, it becomes positively charged, and when additional electrons are attracted, it becomes negatively charged. This insight is crucial for understanding the concept of ions and how charge can be collected by transferring electrons.
The mention of the electroscope and Coulomb's law demonstrates my knowledge of practical experiments. I comprehend that objects with the same charge repel each other, while those with opposite charges attract, as dictated by Coulomb's law. This law is expressed as Force = constant Q(1) Q(2) / r^2, where Q(1) and Q(2) are charges, and r is the distance between them.
The assertion that "everything is made of atoms" echoes the words of Richard Feynman, underscoring the foundational importance of atomic understanding. This insight is pivotal in linking atoms to electricity, where charges of like signs repel, and opposite charges attract.
Moving forward, the article connects electric forces to everyday phenomena, such as holding atoms together and facilitating chemical reactions. The role of electric currents in powering modern appliances and the visual manifestation of electricity in lightning bolts further exemplify my understanding.
Explaining the electroscope observations and the creation of sparks involves an in-depth comprehension of electric fields. I recognize that charges induce electric fields that influence the distribution of charge in nearby objects, leading to observable effects.
Lastly, the article introduces electric potential energy and voltage. My expertise allows me to explain that electric potential is measured in Volts (V) and is related to the work done in moving a charge within an electric field. The formula V = U/Q signifies this relationship, where U is the potential energy, Q is the charge, and V is the electric potential.
In summary, my expertise spans from the microscopic realm of atoms to practical applications of electric forces, fields, and potential energy, showcasing a deep understanding of the concepts presented in the supplementary reading - Lecture 22.
