Chapter 8: Electricity and Magnet PDF

Chapter 8: Electricity and Magnets Physics 100 Lecture Note 1  Matter & electricity Composition of matter and electricity ▪ Matter is a pile of atoms ▪ An atom is a pile of electrons, protons and neutrons ▪ An atom normally has an equal number of electron...

Chapter 8: Electricity and Magnets Physics 100 Lecture Note 1  Matter & electricity Composition of matter and electricity ▪ Matter is a pile of atoms ▪ An atom is a pile of electrons, protons and neutrons ▪ An atom normally has an equal number of electrons and protons ▪ An electron has a much smaller mass than a proton ( protons and neutrons have about the same mass) ▪ All the electrons in the university are identical and so are protons and neutrons ▪ Electrons and protons have an electric charge The electron has a negative charge The proton has a positive charge ▪ The electron and the proton have the same amount of charge Physics 100 Lecture Note 2  Electric charges ▪ The electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter ▪ Electric charge comes in two types, called positive and negative. ▪ An object is negatively charged if it has excess electrons. ▪ An object is positively charged if it has a deficiency of electrons. ▪ An object is uncharged or neutral when the positive and negative charges in it are balanced. ▪ Two substances (objects) that are similarly charged experience a mutual repulsive force: ▪ Two negatively charged objects repel each other. ▪ Two positively charged objects also repel each other. ▪ Two objects that are oppositely charged experience an attractive force. ▪ The SI unit of electric charge is the coulomb (C). ▪ One coulomb is roughly the charge of 6.241509 × 1018 Physics 100 Lecture Note 3  The electric field ▪ If a small charge is brought near another charge, the charge feels a force: Attractive if different Repulsive if similar ▪ Since the small charge feels a force at a distance from the other charge, we imagine that the region surrounding the charge is filled with some kind of a force field, called the electric field. ▪ The strength of the electric field near a charge is the force that would be exerted on a stationary test particle of unit charge. ▪ The sense of the electric field lines indicates which way a small positive charge would accelerate if brought near the charge. ▪ The density of the electric field lines represents the strength of the force (strong near the charge and weakens away from it). Physics 100 Lecture Note 4  Electric force between charges ▪ Charges influence each other. That is they exert forces on each other. ▪ The force between two charges acts in the following way: The force between similar charges is repulsive (similar charges move from each other) The force between opposite charges is attractive. The bigger the amount of charges, the larger the force between them. The force between any two charges decreases when the distance between them becomes larger. Figure: Electric field of two charges These observations are summarized by the following Coulomb’s law. Physics 100 Lecture Note 5  Electric Force and Charges Central rule of electricity Opposite charges attract one another; like charges repel. Physics 100 Lecture Note 6  Electric Force and Charges Protons Positive electric charges Repel positives, but attract negatives Electrons Negative electric charges Repel negatives, but attract positives Neutrons Neutral electric charge Physics 100 Lecture Note 7  Electric Force and Charges Fundamental facts about atoms 1. Every atom is composed of a positively charged nucleus surrounded by negatively charged electrons. 2. Each of the electrons in any atom has the same quantity of negative charge and the same mass. 3. Protons and neutrons compose the nucleus. Protons are about 1800 times more massive than electrons, but each one carries an amount of positive charge equal to the negative charge of electrons. Neutrons have slightly more mass than protons and have no net charge. 4. Atoms usually have as many electrons as protons, so the atom has zero net charge. Physics 100 Lecture Note 8  Electric Force and Charges Ion: Positive ion—an atom losing one or more electrons has a positive net charge. Negative ion—an atom gaining one or more electrons has a negative net charge. Physics 100 Lecture Note 9  Coulomb’s Law The content of the law ▪ The force between two charges is directly proportional to the product of the two charges (𝐹 ∝ 𝑞1 × 𝑞2 ) ▪ The force between two charges is inversely proportional to the square of the 1 distance between them (𝐹 ∝ 𝑟 2 ) ▪ The direction of the force is along the line connecting the two charges Coulomb’s Law 𝑞1 𝑞2 𝐹=𝑘× 2 𝑟 ▪ F is the magnitude of force between the force between the charges, 𝑞1 𝑎𝑛𝑑 𝑞2 are the two charges, r is the distance between then and k is a constant equal to 9 × 109. ▪ The electric force is very similar to the force of gravity in the way it depends on the magnitude of the charges and the distance but it is much stronger Physics 100 Lecture Note 10  Examples ▪ Two point charges 𝑞1 = +25 𝑛𝐶 𝑎𝑛𝑑𝑞2 = −75𝑛𝐶, are separated by distance r= 3cm. Find the magnitude of the force 𝑞1 𝑒𝑥𝑒𝑟𝑡𝑠 𝑜𝑛 𝑞2 Solutions: First note that 1 nanocoulomb (nC) is 10−9 coulomb and 1cm = 0.01 m K = 9 × 109 Then 𝑞1 𝑞2 9 × 109 ∗ +25 × 10−9 ∗ −75 × 10−9 𝐹=𝑘× 2 = 𝑟 0.032 = 1.875 × 10−2 𝑁 Physics 100 Lecture Note 11  Coulomb’s Law Examples According to Coulomb's law, a pair of particles that are placed twice as far apart will experience forces that are A. half as strong. B. one-quarter as strong. C. twice as strong. D. 4 times as strong. Physics 100 Lecture Note 12  Coulomb’s Law Examples According to Coulomb's law, a pair of particles that are placed twice as far apart will experience forces that are A. half as strong. B. one-quarter as strong. C. twice as strong. D. 4 times as strong. Physics 100 Lecture Note 13  Coulomb’s Law Differences between gravitational and electrical forces: Electrical forces may be either attractive or repulsive. Gravitational forces are only attractive. Physics 100 Lecture Note 14  Magnets and the magnetic force ▪ Unlike charges, magnets come with two sides (called the north and south poles) ▪ There is a repulsive force between similar poles ▪ There is an attractive force between different poles ▪ Like charges, magnets are surrounded by a field of force (called magnetic field) ▪ Magnetic line of force from closed loops (they stick out from one pole and return to the other) ▪ Like the force between charges, the force between two magnets decreases with Figure: Magnetic fields distance in the same way Physics 100 Lecture Note 15  Sources of magnetism Permanent magnets ▪ The north pole of a magnet is defined as the pole that, when the magnet is freely suspended, points towards the Earth’s North Magnetic Pole ▪ Since opposite poles (north and South) attract, the North Magnetic Pole is actually the south pole of the Earth’s magnetic field ▪ In addition to the electric charge, the electron, proton and neutron have a small bit of magnetism. They are tiny magnets ▪ In most materials, the electrons naturally have random orientations In the result is that their small magnetism cancels itself ▪ There are, however, some materials whose electrons have the tendency to have the same orientation These are magnetic materials, such as iron, Nickel, and cobalt ▪ These materials can be initially magnetized with the help of strong, magnets, but can lose their magnetism when heated Physics 100 Lecture Note 16  Moving charges… electricity ▪ Electricity is about charges in motion ▪ Materials that conduct electricity have some of the electrons loosely attached to their atoms. ▪ These electrons can be driven by an electric force. ▪ Electric generators and batteries produces such electric force to drive electrons in circuits. Figure: Electric circuit ▪ The energy in electricity is a reflection of the motion of charges. Physics 100 Lecture Note 17 Moving charges and magnetism ▪ Moving electrons produce a magnetic field around them. This is in addition to their permanent magnetism ▪ Electricity is moving charges so electricity produces a magnetic field. ▪ Any wire that carries electricity also has some magnetism circling it. ▪ the magnetism around a wire can be made stronger by it around a rod ▪ The result is an electromagnet - a magnet is only a magnet when electricity is on Physics 100 Lecture Note 18  Electricity … charges in motion ▪ There are two types of electricity: direct and alternating current Batteries direct current (DC) The flow of charges is unidirectional ▪ The overwhelming majority of electric generators produce alternating current (AC) In alternating current, the direction of flow of charges is alienating Figure: Prototype of an electric generator Physics 100 Lecture Note 19  Applications of electromagnets An electromagnet Bar magnet Magnetic Filed Heavy duty electromagnet Physics 100 Lecture Note 20 Magnetism of the earth Figure: Earth as a giant magnet Physics 100 Lecture Note 21  Charges and magnets in motion: ▪ A charge at rest does not interact with magnets at rest but interacts with charges at rest ▪ Like wise, a magnet at rest does not interact with charges at rest but interacts with magnets at rest ▪ Things change if either the charge or the magnet is in motion Motion creates a magnetic field around a moving charge As a moving charge behaves both as a charge and also a magnet A moving chare will experience a force if passes near either a charge or a magnet ▪ Motion also creates an electric field around a moving magnet So a moving magnet behaves both as a magnet and also a charge A moving magnet will experience a force if passes near either a magnet or a charge Physics 100 Lecture Note 22  Accelerated charges & Radiation ▪ Charges and magnets in uniform motion have both electric and magnetic fields around them ▪ If a charge or a magnet is accelerated (i.e., speed or direction of motion change), then the shake of some of the electric fields into space ▪ When that happens, we say the charge/magnet is radiating ▪ In the radiation, the fields spread ways from the charge/magnet and into space ▪ Example: Radiation by an accelerated charge Physics 100 Lecture Note 23 https://phet.colorado.edu/sims/html/charges-and-fields/latest/charges-and-fields_en.html https://phet.colorado.edu/sims/html/coulombs-law/latest/coulombs-law_en.html https://phet.colorado.edu/sims/html/balloons-and-static-electricity/latest/balloons-and-static- electricity_en.html https://phet.colorado.edu/sims/html/faradays-law/latest/faradays-law_en.html Physics 100 Lecture Note 24