Engineering Physics: Electricity and Magnetism - Assignment Example

Physics Chapter 21 Can you speak of the capacitance of an isolated conducting sphere The lines of force of a charged spherical conductor are radial .the charge on the sphere acts as if it is concentrated at the centre of the sphere and that that the surface of the sphere is just a number of points external to charge itself. Let R be the radius of a sphere having charge Q. the potential at a point on the surface due to the charge is Q/R. since potential inside a conductor is the same everywhere, this expression itself gives the potential of any point in the interior of the sphere. But the potential of a conductor is Q/C where C is the capacitance. Therefore C=R. hence the capacity of an isolated charged sphere in air is numerically equal to the radius in centimeters. A capacity of x cm. means that capacity of sphere of radius x cm. in e.s. units. If the medium is one of s.i.c. equal to K, the capacity of the sphere is KR. 2) How does the direction signal device on your car make use of a capacitor The capacitor in the direction signal receives current from the battery. The signal light (or alarm) derives current from the capacitor. Initially when the switch is on the current from battery flows into the capacitor and charges it. After the capacitor is charged the power from the capacitor is used. Gradually the capacitor gets discharged and the light becomes dimmer and dimmer. In the meantime the capacitor again derives energy from the battery and gets charged. This chain process continues until the switch is off. 3) If an ammeter is connected in series to the circuit what will it indicate Why In this case it will indicate the amount of current passing through it .this is because when the ammeter is connected in the series the current passing though the circuit will also pass through it. The current passes through the needle in the ammeter and it will deflect. The deflection here is proportional to the current flowing .the ammeter is designed in such a way that the current passed through the device will be shown a readings in it. 4) Discuss the statement, "If a capacitor contains equal amounts of positive and negative charge it should be neutral and have no affect whatsoever" The energy stored in the capacitor is a function of its charge. When it contains equal number of positive and negative charge the net charge gets nullified and it becomes neutral. 5) In tuning in you favorite radio station, you adjust the tuning knob, which is connected to a variable capacitor. How does this variable capacitor work A variable condenser consists of two sets of aluminum plates. One set which is fixed is called stator and other set which is movable is called rotor. The rotor can be adjusted by turning the knob and thus we get variable capacitance. 6) If the potential difference across a capacitor is doubled what does this do to the energy that the capacitor can store If the potential difference is doubled, the energy stored goes up by a factor of four; energy stored is proportional to the square of the pd i.e. E = CV. 7) Is more energy stored in capacitors when they are connected in parallel or in series Why The energy stored in capacitors is more when they are connected in parallel. This can be derived as follows .the energy of a capacitor is directly proportional to tits capacitance and directly proportional to the square of the voltage. For a given set of capacitors the capacitance is more when they are connected in parallel than in series .similarly when capacitors are connected in parallel the voltage applied at both the ends is same for each capacitors. But in the case of series connection the voltage gets divided .so when compared to series the parallel connection offers more capacitance and voltage. Thus energy also is more in parallel connection than in series connection. 8) If the applied potential difference across a capacitor exceeds the dielectric strength of the medium between the plates, what happens to the capacitor When the potential difference exceeds the dielectric strength of the medium the medium ceases to act as an insulator and a spark or arc occurs at this potential which burns the dielectric material. 9) Can a coaxial cable have a capacitance associated with it Yes a coaxial cable can have a capacitance associated with it. Chapter 22 1) Should there be a law similar to Coulomb's law of electrostatics that shows the force between magnetic poles What would be the advantages and disadvantages of such a law Yes there is law framed by Coulomb in 1820 which states that the force of attraction or repulsion between isolated point magnetic poles is proportional to the pole strength and inversely proportional to the square of the distance between them .the main advantages of this law was that it was useful in calculating the magnetic strength of a material .the main drawback was that the poles of a thin magnet are assumed to be point poles as it is not possible to get isolated poles in practice. 2) In the very early days of nuclear physics, nuclear radiation was described in terms of alpha, beta, and gamma particles. How did Rutherford use a magnetic field to distinguish among these particles The magnetic field was used to distinguish the particles in the following way. A magnetic field was applied perpendicular to radioactive element emitting a narrow beam of radiation. The rays which deflected towards the negative plate were discovered as alpha particles and the particles which deflected to the positive were the beta rays and the particles which were undeflected were the gamma rays. 3) A charge in motion in a long straight wire with a drift velocity v subscript degenerates a magnetic field around the wire. If you were to move parallel to the wire at the same velocity, would you still observe a magnetic field If not, where did the field go According to Flemings left hand rule a moving charge produces a magnetic field perpendicular to the direction of motion. But since the charge is degenerating the magnetic field it means that the magnetic field created by the moving charge is in opposite direction to the existing field around the wire .so due to this the magnetic field has degenerated. So we would not observe any magnetic field as we move along with the charge parallel to it. 4) Since a moving electric charge creates a magnetic field, does moving a magnet create an electric field Yes a moving magnetic, carrying a magnetic filed, will create an electric field 5) 5) Electric fields begin on positive electric charges and end on negative electric charges. If there are no isolated north and south poles, do magnetic fields begin and end anywhere or are they always continuous Describe the magnetic field of a bar magnet from this point of view. In absence of isolated north and South Pole the magnetic filed is continuous. The magnetic field of a bar magnet starts from the North Pole and tend towards the South Pole outside the magnet. But inside the magnet this is vice versa 6) What is meant by an electromagnet Electromagnet is a coil of wire wound about a magnetic material, such as iron, that produces a magnetic field when current flows through the wire. 7) How can a solenoid be used as a switch Give an example of such a use in your car. Solenoids are used in cars in the ignition system .it is also called as starter relay The starter solenoid receives a large electrical current from the car battery and a small electrical current from the ignition switch. When the ignition switch is turned on (when the key is turned to start the car), the small electrical current tells the starter solenoid to relay the large electrical current to the starter motor. 8) How can you use a magnetic field to separate isotopes of a chemical element The isotopes of a chemical element can be separated using magnetic fields. This works on the principle that charged particles are deflected in a magnetic field .also the amount of deflection depends on the particle mass. Since isotopes vary in their masses they can be separated by this method 9) How can you make a bar magnet There are different methods to make a bar magnet like Single touch method Method of divided touch Electrical method 10) What causes the earth's magnetic field Is the field constant or does it changes with time Is it possible for the earth's poles to flip, that is, for the North Pole to become the south and vice versa The Earth is a giant magnet. Its hot liquid centre core contains iron, and as it moves, it creates an electric current that causes a magnetic field around the Earth. This is the cause for earth's magnetic field. The earth's magnetic field is not constant and it changes with time. Yes it is possible for the earth's magnetic field to flip. Earth's magnetic field has flipped many times over the last billion years, according to the geologic record. But only in the past decade have scientists developed and evolved a computer model to demonstrate how these reversals occur. 11) If you heat a bar magnet it loses its magnetism. Why On heating a bar magnet ,the kinetic energy of the atoms increase and the alignment gets disturbed .with this energy they move randomly and the order of arrangement gets disturbed and eventually the magnet gets demagnetized 12) What does magnetism have to do with the Van Allen belts The Van Allen Belts are toroidal (doughnut shaped) regions of magnetically trapped highly energetic charged particles. They are formed due to earth's geomagnetic field 13) What is magnetohydrodynamics Magnetohydrodynamics (MHD) (magnetofluiddynamics or hydromantic), is the academic discipline which studies the dynamics of electrically conducting fluids. Examples of such fluids include plasmas, liquid metals, and salt water. The word magnetohydrodynamics (MHD) is derived from magneto- meaning magnetic field, and hydro- meaning fluid, and -dynamics meaning movement. The field of MHD was initiated by Hannes Alfvn, for which he received the Nobel Prize in 1970. Chapter 23 1) Describe the concept of magnetic flux. Could you also define an electric flux for an electric field Magnetic flux is the product of the average magnetic field times the perpendicular area that it penetrates. It is a quantity of convenience in the statement of Faraday's Law and in the discussion of objects like transformers and solenoids. In the case of an electric generator where the magnetic field penetrates a rotating coil, the area used in defining the flux is the projection of the coil area onto the plane perpendicular to the magnetic field. For a closed surface, the sum of magnetic flux is always equal to zero (Gauss' law for magnetism). No matter how small the volume, the magnetic sources are always dipole sources (like miniature bar magnets), so that there are as many magnetic field lines coming in (to the South Pole) as out (from the North Pole). So magnetic flux is given as = B.A Where B is the magnetic field and A is the area perpendicular to magnetic field B The total number of lines of force emanating from a charge is called electric flux and denoted by a Greek letter . A unit charge is supposed to emanate one flux. Thus in case of an isolated charge q coulomb the flux =q and is independent of the nature of the medium .the unit of flux is that of charge i.e. Coulomb. 2) If changing the magnetic flux with time induces an electric field, does changing the electric flux with time induce a magnetic field Yes the changing electric field corresponds to a changing electric flux and thus a magnetic field is created according to the correction term introduced by Maxwell in the Ampere's law. 3) Is it possible to change both the area of a loop and the magnetic field passing through the loop and still not have an induced e.m.f. in the loop Yes it is possible. 4) Discuss Lenz's law The direction of the induced emf or the induced current can be found by Lenz law. It states that the direction of the induced emf is such that it tends to oppose the change the flux which induces it 5) Can an electric motor be used to drive an AC generator, with the output from the generator being used to operate the motor It is possible only when the efficiency of both the machines is 100%. Otherwise the output of the generator will not be enough to drive the motor. 6) Discuss how energy is stored in the magnetic field of a coil. Compare this to the way energy is stored in the electric field of a capacitor. Energy is stored in the magnetic field of a coil in the form of induced EMF. This induced Emf produces current. This induced EMF drives a current around the loop in such a direction as to oppose the change associated with the changing current that gave rise to it. In case capacitor some work is done to charge the capacitor and this work is stored as energy in the capacitor 7) If changing an electric field with time produces a magnetic field, and changing a magnetic field with time produces an electric field, is it possible for these changing fields to couple together to propagate through space Yes, it is possible. The best example is electromagnetic radiations which have varying electric and magnetic acting upon each other perpendicularly. Works cited Electricity and Magnetism - Narayanamurti and Krishna Rao Engineering Physics- R.K Gaur and S.L. Gupta Read More