Monday, March 4, 2013
In electricity and magnetism // //
Electricity (Homework 4)
Current = Voltage/ Resistance
40. Calculate the power of a hair dryer that operates on 120 V and draws a current of 10 A.
$Power = Voltage*Current = U*I = 120*10 =1200 W$
70. Why is charge usually transferred by electrons rather than by protons?
A transferred charge means that a charge is taken away from an atom and moves to s second atom and so on. Since the forces that keep together the electrons in the atoms (electric forces) are much weaker than the forces that keep together the protons in a atom (nuclear forces) than the charge is transferred by electrons rather than protons.
72. Why do clothes often cling together after tumbling in a clothes dryer?
Because the became electrically charged by the friction between them. Different materials become electrically charged with different (opposite) charges. Opposite charges attract each other.
86. What happens to the brightness of light emitted by a light bulb when the current in the filament increases?
Since the electrical power delivered is proportional to the current, then the electrical power increase. Since this electrical power is transformed into light with in a certain proportion, then the brightness of the light emitted by the bulb will increase.
88. Why are compact fluorescent lamps (CFLs) more efficient than incandescent lamps?
CFL are more efficient than the incandescent lamps because the fluorescence mechanism of emitting light by atoms is in itself more efficient than incandescence mechanism of emitting light. This first mechanism implies atoms of the emitting material are excited by impact with moving IONS and then un-excite emitting photons, the second mechanism implies the moving ELECTRONS collide with atoms, excite them and then these atoms un-excite emitting photons. Since mass of moving IONS is larger than mass of moving ELECTRONS and then the efficiency of excitation the atoms is higher in the first mechanism.
92. Are automobile headlights wired in parallel or in series? What is your evidence?
The automobile headlights are wires in parallel. The evidence of parallel wiring is that all the bulbs have the same working Voltage (12 V- battery voltage). If they were wired in series, the current through bulbs had been the same and the working voltage had been less than 12 V, the battery voltage.
108. Your tutor tells you that an ampere and a volt really measure the same thing, and the different terms only make a simple concept seem confusing. Why should you find another tutor?
Let’s compare the flow of the electric current to a river. Then the voltage would be equivalent with the height of a water fall and the current with the quantity of water that falls. There is no equivalence (in other words it is not the same thing) between the height of the fall and the quantity of water that falls.
110. Does more current “flow” out of a battery than into it? Does more current “flow” into a light bulb than out of it? Explain.
The answer is no. Since the electric current represents the movement of electric charges, and since there is no charge created outside the battery or inside the light bulb then the current is the same in and out.
112. Does a lamp with a thick filament draw more current or less current than a lamp with a thin filament? Defend your answer.
The current in the filament is inversely proportional to the resistance of the filament as ohm law states. $I = U/R$. Since Resistance is inversely proportional to the thickness ($S$- transverse area of filament) $R = \rho*L/S$ then this implies $I$ proportional to $S$ (thickness). It means the thicker the filament, the greater current.
120. As more and more bulbs are connected in series to a flashlight battery, what happens to the brightness of each bulb? Assuming that the heating inside the battery is negligible, what happens to the brightness of each bulb when more and more bulbs are connected in parallel?
The brightness of a bulb is proportional to the current through it. If you connect in series, the total resistance increases ($R = R1+R2+R3+...$). The current $I = U/R$ ($U$ is the battery voltage) decreases, and therefore the brightness decreases.
When you connect in parallel bulbs to a battery, if the battery is IDEAL then the voltage and current through the bulbs stays the same and therefore the brightness stays is the same.
31. Write a letter to a relative or friend saying that you have discovered the answer to what has been a mystery for centuries— the nature of light. State how light is related to electricity and magnetism.
Beginning with the 18th century laws of electrostatics were discovered (electrical charges create an electric field) . At the beginning of the 19th century a new chapter of physics began to be studied, i.e. the moving electric charges (which create a magnetic field) became to be studied and the chapter in physics that deals with this was named electrodynamics. At the end of 19th century electrodynamics discovered the laws of by which an electrical VARIABLE field transforms itself into a magnetic VARIABLE field, and named these laws the Maxwell laws. This way the electric and magnetic fields were reunited into a single field ELECTROMAGNETIC field. The Maxwell laws predict for the speed of an electromagnetic field the same value as the speed of light has. Light by its nature is electromagnetic and has the same laws as Maxwell laws.
48. All atoms have moving electric charges. Why, then, aren’t all materials magnetic?
The answer is that the electrons moving inside the atoms have also a property named SPIN. The electric currents generated by the movement of the electrons depends on the value of the electron spins. Of these spins couples themselves (are parallel in other words) then the material is magnetic otherwise (if the spins are opposite, as in the majority of materials) and the moving electrons does not couple each other the material is nonmagnetic.
52. What kind of force field surrounds a stationary electric charge? What additional field surrounds it when it moves?
See answer to question 31.
A stationary electric charge creates am ELECTRIC field around it. A moving electric charge is equivalent to a current and creates a MAGNETIC field around it. Both these fields are reunited into a single filed named ELECTROMAGNETIC field.
92. Discuss why a motor also tends to act like a generator.
A motor is constructed of rotor windings through which current flows creating a magnetic field. This field interacts with a second magnetic field (of the stator) created by permanent magnets and creates motion (i.e. applies an internal force to exterior). This a a motor.
A generator is constructed of the same rotor windings that moves (because of an external force) into the same second magnetic field created by the permanent magnets of the stator. As Maxwell laws says a variable magnetic field (because of the movement of the rotor) creates an electric field. Therefore in the windings there will be an electric current. This is a generator.
In other words a motor transforms the electrical force in mechanical force, and the same device can perform in an opposite direction to transform mechanical force into electric force.