Physical Science Review Electricity and Magnetism

Transcription

1 Victor is going to investigate static and moving charges using the following objects: Some silk cloth A glass rod Some copper wire Some electric bulbs Some metallic spheres Some dry cells Some bits of paper With these objects, he performs the following experiments: Experiment 1 Victor rubs the glass rod with the piece of silk cloth and brings it near the bits of paper. The paper bits are attracted toward the glass rod. Experiment 2 Victor connects a charged glass rod to a metallic sphere through copper wire. Experiment 3 Victor places a charged glass rod near the two uncharged metallic spheres in contact with each other. Experiment 4 Victor connects two identical bulbs in a parallel arrangement. He then connects a 20V cell across this arrangement.

2 1. In experiment 4, what is the amount of current flowing through R1 and R2? a. 1 Amp through each of R1 and R2 b. 2 Amps through each of R1 and R2 c. 3 Amps through each of R1 and R2 d. 4 Amps through each of R1 and R2 Notes: The correct answer is a 1 amp through each R1 and R2 Because there is 2 Amps running to the split and the fact that both resistors provide equal ohm (Ω) then the current will split evenly assuming that the wires are the same. 2. How many volts are provided by the cell in experiment 4? a. 10 V b. 20 V c. 30 V d. 40V Notes: 20 Volts. You can identify this answer in two different ways. 1 You can look at the diagram and notice the power source is 20V or 2. You can read the experiment description. 3. In experiment 1 if the paper buts were not attracted to the glass rod prior to rubbing it with the silk cloth, how did the silk cloth enable the glass rod to attract the particles of paper after it was rubbed? a. Electrons were transferred from the glass rod to the silk cloth leaving the glass rod with a mostly positive charge. b. Protons were transferred from the glass rod to the silk cloth leaving the glass rod with a mostly negative charge. c. The silk cloth charged the glass rod via electromagnetic induction. d. Rubbing the glass rod with the silk cloth altered the domains of the glass rod forcing them to all point the same way, giving the glass rod a temporary magnetic field. Notes: Protons are absolutely never transferred or changed unless there is some sort of Nuclear fission or fusion event which basically concludes in an enormous release of energy, so it is clear to say that that didn t happen. We know that electrons are able to flow, we call this electricity. By electrons (negatively charged subatomic particles) leaving the class rod, the glass rod will contain more protons than electrons which will give it a positive charge. 4. What happens in experiment 2 to when the positively charged rod is attached to the metal sphere by a copper wire? a. The positive charge is transferred to the metal sphere via charging by conduction. b. The metal sphere becomes positively charged as it loses electrons through the copper wire to the charged glass rod via charging by conduction. c. Nothing happens because there is not a potential difference between the two objects to initiate the flow of current. d. The positive charge is transferred to the metal sphere via charging by conduction. Notes: Charging by conduction refers to the transfer of charge through direct contact, so we know it must charge via conduction due to the attached copper wire. As discussed in the answer to #3 we know that protons never flow or change, but electrons do; therefore, if electrons flow from the metal sphere through the copper wire towards the positively charged rod (due to their attraction to positive charges [opposites attracts]) then the metal sphere the electrons left from will become more positive due to its protons outnumber its electrons after they leave. 5. What happens in experiment 3 when the charged object is brought near the two sphere system? a. The charged rod charges the spheres via charging by induction, protons move to one side of the two sphere system causing one sphere to become negatively charged and one sphere to become positively charged. b. The charged rod has no effect on the two sphere apparatus. The two spheres allow electrons to flow freely and cancel out any attempt to transfer a charge to the two sphere system. c. The charged rod charges the spheres via charging by induction, electrons move to one side of the two sphere system causing one sphere to become negatively charged and one sphere to become positively charges. d. The charged rod charges the spheres via charging by conduction, electrons move to one side of the two sphere system causing one sphere to become negatively charged and one sphere to become positively charges. Notes: Electrons are free to move between the two spheres. When the positively charged rod is brought near but not touching the spheres (we know it is charging by induction). Electrons will flow from sphere B into sphere A as they are attracted to the positively charged rod. This will make sphere A negatively charged due to the influx of electrons and leave sphere B positively charged due the absence of electrons. 6. Which graph shows the direction of flow of electrons in experiment 4? a. c. b. d.

3 Notes: B and D are Invalid graphs; They both represent either the positive (b) or the negative (d) aspects of the Alternating Current graph (a). Experiment 4 involves the use of a 20 Volt power source (20V battery) therefore we know that it must be a Direct Current Circuit. The graph for Direct Current is Graph c. 7. Which diagram shows the correct arrangement of charges in experiment 3 after the spheres are separated from each other and the glass rod is removed? A. B. C. D. Notes: A is the correct answer as discuss in question #5. 8. Which diagram represents the arrangement of charges after the copper wire has been attached to both the glass tube and the metallic sphere in experiment 2? a. d. b. Notes: D is the best answer choice. As electrons migrate from the sphere through the copper wire to the positively charged rod, the sphere will become more and more positive as a whole. c. 9. A balloon acquires a charge when rubbed across fur. Why does the electrically charged balloon now attract small bits of paper? a. Excess charges on the balloon jump to the bits of paper. b. The charge on the balloon polarizes the charge on the bits of paper. c. Charges on the balloon attract charges of opposite signs on the bits of paper. d. The electrical charge on the balloon induces magnetic attraction with bits of paper. Notes: The charged balloon will charge the bits of paper via charging by induction; therefore, polarizing (creating a + and side) the paper allowing it to become attracted to the charge of the balloon.

4 10. Andrea constructed an electromagnet using a steel rod and a copper wire, as shown in the figure below. Which modification could Andrea make in the electromagnet to strengthen the magnetic field? a. Replace the steel rod with a glass rod. b. Increase the resistance in the copper wire. c. Remove the steel rod from the copper wire. d. Increase the number of coils around the steel rod. Notes: There are two ways to increase the power of an electromagnet: 1 : increase the number of coils & 2. Increase the voltage/current What are two ways to make the field of an electromagnet stronger? a. increase the voltage, decrease the current b. wrap more wire around the iron core, increase the voltage c. wrap more wire around the iron core, decrease the current d. replace the iron core with copper, increase the voltage Notes: See the explanation to the answer of questions number Which way will the loop of wire move when the current is flowing? a. Rotate around an axis parallel to the z direction b. Rotate around an axis parallel to the x direction c. Out of the magnetic field in the y direction d. Out of the magnetic field in the z direction Notes: The coil will rotate within the magnetic field, thus eliminating answer choice c & d. The coil will rotate in a direction parallel to x and perpendicular to the magnet pictured in the figure 13. The circuit below has three resistors. The resistances in the circuit are R1= 2 ohms, R2= 4 ohms, and R3= 6 ohms. What is the total resistance in the circuit? a. 2 b. 6 c. 10 d. 12 Notes:The overall resistance is equal to the sum of all resistances (Ω) 14. The figure below shows a circuit in which resistors R 1, R 2, & R 3 are connected in parallel. The battery has a potential difference of 18 volts. What will be the flow of current of the resistance in R 1, R 2, & R 3 are 3Ω, 6Ω, 9Ω respectively? a. 3 A, 6A, 9A b. 6A, 3A, 2A c. 1A of current through each of the resistors d. 18A of current through each of the resistors Notes: Use the formula V=IR (Voltage = Current [amps] x Resistance [Ω] for each of the Resistors R 1, R 2, & R 3

5 15. A series circuit has a resistor of 5Ω. What is the change in the amount of current flowing through the circuit if a resistor of 10Ω is also connected to the circuit? a. The value of current flowing in the circuit becomes twice the original value. b. The value of current flowing in the circuit reduces to half of the original value. c. The value of current flowing in the circuit becomes a third of the original value. d. The value of current flowing in the circuit becomes three times the original value. Notes: The resistances (Ω) is tripled therefor the current is the reciprocal which is a third. Current and Ohms are directly related in terms of Ohm s Law. Therefore, we know that if we triple one then the other will be cut by equal quantity. 16. What is the direction of flow of electrons in a simple direct-current circuit? a. Electrons flow from the negative terminal of the battery to the positive terminal in a direct-current circuit. b. Electrons flow from the positive terminal of the battery to the negative terminal in a direct-current circuit. c. Electrons flow from the positive terminal of the battery to the negative terminal and then from the negative terminal to the positive terminal in a cyclic manner in a direct-current circuit. d. Electrons flow from the negative terminal of the battery to the positive terminal and then from the positive terminal to the negative terminal in a cyclic manner in a direct-current circuit. Notes: Direct Current means electricity can only flow in one direction, thus eliminating answer choices c & d. A is the best answer because electrons flow from the anode to the cathode. Oxidation occurs at the negative terminal (production of electrons) and Reduction occurs at the cathode (addition of electrons). 17. What is the acceleration of a car that goes from 40km/hr to 80 km/hr in 2 s? 20 m/s You apply a force of 18 N on to the end of a lever to open a paint can lid. The resistance of the lid is 9 N. Calculate the MA Three of your friends are all sitting on one end of a seesaw. The combined weight is 275 N. The length from the fulcrum to your friends is 2.5 m. The rest of the seesaw (from the fulcrum to you) is 4.5 m. What is the MA? Find the volume of a rectangular shaped solid that has a length of 2.5 cm, a height of 6.37 cm, and a width of 1.79cm cm When an F-14 airplane takes-off on an aircraft carrier it is literally catapulted off the flight deck. The plane s final speed at take-off is 68.2 m/s. The F-14 starts from rest. The plane accelerates to its final speed at takeoff in 2 seconds from rest, and has a mass of 29,545 Kg. What is the total force that gets the F-14 in the air? 1,007,484.5 Newtons 22. A vehicle travels 2345 m in 315 s toward the evening sun. a. What is its speed? 7.4 m/s b. What is it velocity? 7.4 m/s toward the evening sun 23. A rock has a density of 4 g/ml and a mass of 16 grams. What is the volume this rock occupies? 4 ml 24. An unknown substance from planet X has a density of 10 g/ml. It occupies a volume of 80 ml. What is the mass of this unknown substance? 800 grams 25. A cube made of an unknown material has a height of 9 cm. The mass of this cube is 3,645 grams. Calculate the density of this cube given this information. 5g/cm When resistors are connected end to end in a circuit, they are said to be in series. The total resistance equals the sum of all the resistances. Suppose a string of lights has 25 bulbs. Each bulb has a resistance of 8.0Ω. If the string is plugged into a 120-V outlet, how much current is in the entire set of lights? Amps 27. A chandelier has 10 sockets, each of which holds a 60.0-W light bulb. Each light bulb has a resistance of 240Ω. However, the chandelier is wired so that the overall resistance provided by the 10-bulb circuit is only about 24.0Ω. If the voltage across the chandelier s circuit is 115 V, how much current is in the chandelier? 4.79 amps

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