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Unit packet Electrostatics II and Circuits 1. Sketch the electric field around a positive point charge. Which way do the field lines align? a) Place two points in the above field (# 1) labeled A, B. Place ‘A” at a location of greater field strength than ‘B’. What is true about the field lines at ‘A’ compared to B? b) Indicate all of the locations in the electric field above that are at the same field strength as ‘A’. c) Place a + (positive charge) in the field above and indicate the direction of the force it experiences due to the 2. Repeat # a-c above for a negative point charge electric field 1 3. Define test charge. What is the purpose of this concept. 4. Consider the electric field between the two charges above. Three charges are placed in the electric field as shown. Indicate the initial direction of motion of each charge. 5. Sketch the electric field between two oppositely-charged parallel plates. How does this electric field differ from # 1-2? 6. Place a both a positive and a negative charge in the middle of the parallel-plate field. Indicate the direction of their motion. 2 7. A uniform parallel-plate electric field of field strength 0.5 N/C has a plate separation of 5 cm. a) Make a sketch of this b) Consider a 1.0 nC test charge in the electric field. How does the force exerted on this charge vary in this field? c) How does the speed of this charge vary in this field? d) How does the acceleration of this charge vary in this field? 8. A proton is in a uniform electric field of 125 N/C. If it requires 8.0 x 10-19 J of work to move the proton from one plate to the other, determine the distance between plates and which plate the proton was moved to. 3 9. Two oppositely charged parallel plates at a distance of 18 cm set-up a uniform field of 480 N/C. Determine the maximum electric potential energy of each of the following charges in this electric field: a) Electron b) Proton c) + 2.1 nC d) -5.0 µC 10. A proton is placed in a uniform electric field of 50 N/C at its location of maximum electric potential energy. If the plate separation is 5 x 10-3 m, then a) Sketch the set-up b) If the proton is allowed to impact the opposite plate, calculate the impact velocity 4 11. Explain the difference between voltage and current using energy concepts. 12. When you flip the light switch turn on a lamp, the light is seen immediately. Explain. 13. What are two fundamental ways to increase the current in a circuit? 14. What characteristics of a conductor affect its resistance to electric current? What materials make the best wires? 15. An appliance is wired in circuit branch across which 120 V is applied. The current is measured to be 8 Amps, but after a few minutes the current drops to 6 Amps. Explain. 5 16. The home oven and stove is on a 120-V AC circuit and draws about 8.0 A of current under normal conditions. What is the resistance of the oven at this temperature? 17. A calculator requires 0.16 A of current against a resistance of 19 ohms. What total battery voltage is required for the calculator? (How could this be accomplished?) 18. A flashlight with a resistance of 12 ohms runs on 3 1.5-V C-cells. How much current does the flashlight draw from the batteries? 19. A lamp draws 0.5 A when plugged into the 120-V outlet. Calculate the lamps resistance and the power consumption of the bulb. 20. A 12-V car battery is used to power a dashboard lamp with a resistance of 33 Ω. Calculate the current in this circuit. 6 21. A 100-W bulb is connected in a circuit to the 110-V wall outlet. a) What is the current through the bulb? b) What is the resistance of this bulb? c) Would these values change for a 50-W bulb? Calculate and explain. (For the problems below , assume that electricity costs 10 cents per kW-hr) Also 1 month = 30 days 22. A furnace which runs on electricity has a resistance of 5-ohms. It is connected to a 240V outlet. If it turns on about half the time, what is the electric bill for the furnace alone in a 30-day month? 23. A 100-W bulb is left burning all day. a) What is the energy cost for the whole day? b) What is the energy cost for a whole month? 7 24. An air conditioner uses 2200W of power. If it is used for 6 hours per day for a week, what is the electrical cost per week to use the air conditioner? 25. A microwave oven uses 1400 W, where a conventional oven uses about 1800 W. A microwave re-heats leftovers in 2 minutes, where in the conventional oven it would take 10 minutes. a) What is the electrical cost to use the microwave oven? b) What is the cost for the conventional oven? c) What is the savings? 26. Resistor 1 = 22 Ohms is wired in series with Resistor 2 = 15 Ohms. The current is 0.20 A. A) sketch schematic b) Determine battery voltage C) Determine voltage drop across each. d) Determine total power, Power for R1 and power for R2 8 27. Two lamps are in series across 24 V DC. Lamp 1 has R = 22 Ohms and lamp 2 has R = 5 Ohms. a) Sketch schematic b) Determine circuit current c) Determine voltage drop across each lamp d) Which lamp will be brighter when they are connected in series? 9 28. A 9.0 V battery provides power to two resistors connected in series. The current in the circuit is 0.18 A. One resistor has a voltage drop of 6.3 V. a) Sketch schematic b) Determine the power dissipated by each resistor 29. Three resistors are connected in series: R1 = 92 Ohms, R2 = 150 Ohms, R3 = 220 Ohms. a) What is the maximum current that can flow in this circuit if the resistors are rated at 5.0 W (can’t exceed 5.0 W)? b) What is then the maximum applied voltage for this circuit? Sketch the circuit schematic. c) Calculate the voltage drop across each resistor. d) Calculate the power supplied and the power dissipated by each resistor. 10 30. Resistor 1 = 250 Ohms, resistor 2 = 150 Ohms, Resistor 3 = 350 Ohms are connected in parallel to a 24 .0 V battery. a) Sketch the schematic b) Determine the current in each branch. c) Calculate the total circuit current. d) Calculate the power output of the battery and the power dissipated by each resistor, as well as the total power used by the resistors. 31. Resistor 1 = 50 Ohms, Resistor 2 = 25 Ohms, resistor 3 = 150 Ohms are connected in parallel. The total circuit current is equal to 1.3 A. a) Determine the circuit equivalent resistance b) Calculate the voltage provided. c) Calculate the branch currents 11 32. Two lightbulbs, one at 25 W and one at 75 W, are connected in parallel to outlet voltage of 120-V AC. a) Sketch schematic b) What is the voltage across each bulb? c) Calculate the current in each branch (bulb). d) Which bulb will burn brightest? 12 33. A 100-W bulb is plugged into 120-V outlet AC. A) Sketch the schematic B) Calculate current in the circuit and the resistance of the bulb. Next, a 60-W bulb is added in parallel configuration. d) Sketch schematic. e) Calculate the current through the 60-W bulb, and the resistance of the 60-W bulb. f) Did the 100-W bulb increase, decrease or stay the same in: - Current - Resistance - Power - Voltage drop g) h) i) j) k) l) Next, remove the 60-W bulb from the parallel configuration, and wire it in series with the 100-W bulb. Calculate the current through the 60-W bulb Voltage drop across 60-W bulb Power dissipated by 60-W bulb Current through 100-W bulb Voltage drop across 100-W bulb Power dissipated by 100-W bulb. 13