South Pasadena A.P. Physics Chapter Electric Current & DC Circuits Date / / Period Electricity Practice Test

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1 South Pasadena A.P. Physics Name Chapter Electric Current & DC Circuits Date / / Period Electricity Practice Test Electric Current I = Q/t 1. A charge of 30 Coulombs passes through a 24-ohm resistor in 6.0 seconds. What is the current through the resistor? I = 30 C/6 seconds = 5 Amps 2. Charge flowing at the rate of 2.50 x elementary charges per second is equivalent to a current of...? 2.50 x e /second x 1.6 x C = A 3. The current through a light bulb is 2.0 amperes. How many coulombs of electric charge pass through the light bulb in one minute? 2.0 A x 1C x 60 s x 1 min = 120 Coulombs 1 A-s 1 min 4. A 1.5 volt, AAA cell supplies 750 milli amperes of current through a flashlight bulb for 5 minutes, while a 1.5 volt, C cell supplies 750 milli amperes of current through the same flashlight bulb for 20 minutes. Compared to the total charge transferred by the AAA cell through the bulb, the total charge transferred by the C cell through the bulb is.. e a) Half as great c) The same b) Twice as great d) Four times as great 20 min./ 5 min. = four times more charge or I 1 = 750 ma x 1 A x 1 C x 5 min. x 60 s = 225 C 1000 ma 1A-s 1 min. 5. The current traveling from the cathode to the screen in a television picture tube is 5.0 x 10 5 amperes. How many electrons strike the screen in 5.0 seconds? 5.0 x 10 5 amps x 1C x 5 s x1 e = 1.6 x e 1 A-s 1.6 x C Table Material of Resistivities at 20 C (Ω m) Resistivity at 20 C R Aluminum 2.82 x 10 8 Copper Gold Ni-chrome Silver Tungsten 1.72 x x x x x A 3.50 m length of wire with a cross sectional area of 3.14 x 10 6 m 2 at 20 C has a resistance of Ω. Determine the resistivity of the wire and the material it is made from. Resistivity = R x A = Ω x 3.14 x 10 6 m 2 L 3.50 m = 5.60 x 10 8 Ω m Material = Tungsten 7. The electrical resistance of a metallic conductor is inversely proportional to its a) Temperature b) Length I 2 = 750 ma x 1 A x 1 C x 20 min. x 60 s = 900 C 1000 ma 1A-s 1 min. 900 C / 225 C = 4 x more charge c) Cross sectional area d) Resistivity

2 8. At 20 C, four conducting wires made of different materials have the same length and the same diameter. Which wire has the least resistance? a) Aluminum c) Ni-chrome b) Gold The one with least resistivity d) Tungsten 9. Referring to the table above, explain why (a) you think copper is mostly used as the material for wiring most circuits and (b) why is tungsten the most common metal used for the filaments in light bulbs? (a) Copper is less costly than silver but still has a small resistivity which is desirable in a conductor. It has a slightly higher resistivity than silver but is still smaller than other metals. (b) Tungsten has one of the highest values for resistivity which is great for an incandescent light bulb, because this produces a higher resistance which produces more light. It also has a very high melting point which is also good for a filament, so that it does not melt easily. 10. A length of copper wire and a 1.00 meter long silver wire have the same cross sectional area and resistance at 20 C. Calculate the length of the copper wire. R Cu = ρ L / A = R Ag = ρ L / A L Cu = ρ Ag L Ag A Cu = 1.59 x 10 8 Ω m x 1.00 m A Ag ρ Cu 1.72 x 10 8 Ω m = 0.92 m 11. A 10 m length of copper wire is at 20 C. The radius of the wire is 1.0 x 10 3 meter. (a) Determine the cross sectional area of the wire. A = πr 2 = π (1.0 x 10 3 m) 2 = 3.14 x 10 6 m 2 (b) Calculate the resistance of the wire. R Cu = ρ L / A = (1.72 x 10 8 Ω m) x (10 m) 3.14 x 10 6 m 2 = 5.47 x 10-2 Ω = 5.5 x 10-2 Ω 12. The current in a wire is 24 amperes when connected to a 1.5 volt battery. Find the resistance of the wire. R = 1.5 V/24 A = or Ω 13. In a simple electric circuit, a 24 ohm resistor is connected across a 6 volt battery. What is the current in the circuit? I = 6 V/24 Ω = 0.25 A 14. What is the current in a 100 ohm resistor connected to a 0.40 volt source of potential difference? I = 0.40 V/100 Ω = A 15. A constant potential difference is applied across a variable resistor held at constant temperature. Which graph below best represents the relationship between the resistance of the variable resistor and the current through it? Answer is (1) since I is indirectly proportional to resistance, so as R gets big, I gets small.

3 South Pasadena A.P. Physics 1 ANSWERS Practice Test Terms and Symbols Match the following units or symbols with their appropriate names: Name Period 18 Electric Currents & 19 DC Circuits Match the following units or symbols with their appropriate names: 1. Ohms c a) Current 2. Volts b & d b) Electric Potential 3. Amperes a c) Resistance 4. Ω c d) Battery 5. I a 6. VVVV c 7. ا b & d * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * 8. What is the definition (or formula) for 9. If 10 coulombs of charge pass through a resistor in 5 seconds, what is the current? 10C/5seconds = 2 amps a) watt 1 Watt = 1 J/sec b) volt 1 Volt = 1 J/Coulomb c) ampere 1 Ampere = 1 C/sec d) ohm 1 Ohm = 1 V/A Match the following Terms with their correct definitions A) Resistor B) Battery C) Fuse D) Voltmeter E) Short-Circuit C A E B D 10. This protects an electrical circuit from an overload or surge of current. 11. This device can influence the amount of current that passes through a circuit. A tolerance value is usually stated on this device. 12. This causes electrical current to take an alternate pathway through a circuit. 13. This can come in different sizes and is analogous to water pressure in a garden hose. 14. This is placed in parallel in a circuit, to make measurements of a circuit. Write the names listed below in the correct box: Voltmeter Light Bulb Battery Switch Resistor Resistor Battery Voltmeter V Light Bulb Switch

4 Ohm s Law Solve the following problems: 1. A simple circuit consists of a 12-volt battery and a resistor of 100 ohms. What is the current passing through the circuit? 12V/100 Ω =0.12 Amps 2. A certain toaster has a heating element with a resistance of 14 ohms. We connect it to a 120 Volt outlet. a) What is the current that is drawn? 120 V/14 Ω = 8.6 A b) What is the power dissipated? 8.6 A x 120 V = 1032 W or 1030 Watts 3. The resistance from hand to hand across the human body, if it is dry, is 100 k Ω. If you apply a 120 volt difference across your hands, what current goes through your body? I = 120 V / 100,000 Ω = A or 1.2 ma 4. For a human to have a heart attack, the current through the body must be 100 to 200 ma. If the body is affected by a 120-V potential of alternating current, what resistance would be required for a current of 100 ma? R = 120 V / 0.1 A = 1200 Ω How might this resistance be achieved? By having moist or sweaty fingers which decreases resistance 5. What potential difference or voltage is required to cause 2.0 A to flow through a resistance of 8.0 Ω? 8 Ω x 2.0 A = 16 Volts 6. If the voltage impressed across a circuit is constant but the resistance doubles, what change occurs in the current? Current halves or decreases to one-half 7. a) How much current flows through your fingers if you touch them to the terminals of a 12-volt lawn garden battery, if the resistance between your fingers is 1200 ohms? I = 12 V/1200 Ω = 0.01 A Electric Power 1. How many joules/coulomb are given to charges that flow in a 120-volt circuit? 120 J/C 2. Recalling that Work = force x distance and is measured in joules, a) what is the unit of power? Watts b) Power is the at which work is done. Rate 3. To calculate electric power, we use the formula; Power = I x V or I 2 R 4. A 60-V cell is connected to a 200 Ω resistor and a switch. The switch is closed for 90 minutes. a) What is the current through the resistor while the switch is closed? I = 60 V / 200 Ω = 0.3 A b) What is the power dissipated while the switch is closed? P = 0.3 A x 60 V = 18 Watts c) What is the heat dissipated (in Joules) while the switch is closed? Heat = 18 W x 90 min x 60 sec/min = 9.72 x 10 4 Joules

5 5. How many watts of power would be used by a calculator, that operates on 8.0 volts and 0.1 amperes of current? P = 0.1 A x 8.0 V = 0.80 Watts 6. If the calculator is used for one hour, how much energy in kilowatt-hours does it use? Energy = 0.80 Watts x 1 kw/1,000 W x 1 hour = kw-hours D.C./A.C. & Power 1. If you made a graph of current vs. time with time on the horizontal axis, a) what shape would the graph have for direct current from a battery? b) what shape would the graph have for alternating current from an outlet? I I Straight line Sine Wave time (DC) time (AC) 2. If you use a 1200-watt hair dryer operating on an A.C. line of 120 volts, will the dryer work if the current is limited to 15 amperes by a safety fuse? YES P = IV I = P/V = 1200 Watts/120 Volts = 10 amps 3. If you used two identical hair dryers on the same A.C. line above, would both be able to operate? NO 10 Amps + 10 Amps = 20 Amps, which is greater than 15 amps 4. What is the function of the third prong in a household electric plug? Acts as a ground This prong is on top of the outlet 5. How many amperes flow through a 60 watt bulb when 120 volts are impressed across it? 60 W/ 120 V = 0.5 amps

6 Light Bulbs 1. Draw a picture using a battery, bulb and wire that shows a successful way to make the light bulb light up and an unsuccessful way to make the light bulb light up. The successful way: one wire should touch the bottom of the bulb and the other wire touches the side of the bulb. There are several unsuccessful ways to light the bulb. I will rely on your memory from the activity, for which ways did not work. 2. If you have a three-way bulb (50 W, 200 W & 250 W), explain how only two filaments would be needed to provide these three values and draw a picture of how these two filaments might be different? One filament would be thinner and be for the 50-Watt bulb. The other filament would be thicker and be for the 200-W bulb. When both filaments are working, that makes it 250-Watts (50 Watts Watts) 3. Draw a circuit diagram showing 2 light bulbs connected in series with a 3.0 Volt battery. Light Bulb 3.0 Volts Light Bulb 4. a) Which of the substances in the table below would be the best material to be used for a light bulb filament? D would be the best material, since it has the highest melting point Material Melting Point º C A 1959 B 2200 C 2893 D 3422 b) What is most likely the name of the material that you selected? Tungsten Metal (Elemental Symbol is W) 5. You have three wires of copper, A, B & C, which have diameters of 0.24 mm, 0.53 mm and 0.39 mm, respectively. Place them in order of least resistance to greatest resistance. First would be.53 mm, then.39 mm and finally.24 mm or B < C < A

7 Resistors in Series 1. What is the є or voltage of the battery? 4 V 2. What is the voltage drop across the 10 ohm resistor? 2 V 3. What is the current passing through the 2 ohm resistor? 0.2 A (same everywhere) 4. What is the sum of the voltage drops across the 2 Ω and 8 Ω resistors? 2 V 5. A resistor is listed as 100 ohms with a tolerance of ± 5%. What is the range of values of resistance that you would expect with this resistor? 95 Ω 105 Ω (100 Ω ± 5Ω) 6. Three resistors of 5 Ω, 15 Ω and 25 Ω are connected in series. What is the total value of their resistance? 45 Ω 7. Three identical resistors of 100 Ω are connected in series to a battery. A current of 12 Amps flows from the battery. How much current flows through any one of the resistors? 12 Amps 8. What is the voltage of the battery from the previous problem? 12 A x 300 Ω = 3600 V 9. If one of the light bulbs goes out (breaks) what will happen to the other two light bulbs? They will go out!

8 Resistors in Parallel Refer to the Diagram for Questions 1-3 (circle the correct responses) Five identical light bulbs are connected to a 120 V power supply. Each light bulb has a resistance of 15 Ω. The switch is closed. 1. What is the net resistance of the circuit? (A) 3 Ω (B) 30.1 Ω (C) 40 Ω (D) 75 Ω 2. What is the current in the light bulb L 1? (A) 1.6 A (B) 3 A (C) 8 A (D) 40 A 3. Which light bulb or bulbs could burn out without causing any others to go out? (A) Only L 1 (B) Only L 2 (C) Only L 4 (D) Only L 5 4. Three resistors of 5 Ω, 15 Ω and 25 Ω are connected in parallel. What is the total value of their resistance? 3.3 Ω 5. Three identical resistors of 100 Ω are connected in parallel to a battery. A current of 12 A flows from the battery. How much current flows through any one of the resistors? 4 A 6. What is the voltage of the battery from the previous problem? 400 V 7. If one of the light bulbs goes out (breaks) what will happen to the other two light bulbs? 8. Draw a circuit diagram of the circuit in question #5 above: Nothing

9 9. A 6.0 Ω and a 12 Ω resistor are connected in parallel to a 36-volt battery. a) What is the current traveling through each resistor? I 1 = 36 V/6 Ω = 6 amps I 2 = 36 V/12 Ω = 3 amps (I T = 36 Volts/4 Ω = 9 amps, and this is the sum of 6 amps and 3 amps) b) What is the power dissipated by the 6.0 Ω resistor? P = IV = 6 A x 36 V = 216 watts Miscellaneous 1. If you connected two 1.5 V batteries in series, what would be the total voltage? 3.0 Volts 2. Draw a diagram with the electrodes of each battery labeled + and + + and show how the two batteries should be connected. 3. If two 4.0 Ω resistors are connected in parallel and then this combination is connected in series with a 3.0 Ω resistor, what is the total resistance of the three resistors? R parallel = 2 Ω R series = 3 Ω R Total = 5 Ω 4. Two resistors of 5.0 Ω and 9.0 Ω are connected in parallel. A 4.0 Ω resistor is then connected in series with the other two. A 6.0 V battery is then connected to the series-parallel combination. What is the current through the 5.0 Ω resistor? Tough Question amps 5. Which of the following factors will affect the resistance in a wire? (1) the length of the wire (2) the thickness of the wire (3) the composition or type of material in the wire (4) the temperature of the wire a) 1 and 2 c) 1, 2, and 3 b) 2 and 3 d) all of these 6. A light bulb filament is usually made up of which of the following metals? a) silver c) iron b) aluminum d) tungsten 7. A light filament should have a low or high melting point? a) low b) high c) it depends on what the light bulb is used for 8. As the length of a wire increases, its resistance does what? a) it also increases b) it decreases

10 9. A light bulb has a current of 0.5 amps running through it when it is connected to a 120-volt electrical outlet. If the voltage source were changed from 120 volts to 10 volts, then the current reduces to only 0.2 amps. The new resistance of the light bulb... a) would increase b) would decrease c) would stay the same 10. What is the resistance of the circuit above, when the current flowing is 0.5 amps? 240 Ω R = V/I = 120 Volts/ 0.5 A = 240 Ω 11. What is the resistance of the circuit above, when the current flowing is 0.2 amps? 10 V/0.2 A = 50 Ω Concepts Circle the best answer: SERIES CIRCUITS 1. In a Series Circuit, how does the voltage across the various resistors compare to the battery voltage? a) They are the same values as the battery voltage. b) The voltages have different values compared to the battery voltage. 2. In a Series Circuit, how does the current through the various resistors compare to the source current? a) It is the same everywhere in the circuit. b) The current varies through each resistor. PARALLEL CIRCUITS 3. In a parallel circuit, how does the voltage across the various resistors compare to the battery voltage? a) They are the same values as the battery voltage. b) The voltages have different values compared to the battery voltage. 4. In a parallel circuit, how does the current through the various resistors compare to the source current? a) It is the same everywhere in the circuit. b) The current may vary through of the branches where there is a resistor. Fill in the Chart Below for a 3 Volt Battery Source Resistance Current x Voltage = Power 2 Ω 1.5 A 3 V 4.5 W 2 Ω 1.5 A 3V 4.5 W 1 Ω 3.0 A 3 V 9.0 W