Magnetism

Student Exploration Circuits Vocabulary ammeter, circuit, current, electron, ohmmeter, Ohms law, parallel circuit, resistance, resistor, series circuit, voltage Prior Knowledge Questions (Do these BEFORE using the Gizmo.) Strings of holiday lights can be designed in one of two ways. In some strings of lights, each light is connected to the others along a single wire (in series). In others, each light is attached to its own wire (in parallel). Suppose a single light bulb burns out. How do you think this will affect lights that are strung along a single wire All the lights go out How will a single burned-out bulb affect the string of lights if each light is attached to its own wire Only that single light goes out Gizmo Warm-up The Circuits Gizmo shows a circuit board and a variety of components. Create a circuit with a battery, a light switch, a wire, and a light bulb, as shown. (Click the light switch to turn it to OFF.) Click the light switch to turn it to ON. What happens The light turns on Turn on Show current and select Electron flow. The moving dots represent a current of electronstiny, negatively charged particlesmoving through the wire. Voltage is a measure of how much more potential energy an electron at one end of a battery has than an electron at the other end of the battery. . How does changing the batterys voltage affect the current Changing voltage doesnt effect the current How does changing the batterys voltage affect the brightness of the light The more voltage, the brighter the light Remove the wire. What happens The current stops  Activity A Ohms lawGet the Gizmo ready Click Clear. Create the circuit shown at right. (Use the 10 ohm resistor.) Click on the battery. Set the Selected battery voltage to 10 volts. Introduction Resistors are devices that slow the flow of current in a wire. The resistance of the circuit to current is measured in units called ohms. Question What is the relationship between voltage, current, and resistance Form hypothesis How do you think increasing the resistance in a circuit will affect the current in the wire Increasing resistance would slow down the current alot Observe Turn the light switch On to start the flow of current. An ammeter is a device that measures current in Amperes (A). Drag the Ammeter () to various parts of the circuit. Is the current the same throughout, or does it change Its the same What is the current in the wire now 1.00 A Collect data Measure the current in the circuit using the resistor and voltage combinations given below. VoltageResistanceCurrent10 volts10 ohms1.00 A20 volts10 ohms2.00 A30 volts10 ohms3.00 A VoltageResistanceCurrent50 volts20 ohms2.50 A50 volts100 ohms0.50 A50 volts200 ohms0.25 A Analyze What is the mathematical relationship between voltage (V), resistance (R), and current (I) Express your answer as an equation VIR This equation is known as Ohms law. Test Test your equation with other combinations of voltage and resistance. Modify the equation if necessary. 10V I20 I 0.05 A Apply Use Ohms law to find the resistance of the light bulb in the Gizmo. What is it V10V I 0.67 A R 10V 0.67AR R 14.93 ohms  Activity B Series circuitsGet the Gizmo ready Click Clear. Create a circuit as shown. Click the battery to select it. Set the Selected battery voltage to 10 volts. Question In a series circuit, there is only one path for charge to flow. What are the properties of series circuits Observe Turn the light switch ON and observe the light bulb. Then start replacing the wire segments with new light bulbs. How does each new light bulb affect the others All the other lights get dimmer Use the Ammeter to measure the current in various parts of the circuit. Is the current the same throughout, or does it change The current is the same Form hypothesis How do you think the total resistance of a series circuit is calculated As resistance increases, voltage decreases Experiment Create a series circuit with a 10-volt battery and four 10-ohm resistors, as shown. Measure the current. Based on the voltage and current, what is the resistance of the circuit (Hint Use Ohms law.) VIR 10V 0.25R R40 ohms Ohmmeters measure resistance. Remove the battery and attach the terminals of the Ohmmeter () to the ends of the circuit. What is the resistance 40 ohms Make a rule How do you calculate the total resistance of a series circuit Rtotal R1R2R3 Test your rule using the Gizmo. If necessary, modify your rule. 810 ohm 80 ohm Extend your thinking Replace the battery. Turn on Show current, and remove one resistor. Why would it be a problem if your household appliances were connected in a series circuit The more appliances there are, the weaker the voltage for each one due to the increase of resistance.  Activity C Parallel circuitsGet the Gizmo ready Click Clear. Create a circuit as shown. Click the battery to select it. Set the Selected battery voltage to 15 volts. Question In a parallel circuit, there is more than one path along which charges can flow. What are the properties of parallel circuits Observe Turn the light switch ON and observe the light bulb. Then add two more light bulbs to the circuit, parallel to the first two. How does each new light bulb affect the others Each new bulb has equal brightness to the next bulb Use the Ammeter to measure the current in various parts of the circuit. Is the current the same throughout, or does it change Explain one bulb has the current of the battery Form hypothesis How do you think the total resistance of a parallel circuit is calculated The inverse of the total resistance Experiment With the battery voltage set to 15 volts, measure the current in a parallel circuit with 1, 2, 3, and 4 light bulbs. (In each case, place the ammeter next to the battery.) Use Ohms law to calculate the total resistance of the circuit. Record results below. Number of light bulbs1234Voltage15 volts15 volts15 volts15 voltsCurrent1.00 amp2.00 amp2.99 amp3.99 ampTotal resistance15 ohm7.5 ohm5.02 ohm3.76 ohm Make a rule How would you find the resistance of a parallel circuit with n identical resistors 1/total 1/n1/n Apply What will be the total resistance and current in a parallel circuit with a 15-volt battery and three 10-ohm resistors Test your answers with the Gizmo. Total resistance estimate of 3.33 ohm (-1/total1/101/101/10) Current estimate of 15 volts (Activity C continued on next page) Activity C (continued from previous page) Extend your thinking Household appliances are usually connected in a parallel circuit. Why do you think it might be a problem if too many appliances are turned on at once (Hint current in a wire also produces heat.) It creates too much current which will result in the system over heating and causing some fires Calculate Determining the total resistance of a parallel circuit when there is a variety of resistors is more complex. The total current in the circuit (I) is equal to the sum of currents in each branch INCLUDEPICTURE http//www.explorelearning.com/ELContent/gizmos/ELScience_Deliverable/ExplorationGuides/images/EL_MSPS_Circuit20053.gif MERGEFORMATINET Ohms law (V IR) can be rewritten as I V / R. Substituting this expression into the equation above INCLUDEPICTURE http//www.explorelearning.com/ELContent/gizmos/ELScience_Deliverable/ExplorationGuides/images/EL_MSPS_Circuit20054.gif MERGEFORMATINET (Note Since each branch of the circuit might have a different resistance, we write R1, R2, and so forth. But the voltage is the same across each branch, so V is used for each term.) Divide each side of the equation by V, and you get an expression for the total resistance of the circuit INCLUDEPICTURE http//www.explorelearning.com/ELContent/gizmos/ELScience_Deliverable/ExplorationGuides/images/EL_MSPS_Circuit20055.gif MERGEFORMATINET Practice Determine the total resistance of each of the following parallel circuits. Then use the Gizmo to check your answer. (You can calculate the total resistance from the current and voltage using Ohms law, or use the Ohmmeter to measure the resistance directly.) A parallel circuit with a 20-ohm resistor and a 10-ohm resistor. 6.67 ohm A parallel circuit with two 20-ohm resistors and a 10-ohm resistor. 5 ohm A parallel circuit with a 15-ohm light bulb and a 20-ohm resistor. 8.57 ohm A parallel circuit with two 100-ohm resistors and a 20-ohm resistor. 14.3 ohm A parallel circuit with a 10-ohm, 20-ohm, 100-ohm and 200-ohm resistor. 6.06 ohm T
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