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Transcript

Student ________________________ Lab Date _____________ Lab # _____ Due Date _____________ ELECTRIC CIRCUITS PURPOSE: To study the principles of series and parallel circuits, voltage, current flow, electrical resistance and power that are used in electricity MATERIALS: Snaptricity lamp board, ammeter, digital voltmeter, wiring, switch PROCEDURE: The electric boards we will use in this lab show the effect of different kinds of series and parallel electric circuits via the brightness of light bulbs acting as resistances. To use the digital voltmeter make sure you use these settings: the red lead of the multimeter has to go into the opening next to V, and the dial of the multimeter should point to 20 V (DC , not AC ~). Most importantly, a voltmeter is always connected in parallel to a part of a circuit. The picture of this connection looks like this: To use the multimeter as an ammeter make sure you use these settings: the red lead of the multimeter has to go into opening next to A, 10 A and the dial of the multimeter should point to 10 A (DC , not AC ~). We always an ammeter connect in series with a part of a circuit. In other words, to measure the electric current in a bulb you should break the circuit at a point next to the bulb and connect the ammeter as part of the circuit. The picture of this connection looks like this: SERIES CIRCUIT The ﬁrst principle to understand about series circuits is that the amount of current is the same through any component in the circuit. This is because there is only one path for electrons to ﬂow in a series circuit. From the way that the battery is connected, we can tell which direction the current will ﬂow. We know that charge ﬂows from positive to negative, by convention. We know that in a series circuit the current has to be the same in all components. So we can write: 𝐼 = 𝐼1 = 𝐼2 = 𝐼3 . We also know that total voltage of the circuit has to be equal to the sum of the voltages over all three resistors. So we can write: 𝑉 = 𝑉1 + 𝑉2 + 𝑉3. When there is more than one resistor in a circuit, we are usually able to replace all resistors with a single resistor whose eﬀect is the same as all the 1 resistors put together. The resistance of the single resistor is known as equivalent resistance. For 3 resistors in series the equivalent resistance is: 𝑅𝑒𝑞 = 𝑅1 + 𝑅2 + 𝑅3 PARALLEL CIRCUIT The ﬁrst principle to understand about parallel circuits is that the voltage is equal across all components in the circuit. This is because there are only two sets of electrically common points in a parallel circuit, and voltage measured between sets of common points must always be the same at any given time. For a parallel circuit the following is true: 𝑉 = 𝑉1 = 𝑉2 = 𝑉3. The second principle for a parallel circuit is that all the currents through each resistor must add up to the total current in the circuit: 𝐼 = 𝐼1 + 𝐼2 + 𝐼3 1 1 1 1 For 3 resistors in parallel, the equivalent resistance is: 𝑅 = 𝑅 + 𝑅 + 𝑅 𝑒𝑞 1 2 3 PART A - SERIES CIRCUIT 1. Build a circuit that consists of three light bulbs, a switch and 4.5 V battery. What is the direction of the electron current? _______________________ DO NOT flip the switch until the teacher checks your circuit! 2. Connect voltmeter in parallel. Measure the voltage in the battery and in each bulb. Vbattery (V) V1 (V) V2 (V) V3 (V) 3. Connect ammeter in series. Measure the total current in the circuit and in each bulb. Itotal (A) I1 (A) I2 (A) I3 (A) 4. Take a paper clip and extend it so it becomes a long wire. Make two contacts around the middle bulb. Explain what you see. __________________________________________ 5. Disconnect the middle bulb from the circuit. What happens? ______________________ ______________________________________________________________________________ PART B 1. Build a circuit that consists of three light bulbs, a switch and 4.5 V battery. What is the direction of the conventional current? ___________________ DO NOT flip the switch until the teacher checks your circuit! 2. Connect voltmeter in parallel. Measure the voltage in the battery and in each bulb. Vbattery (V) V1 (V) V2 (V) V3 (V) 2 3. Connect ammeter in series. Measure the total current in the circuit and in each bulb. Itotal (A) I1 (A) I2 (A) I3 (A) 4. Take a paper clip and extend it so it becomes a long wire. Make two contacts around the middle bulb. Explain what you see. __________________________________________ 5. Disconnect the middle bulb from the circuit. What happens? ______________________ ______________________________________________________________________________ CALCULATIONS & CONCLUSIONS PART A - SERIES CIRCUIT 1. What is your conclusion about the total voltage of a battery and the voltage drops in each resistance for a series circuit? Explain any discrepancies. Vtotal = V1 V2 V3 ______________________________________________________________________________ ______________________________________________________________________________ 2. What is your conclusion about the total voltage of a battery and the voltage drops in each resistance for a series circuit? Explain any discrepancies. Itotal = I1 I2 I3 ______________________________________________________________________________ ______________________________________________________________________________ 3. Use Ohm’s law to calculate the resistance in the circuit and in each bulb. Req () R1 () R2 () R3 () Req = R1 R2 R3 ______________________________________________________________________________ ______________________________________________________________________________ 4. Use power formula to calculate the power developed by the battery and by each bulb. Ptotal (W) P1 (W) P2 (W) P3 (W) What is your conclusion about the total power developed by the battery and the power developed in each bulb? Explain any discrepancies. 3 Ptotal = P1 P2 P3 ______________________________________________________________________________ ______________________________________________________________________________ 5. Which conservation law seems to apply for series circuits? ______________________________________________________________________________ ______________________________________________________________________________ PART B - PARALLEL CIRCUIT 1. What is your conclusion about the total voltage of a battery and the voltage drops in each resistance for a parallel circuit? Explain any discrepancies. Vtotal = V1 V2 V3 ______________________________________________________________________________ ______________________________________________________________________________ 2. What is your conclusion about the total voltage of a battery and the voltage drops in each resistance for a parallel circuit? Explain any discrepancies. Itotal = I1 I2 I3 ______________________________________________________________________________ ______________________________________________________________________________ 3. Use Ohm’s law to calculate the equivalent resistance (of the circuit) and in each bulb. Req () R1 () R2 () R3 () 1 𝑅𝑒𝑞 = 1 1 1 𝑅1 𝑅2 𝑅3 ______________________________________________________________________________ ______________________________________________________________________________ 4. Use power formula to calculate the power developed by the battery and by each bulb. Ptotal (W) P1 (W) P2 (W) P3 (W) Ptotal = P1 P2 P3 5. Which conservation law seems to apply for parallel circuits ? Is that different from series circuits? ______________________________________________________________________________ ______________________________________________________________________________ 4