Lab-1 Instructions

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University of Toronto *

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110

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Electrical Engineering

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Dec 6, 2023

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E C E 1 1 0 : L a b - 1 P a g e 1 of 5 University of Toronto Objective i) To demonstrate that there are two types of electric charges in nature. ii) To become familiar with the proto-board. iii) To become familiar with power supply and Digital Multi-Meter (DMM) (Volt/Ampere/Ohm meter). Instruments 1) Proto-board (also called breadboard), 2) Laboratory DC Power supply, 3) DMM: an instrument capable of measuring voltage, current, and resistance. 4) Wimshurst machine (electrostatic charge generator). 5) Electrostatic kit. LAB PREPARATION QUESTIONS Study the instructions for each part of this lab exercise and review the materials related to the electrostatic charges in your textbook. Please answer the questions below and bring them to the lab: a) Provide an expression for the electric field vector at the point “A , ” located at the midpoint between the two oppositely charged spheres with charge q (see Fig. 1). b) Draw the electric field lines associated with the two oppositely charged spheres of Fig. 1. c) Dielectric strength is the maximum electric field that a dielectric material can withstand without breaking down (i.e., without failure of its insulating properties). What voltage is required to break down the air between the two metallic spheres shown in Fig. 1 and create a spark? (Hint: the dielectric strength of air is approximately 3 kV/mm.) Remark: The short circuit current of the Wimshurst machine is about 30 μA. At 1mA, little or no electrical shock is felt, so the high voltage in part “ c ” is safe. However, it cannot be concluded that a high voltage is always safe. EXPERIMENT PART 1: WIMSHURST MACHINE Purpose a) To become familiar with Wimshurst machine. b) To observe the storage and transfer of charges using Leyden jars. c) To show via qualitative observations the following phenomena: ✓ Existence of two types of charges (+ and -). ✓ Existence of electrostatic forces between charged objects. Fig. 1. Two oppositely charged spheres 0.5cm - + A x -q +q x-axis Fig. 2. Wimshurst machine

E C E 1 1 0 : L a b - 1 P a g e 2 of 5 University of Toronto Description The Wimshurst machine, shown in Fig. 2, provides an efficient way of separating electric charges by induction; it is an electrostatic generator capable of throwing long sparks between two discharge-spheres (G) shown in Fig. 3, when Leyden jars (D) are connected to them. This machine consists of two parallel dielectric discs (A), hand driven (B) so that discs rotate in opposite directions about a common axis. Each plate has narrow conducting strips (H) arranged radially, equal distances apart around the rim. Two brushes (I) connected to metal rods (C), one in front and one in back, transfer charges from one side of a disc to the other. Other metal brushes (E) collect these charges and store them in two Leyden jars (D). Attached to these jars are metal rods (F) with discharge-spheres (G) at their ends. When enough charge is collected in the jars and the electric field between the spheres exceeds the dielectric strength of the air, a spark jumps between the spheres (see Fig. 3). The Wimshurst machine was used to power the first generation X- Ray tubes in 1890’s. IF YOU HAVE A HEART CONDITION OR A HEART PACEMAKER, IT WOULD BE WISE NOT TO HANDLE THE LEYDEN JARS. Procedure Step 1 : Use the Wimshurst machine to create electrostatic charges. Multiple Wimshurst machines are available in the laboratory, and they will have to be shared among several teams. a) To transfer the charges to your own station, bring the two Leyden jars to the Wimshurst machine and collect the charges from the discharge-spheres. A few clockwise rotations should be sufficient to produce a spark. Have a Teaching Assistant show you how to transfer the charges safely. Step 2 : To verify that you have collected two types of charge polarities, use the needlepoint support setup shown in Fig. 4: a) Charge the plastic straw with one of the Leyden jars by rubbing the straw to the top of the jar, then place the plastic straw on the needlepoint support. b) Next, charge the metallized ping-pong ball by touching it to the other charged Leyden jar. c) Bring the metallized ping-pong ball close to the straw and observe the force on the straw. d) Write down your observation. Note the nature (attractive or repulsive) of the force exerted on the straw placed in the support. e) What rule have you confirmed? Step 3 : Repeating the experiment with the same charge polarity: a) Discharge the metallized ping-pong ball and the plastic straw by touching them to the metallic part of your station. b) Charge the plastic straw by using a Leyden jar and charge the metallized ping-pong ball by touching it to the same charged Leyden jar. Place the plastic straw on the needlepoint support. c) Bring the ball close to the straw and observe the force on the straw. d) Write down your observation. Note the nature (attractive or repulsive) of the force exerted on the straw in the support. e) What rule have you confirmed? Fig. 4. The needlepoint support setup Needle Straw Support Fig. 3. Sparks hopping between the spheres of a Wimshurst machine

E C E 1 1 0 : L a b - 1 P a g e 3 of 5 University of Toronto f) Discharge the Leyden jars before putting them back in the box. EXPERIMENT PART 2: PROTO-BOARD Purpose : a) To become familiar with the proto-board. b) To become familiar with the connectivity check (beep check). Description : When you have an idea and want to take the idea from a thought to a final design, you first begin by drawing a block or circuit diagram on the paper. You then prototype the circuit in some form that you can easily modify and only then consider assembling a permanent version of it. In Electrical Engineering, perhaps the most common means to prototype a circuit is a “ proto- board” or “breadbaord , ” which is a perforated and pre-connected board that does not need any soldering. You insert your wires and components according to how the rows and columns on the board are connected, and you are very much ready to test your prototype. Figure 5(a) shows a typical proto-board, whereas Figs. 5(b) and (c) show the internal connection map and the assembled platform in the lab, respectively. Procedure : Step 1 : Figure 6(a) shows the direction of connections on the proto-board. One very useful test you may use is the connectivity test, which is a feature incorporated in your multi-meter. Some multi-meters beep (a) (b) (c) Fig. 5. A typical prototyping board (also called proto-board) to digital multi-meter (Volt/Ampere/Ohm meter) to power supply (Black plugs are connected together) (a) Direction of connections (b) Connectivity check Fig. 6. Connections on the prototyping board direction of connections A B C D k Ω Connectivity Check

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E C E 1 1 0 : L a b - 1 P a g e 4 of 5 University of Toronto when there is a direct connection between the ends of your probes, with no interruption or components between them. The DMMs in our lab don't have speakers, and they don't beep. Instead, they show zero resistance on their display. Please read the value of the resistance instead of listening for the beep sound. Zero resistance means there is a short circuit. a) Turn the DMM on and put it the continuity mode by pushing the button with the label shown in right side of Fig. 6(b). b) Run the continuity test for cases A-D of Fig. 6(b). In which case(s) do you expect a short circuit? Write down your observations. c) If you wish to apply a voltage across a resister, which of the two configurations in Fig. 7 show the correct connections? Construct the correct connections on your proto-board. At this point, you do not need to connect the circuit to the voltage source. EXPERIMENT PART 3: POWER SUPPLY AND DIGITAL MULTI-METER (DMM) (VOLT/AMPERE/OHM METER) Purpose : a) To learn how to operate basic equipment such as DC power supply and voltmeter function of a DMM. b) To become familiar with the measurement techniques used in the study of DC and AC circuits. Description : 3.1) Power Supply: A power supply produces a constant voltage, "Volts DC". A typical electronics bench power supply will have an on/off button, rotating knobs to adjust the output voltage and current, and three or more connectors for you to connect the power supply to the device under test. Two output connectors are coloured black (for "negative") and red (for "positive"). The third connector is coloured green and labelled as "ground". In this course, the power supply’s “ negative ” terminal is already connected to its “ ground ” terminal through a metal plate (a "shunt"), as shown in Fig. 8(a), so that your power supply will always produce positive voltage. The rotating knobs labeled as “VOLTAGE” are used to set up the constant voltage at which you want to operate. The knobs labeled as “CURRENT” are used to set the maximum current that you allow your circuit to draw (from the power supply). Limiting the current is a good option in order to protect your circuit, in the case things do not go the way you had expected. 3.2) Digital Multi-Meter (DMM): (a) (b) Fig. 8. The Power Supply with one output on the left and three outputs on the right the metal plate (shunt) Fig. 7. Continuity test Metal plate shunt

E C E 1 1 0 : L a b - 1 P a g e 5 of 5 University of Toronto The DMM can be used for three different types of measurements: voltage, current, and resistance. It also can also be used to test continuity ("the beeper"), as you saw above. Engineers often refer to the DMM as the "voltmeter" when used to measure voltage, the "ammeter" when used to measure current, or the "ohmmeter" when used to measure resistance. These three settings are found in almost every digital (or analog) multi-meter. Figure 9 shows the DMM you have on your bench, where the three aforementioned settings are selected through the push buttons labelled "V", "mA, " and "KΩ" on the panel (you should be able to locate these push buttons on Fig. 9). The voltmeter Mode: The voltmeter measures difference in electric potential (voltage). If the voltage is constant (DC), it will display the corresponding DC voltage value. If, however, the voltage changes with time (AC), then the voltmeter will show a number that is the root mean squared of the alternating voltage. In this experiment, you will measure a DC voltage with the DMM. Procedure : In this experiment, you will use the power supply to generate 5V and measure it using the voltmeter function of DMM. Then, you will change the voltage to 7V and measure it again. Step 1 : Setting up the power supply and measuring the DC voltage: a) Start with both instruments turned OFF and rotate the knobs on the power supply to the minimum setting (that is, rotate to the left). This is how you will start your work every time. b) Do you want to measure AC or DC? Push the appropriate button. c) Do you want to measure voltage, current, or resistance? Push the appropriate button. d) What is the estimated maximum value of your measurement? Push the appropriate scaling button. Step 2 : Making the connection: a) Attach the common/ground of the power supply to the DMM ’s "COM" terminal (COM stands for "common"), which is also shorted to the ground via a metal plate. b) Why would you connect the ground terminal of both instruments first? Write down your reasoning. c) Now connect the "V-  " terminal of the DMM to the positive terminal of the power supply. d) Turn the voltmeter on and then turn on the power supply. e) Read the voltage value measured by the DMM. Using the rotating knobs on the power supply, adjust the voltage level to 5V and the current to the maximum of 0.1 A. f) After writing down your observation (measured voltage), adjust the knob on the power supply to provide 7V. Using the DMM measure the output voltage of the power supply. Write down your observation. g) Turn the power supply off for now and do not touch the settings (to preserve the setting). Fig. 9. The Digital Multi-meter (DMM) (Volt/Ampere/Ohm meter)