Lab-1 Instructions

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School

University of Toronto *

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Course

110

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Electrical Engineering

Date

Dec 6, 2023

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pdf

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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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