Lab 4.1 Electrostatics and Practical Electricity

Transcription

1 Lab 4.1 Electrostatics and Practical Electricity Objectives: Learn about the nature of electric charge and electrically charged objects Use Coulomb s Law to calculate the force between charges Learn how to use a multimeter Learn about how a typical house is wired Lab Materials: Van de Graaff generator Multimeter Kill-a-Watt meter and Hair Dryer Switches Task 1. Problem 1 Concept Questions Electrostatics and Electric Charge A. Two small spheres repel one another electrostatically. Which of the following statements is true? a. At least one sphere must be charged. b. Neither sphere needs to be charged. c. Both spheres must be charged and the charges must have the same sign. d. Both spheres must be charged and the charges must have opposite signs. B. Two identical conducting spheres, one that has an initial charge +Q, the other initially uncharged, are brought into contact. What is the new charge on each sphere? a.) Q b.) Q/2 c.) zero d) +Q/2 e.) +Q C. If you bring a positively charged insulator near two uncharged metallic spheres that are in contact and then separate the spheres, the sphere furthest from the insulator will have: a.) no net charge b.) a positive charge c.) a negative charge D. Charges A and B exert repulsive forces on each other. qa = 4qB. Which statement is true? a.) FA on B > FB on A b.) FA on B < FB on A c.) FA on B = FB on A E. Two point charges exert a 9 μn force on each other. What will the force become if the distance between them is increased by a factor of three? a.) 1 μn b.) 3 μn c.) 27 μn d) 81 μn F. If a positive charge were placed at the origin (the crossing point of the vertical and horizontal lines) of the figure, into which quadrant would it feel a net force? a.) A b.) B c.) C d) D e.) None, it feels no net force. G. A piece of brass is rubbed with a piece of cotton. The brass will have a: a.) positive charge b.) negative charge c.) no charge H. H2O is a dipole. The more electronegative oxygen atom attracts electrons from the hydrogen atom. Thus, the oxygen atoms acquire a partial negative charge and the hydrogen atoms acquire a partial positive charge. The water molecule is "polarized." Which diagram correctly portrays a pair of H2O molecules? Page 1 of 6

2 Task 2. Problem 2 Concept Questions Coulomb s Law and Forces between charged particles A. A point charge of q1 = is placed at the origin and a second point charge, q2= of the same sign is placed along the x-axis at x =. What is the x-coordinate of a third positive charge so that the net force on it is zero? [See online problems for specific numbers]. Complete picture below. q1 q2 x B. A point charge of q1 = is placed at the origin and a second point charge, q2= of the opposite sign is placed along the x-axis at x =. What is the x-coordinate of a third positive charge so that the net force on it is zero? [See online problems for specific numbers]. Complete picture below. q1 q2 x Task 3: Van de Graaff Generator demonstrations a) Hold the discharge sphere near the Van de Graaff Generator. b) Have a volunteer stand on an insulating object and place their hand on the sphere of the Van de Graaff Generator. c) Tape a paper clip to the surface of the Van de Graaff Generator. This creates a point on the otherwise smooth sphere. Because this point concentrates the electric filed here, the majority of leaking charge will leave the sphere at this location. Page 2 of 6

3 Task 4: Problem 3 Concept Questions - Practical Electricity There are three basic properties of electric circuits: Potential units of volts Current units of amps Resistance units of Ohms An analogy with fluid flow is often used. Voltage is potential, and like a water pump, it gives energy to the system. Current (charge per time), measured in amps, it is like the flow rate of the fluid. Resistance, measured in ohms, is like a constriction in the pipe, or friction. It is something that takes energy out of the system. Current: A. What type of current is in your house? 1. AC 2. DC B. What is the frequency of the current? Hz Hz What is a Hz anyway? C. To prevent excessive current (and a hazard), there are circuit breakers which trip if the amperage is too high. A typical circuit breaker trips at how many amps? A A A A Voltage: D. What is the voltage in a typical electrical circuit (the type you plug an appliance into)? V V V V E. Use a multimeter to measure the voltage from a standard outlet. Set your multimeter to Volts and use the select button to change to AC. Measured voltage: F. To avoid excessive current, large appliances use a higher voltage circuit. What is the typical voltage of a clothes dryer circuit? V V V V G. Where is 220 V a common voltage? 1. Europe 2. California 3. Canada 4. The Bennett household H. AC voltage is a sinusoidal function, e.g., vv = vv 0 ssssss(ωωωω). A voltmeter measures root mean square (RMS) voltage. The RMS voltage is found by integrating the square of the function for one period, dividing by the period, and then taking the square root, which results in: VV RRRRRR = VV pppppppp 2 In the USA, the RMS voltage is 120 V, what is the peak voltage? V V V V I. What are you measuring with the voltmeter? Read question H. a. average voltage b. peak voltage c. RMS voltage Volts (V) Time (sec) Page 3 of 6

4 Power: J. A basic equation of electricity is PP = VVVV, where PP is power (watt), VV is voltage (volt), and II is current (amps). You plug a 1500 W hairdryer into the circuit. How many amps does the hairdryer draw? A A A Kill A-Watt / Voltage Regulation We can measure electrical parameters with the Kill a Watt meter. Plug the Kill a Watt meter into an outlet. Measure the voltage and the frequency. Voltage Frequency Connect a hair dryer to the Kill a Watt. Fill in the table below. Switch Position Volts Amps Watts Off Fan Low High K. The voltage regulation is a measure of the ability of a system to maintain a constant voltage and defined as RR vv = VV nnnn VV LL 100, and V nl is the voltage with no load, V fl is the voltage with full load. Using the table above, VV ffff determine the voltage regulation, Rv, in Perkins for a hair dryer on high, % % % Switches L. At times we want two switches to control a single light. Which circuit diagram below would work such that flipping either switch would toggle a light? i. Two Single Pole Single Throw (SPST) switches 2. Two single Pole Double Throw (SPDT) switches M. Which circuit diagram below would enable all three switches to turn the light on and off? 1. Three Single Pole Single Throw (SPST) switches 2. Double Pole Double Throw (DPDT) switch between two SPDT switches Watch the online animation. The neat thing is that you include as many DPDT switches as you want in the circuit. By stringing together DPDT switches, it is possible to have 100 switches control a single light. Page 4 of 6

5 Analogies to electric circuits: Title Fluids Thermal Electric Circuit Pressure = Energy/Volume Temperature = Energy/k Voltage = Energy/Charge Voltage A closed faucet has pressure k is Boltzmann s constant A free electrical outlet has but no flow An isolated body has voltage but no current temperature, but no heat flow Current Volume flow rate = Volume/Time Heat flow rate = Heat/time Current = Charge/Time Resistance Current & Flow rate Laws Ground Resistance represented by a severe constriction or obstruction will produce a pressure drop; Resistance of a wire is represented by pressure loss in the pipe or hose. Poiseuille s Law: Volume Flow rate = Pressure Resistance Conservation of Liquid: There is no net pressure change in any closed loop path A reservoir serves as a pressure reference. A reservoir can supply water to a circuit. Once the pipe is filled with water, the pump can circulate the water without further use of the reservoir. Resistance is provided by insulation, or thermal resistance Heat flow: Heat flow rate = Q t = T A R Conservation: There is no net temperature change (change in internal energy) in a closed cycle. Absolute zero serves as temperature reference. Resistance represented by a resistor will produce a potential drop Ohms Law: Current = Voltage Resistance Conservation of Charge: There is no net potential change in any closed loop path A ground serves as a voltage reference. A ground can supply charge to a circuit. Page 5 of 6

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