Energy. This provides a practical measure of the usefulness of a machine. The useful energy transfer in a generator can be represented by:

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Bernice Harrell
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Sensors: Loggers: Voltage, Current, Motion Any EASYSENSE Physics Logging time: 10 seconds 44a Efficiency of an electric generator Read Machines use energy transfers to achieve a useful job of work.

1 Sensors: Loggers: Voltage, Current, Motion Any EASYSENSE Physics Logging time: 10 seconds 44a Efficiency of an electric generator Read Machines use energy transfers to achieve a useful job of work. No machine is able to transfer all of the input energy into a useful output. Some energy is always wasted. This waste energy usually is in the form of heat, but can also be in others forms. In mechanical machines, friction is the main process involved in producing this waste heat. The measure of the efficiency of a machine is calculated by the following equation: OUT PowerOUT Efficiency = 100 % = 100% Power This provides a practical measure of the usefulness of a machine. The useful energy transfer in a generator can be represented by: Motion Electrical OUT In this experiment, you will use the motor to lift a weight. You will then allow the falling weight to drive the motor as a generator, and measure its efficiency as a generator. Preliminary question 1. Draw a flow chart showing the sequence of energy transfers occurring as the weight falls and drives the motor as a generator. Low voltage power unit Lamp DPDT switch Motor / Generator Weights Mesh guard Motion At least 1m Fig Efficiency of an Electric Generator 44a - 1(V2)

2 Switch Circuit Details 6V d.c Motor generator Voltage Lamp Current Fig Black leads What you need 1. An EASYSENSE logger. 2. A Smart Q Voltage 3. A Smart Q Current 4. A Smart Q Motion 5. Lamp 3.8 V 0.3 A (or similar) in holder. 6. Motor / generator with an axle/pulley for winding up a weight. 7. Low voltage d.c. power supply or equivalent battery pack. 8. Thin string x 100g masses on a hanger or 10 x 10g masses on a hanger. 10. Wires Theory In this experiment you will measure the efficiency of an electric motor / generator PowerOUT Efficiency = 100% = Power OUT 100% Power-Out = V x I This data can be calculated from the current and voltage readings using a Post-log Function. Power-In = rate at which the weight loses gravitational potential energy de p dt = dh mg = dt = mgv 44a - 2(V2)

3 The velocity is derived from the Motion data using a Post-log Function. Key: h = distance from the Motion Sensor to the bottom of the weights. V = voltage across the motor/generator I = the current flowing in the motor/generator, and the lamp E p = gravitational potential energy of the load masses m = mass of the weight to be lifted. g = acceleration due to gravity. v = velocity of the moving weight. mg = weight to be lifted d = delta, difference. dh = the difference in height between two points. What you need to do The generator being driven by the falling weight Note: It is important that a sturdy protective mesh is used to prevent the falling masses from reaching the Motion. A wire mesh letters in tray over the motion will suffice if the masses are not too large. Input 1 Input 2 Input 3 Voltage Current Motion 1. It is advised to try this experiment at least once before placing the Motion under the masses to gauge the speed of the falling mass system. If the masses appear to fall too quickly then the mass should be reduced. 2. Set up the apparatus as shown in the two figures (44.1, 44.2), the first figure shows a suggested layout for the apparatus and the second a circuit layout. 3. Make sure the black leads of the Current and Motion s are connected together. Ensure that the connecting leads to the motor have the correct polarity. 4. You will need a ±100 ma or ±1 A Current, depending on the lamp used. Connect the Voltage to input 1, the Current to input 2, and the Motion to input 3 on the logger. 5. Suspend the hanging weight on the motor so that it is at about 20 cm above the Motion when fully unwound. Check that the Motion is vertically below the weight. 6. Write down the value of the mass of the weight. 7. Wind the weight up by turning the pulley until it is close to the pulley. Release the weights and let it fall under gravity towards the Motion. Make sure the weight is stopped before it can reach the Motion. 8. From the EasySense software s Home screen select Open Setup (or File, Open Setup). Open the file Data Harvest Investigations (Edition 2) \ Setup files \ Physics L3 V2 \ 44a Generator efficiency. 9. Wind the weight up to just below the pulley. 10. Check, again, that the weight is vertically above the Motion. 11. Click on Start to begin logging. Wait a few seconds and drop the weight vertically. 12. The logger will log for 10 seconds and then display the results. Check that the Motion curve has no peaks during the fall. Save the results. Results and analysis You have graphs of Voltage, Current and Distance against Time for generating electricity. Mass of the weight = kg 44a - 3(V2)

4 You will need to generate the following data sets from the logged data using Post-log Functions: 1. Output power Power VI from the current and voltage data. a. The current will be reversed and read negative. Put a (minus) sign in front of a. b. The currents are lower. The readings will be in ma. The value of a needs to be Input power Power mgv from the distance data and the value of the weight lifted. a. The value for a = mg. Check that you use the correct value for m. b. The weight is falling. You will have to put a (minus) sign in front of your value for because the velocity is negative. 3. Efficiency % from the Power VI and the Power mgv data sets. a, 1. To generate the input Power mgv data set From the Tools menu select the Post-log Function. Select Formula function: a dx/dt. Make x = Distance data. Parameter, make a = m 9.81 (use your value of the mass, m) Make the name Power mgv, keep the units as watts and decimal places as 3. Use Smoothing (Tools menu) if necessary. 2. To generate the output Power VI data set From the Tools menu select Post-log Functions Select Preset function: Electricity, Power Select the Voltage and Current data channels. Make the name Power VI, keep the units as watts and decimal places as 3. Parameters: Check the information in the white panel to decide on the value for the multiplier (it will depend on which Voltage and Current you used). Use Smoothing (Tools menu) if necessary. 3. To generate the Efficiency data set From the Tools menu select the Post-log Function. Select Formula function: a x/y. x = Power mgv y = Power VI a = 100 to convert to % Decimal places = 3 Name = Efficiency Unit = % Use the Auto scale or Sensor Settings to adjust scales of the data sets produced as required. Questions Use the derived graphs of Power VI (input), Power mgv (output), and Efficiency vs. Time to answer the following questions. Answer separately for the motor and for the generator. 1. What is the efficiency of the motor during the middle of the lift? (use Values) 2. The velocity of the weight remains approximately the same for a length of time. What is the range of values for the efficiency during that time? 3. The efficiency is significantly less than 100%. Explain why this is so and account for the energy losses. 4. The area interval. under a power time curve is equal to the total energy transferred during a particular time 44a - 4(V2)

5 Use Area to measure the energy input and the energy output while the weight is travelling at a steady velocity. (Note having selected the area under one curve. Click to the left of the vertical axis to change from Power VI to Power mgv the new area will be for the same time interval.) Calculate the overall efficiency from this energy data. Extension How does increasing the weight effect the efficiency? How does increasing the current rating of a lamp change the efficiency of the generator 44a - 5(V2)

Newton s Second Law. Newton s Second Law of Motion describes the results of a net (non-zero) force F acting on a body of mass m.

Newton s Second Law. Newton s Second Law of Motion describes the results of a net (non-zero) force F acting on a body of mass m. Newton s Second Law Newton s Second Law of Motion describes the results of a net (non-zero) force F acting on a body of mass m. F net = ma (1) It should come as no surprise that this force produces an