物理演示实验1

Experiment to Measure the Acceleration Due to Gravity Introduction To measure this acceleration we will drop a magnet and measure the time taken for the magnet to travel between two points. This information used with the distance formula below will allow us to calculate the acceleration of the magnet due to gravity. s = ut + ½ at2 Equipment required ​ Thin enamelled wire for the coils. ​ cardboard ​ A plastic tube, at least 80 cm in length. ​ Clamp and stand. ​ A magnet. ​ ADC-40, running Picoscope. Experiment Set up ​ Wrap two pieces of cardboard around the plastic tube about 60 to 70 cm apart. ​ Wind two coils ( 150 turns each ) around the cardboard tubes, using the enamelled wire. The two coils should then be connected in series, as shown in figure 1. ​ Once the coils are wound and attached securely the plastic tube can be mounted vertically using a clamp and stand. ​ A foam pad should then be placed between the plastic tube and the bench to protect the falling magnet and the bench. ​ A few prelimary runs need to be done to make sure Picoscope is setup correctly and capturing all the required information. These settings depend on the distance between the two coils on the plastic tube, but an example of the settings can be seen on the screen shots for this experiment in figures 2 and 3. Figure 1: diagram showing the arrangement of the two coils Carrying out the experiment 1.​ Measure and note the distance between the two coils. 2.​ Start PicoScope to capture the data (using the single trigger event). 3.​ Drop the magnet into the tube. 4.​ Save the waveform. 5.​ Repeat steps 2 to 4, until you have 4 to 5 sets of results. Analysing the results To use the distance formula the time taken (t) to travel distances (s) to each coil have to be found. s = ut + ½ at2 The two distances of the coils are taken from the start point to the top of each coil. The start point is taken from the front face of the magnet, this is illustrated in figure 2. Figure 2: diagram showing measurements for the coils To find the time taken for the magnet to travel between the two coils: 1.​ Place the cursor on the first peak of the waveform from the experiment. 2.​ Picoscope will display the time (t1) of the first peak (shown as 13.39 ms in the screenshot below). 3.​ Place the cursor on the second peak of the waveform from the experiment. 4.​ Picoscope will display the time (t2)of the second peak. 5.​ Subtract t2 from t1 to find the time taken to travel from the first coil to the second. Figure 3: Measurement taken using PicoScope now using s = ut + ½ at2 For the magnet falling to the first coil, where t is the time taken to reach the first coil. s1 = 0 + ½ at2 For the magnet falling from the start position to the second coil, (where dt = t2-t1). s2 = 0 + ½ a(dt + t)2 Solving the two equations for ‘a’ will arrive at a value for the acceleration due to gravity (g). Questions 1.​ Was the value for ‘g’ the value you expected. If not, can you think why the value for ‘g’ is less or greater than what you expected. Further study ​ Faraday’s Law states that the induced e.m.f. is proportional to the rate of cuting of field lines. Since the same magnet is passing through two identical coils it would be possible to investigate if the induced e.m.f. — measured durectly from the trace is indeed proportional to the speed of the magnet — calculated using the data already obtained. Results Using s = ut + ½ at2 For the magnet falling to the first coil, where t is the time taken to reach the first coil. 0.145 = 0 + ½ at12 For the magnet falling from the start position to the second coil, where t again is the time taken to reach the first coil. 0.675 = 0 + ½ a(0.2026 + t)2 Solving the two equations for ‘a’ will arrive at a value for the acceleration due to gravity (g). a = 9.5 m/s2 Answers to questions 1.​ Was the value for ‘g’ the value you expected. If not, can you think why the value for ‘g’ is less or greater than what you expected. ​ Air resistance will impede the magnet as it falls ​ The induced emf represents electrical energy — this cannot have been created out of nothing — it is obtained at the expense of the magnets kinetic energy. ​ Lenz’s law is another way of looking at thesame thing — the induced emf always oposses the motion of the magnet — thereby slowing it down.