LAB 3 Fun with AC Circuits Physics 2020 Spring 2004 In the previous lab we studied some simple DC circuits where all the voltages and currents did not change with time What happens when the voltages and currents do change with time It turns out that we live in a world where most circuits are in fact AC instead of DC In this lab we will explore two different applications of AC circuits electrical impulses from our own hearts and transformers Feel free to ask your TA why we have AC circuits rather than DC PRELAB QUESTIONS there are more pre lab questions on p 3 0 Read the entire lab 1 The picture to the right is a typical display you may see on an oscilloscope What are the period frequency and amplitude of this voltage signal Assume that the volts div setting is 50 V div and the time div setting is 5 msec div NOTES AND PRECAUTIONS 1 In all your measurements and observations include a careful picture of the display and be sure to indicate the horizontal and vertical scales in SEC DIV and VOLTS DIV respectively 2 When connecting the battery to the coil of wire that you will make be sure not to leave the wire connected for too long This will drain the battery and guess who will have to pay for a new one 3 Be sure to listen to your TA for other precautions PART I MEASURING SIGNALS FROM YOUR HEART 45 mins Every living person s heart produces electrical signals that can be measured on the surface of the skin An EKG electrocardiograph is an instrument that can measure these signals and produce a visual image and sometimes an audible sound as in the beep beep beep you hear in movies of the signal your heart produces Your goal for this lab is to obtain an accurate recording of this voltage signal from one of your lab mates and to determine what things affect this signal Why the acronym EKG instead of ECG Actually both are used but EKG comes from the German word Elektrokardiogram It has stuck around and is used more often for historical reasons Our makeshift electrocardiograph is made from an oscilloscope which is a very useful tool for measuring voltages that are changing in time Think of the oscilloscope just like the DMM we used in the last lab it measures voltage but now it maps it out in time The screen on the scope displays voltage vs time voltage on the vertical axis and time on the 1 horizontal axis The grid that you see on the screen is used to measure the voltage and time of your signal think of it like graph paper Each little box on the grid is called a division and you can adjust the scale of the voltage and time axes with the volts div and the sec div knobs respectively For example if the volts div knob is set to 5 this means that each box on the grid is equal to 5 volts These are just the basics on what this device is used for but your TA will give you a short intro on how to properly use an oscilloscope Your heart is a complicated electrochemical machine that produces time varying voltages as it beats These heart voltages produce small voltage differences between points on your skin that can be measured and used to diagnose the condition of your heart Usually nine electrodes positioned at various points of the patient s body are used when recording a full electrocardiogram However in this lab we will only use two electrodes to measure the voltage between your right and left hands A typical plot of voltage difference between two points on the human body vs time is shown above The P deflection corresponds to the contraction of the atria at the start of the heart beat The QRS group corresponds to the contraction of the ventricles The T deflection corresponds to a recovery or re polarization of the heart cells in preparation for the next beat Every heart pattern is slightly different and the interpretation of an EKG requires experience with many patients The EKG apparatus that you will use consists of two electrodes an amplifier and a storage oscilloscope Signals travel from the hands one placed on each electrode to the EKG amplifier and then on to the oscilloscope The voltage that is measured is the potential difference between the two electrodes However the voltage difference between your hands is inconveniently small to measure directly To compensate for this the signal from the electrodes is given a boost by the amplifier Enough talk for know let s get to business A You should have a battery at your lab station Play around with the battery and make sure you understand how to measure its voltage correctly using the oscilloscope B The voltage displayed on the oscilloscope differs from the input voltage of your hands by an amplification factor To find this factor switch the amplifier mode switch to calibrate and then press the red button this produces an input voltage whose peak positive value is 1 mV 10 3 volts Measure the voltage on the oscilloscope and determine the amplification factor of your amplifier 2 C Have one person in your group sit down in front of the electrode assembly with his or her hands wrapped gently around the electrodes palms down Adjust the settings on the oscilloscope until you can see a full EKG signal on the screen Draw a picture of the EKG in your lab notebook and record any specific details of the signal that you can measure Using the amplification factor what is the actual voltage produced by your lab mate s heart D Now grab and squeeze the electrodes with your hands What happens to the voltage signal and what could explain this E Now adjust the oscilloscope so that you can see at least 2 peaks on the screen and record your observations Figure out a way to measure the pulse rate of your lab partner from this data F After everyone in your lab group feels they have a solid understanding of what you are measuring and how you exactly do it predict what would happen if two people holding hands were to touch the electrodes that is on person grabs it with their right hand and the other person grabs the other electrode with their left hand Record your prediction and test it to see if you re right PART II ELECTROMAGNETS AND TRANSFORMERS 45 mins PRELAB QUESTIONS 2 What do transformers do and why are they useful 3 Does a transformer work for DC or AC current What would happen in a transformer if you plugged it in to a DC circuit Why Simply put an electromagnet is just a coil of wire That s it just a bunch of wire You may think that this will be the most boring lab in the world because you re going to play around with a bunch of wire right Well it turns out
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