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236+Manual+02+EKG - EKG Physics 236 Spring 2013 An EKG...

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An EKG trace from a healthy patient. EKG Physics 236 Spring 2013 1 Introduction An electrocardiogram 1 (EKG) is a measurement of the heart’s electrical activity as a function of time. The name comes from the German Elektrokardiogramm ; thus the acronym EKG is often used since ECG sounds a lot like EEG (electroencephalogram) over a hospital PA system. An EKG is one of an extremely large number of real world applications of measuring a potential difference. Electrodes are attached to a patient’s skin and the voltage difference between pairs of electrodes is measured. In our experiment, we will use three electrodes. (Clinical EKG tests often use 12.) The EKG sensor combines the potential of the three electrodes into a single trace. Using this data, we will determine the heart rate and orientation of the dipole of the heart for a "patient" from each lab group. We will also use the EKG sensor to examine the electrical activity of a patient’s jaw muscle. 2 Theory Before starting this lab, you should be familiar with the following physical concepts. If you need to review them, or if you haven’t yet discussed them in your lecture course, consult the indicated sections in the 235 coursepack [2]. Potential, §22.4 Electric dipoles, §21.2 2.1 Voltage a.k.a. Potential The potential energy per unit charge at any point in an electric field is the electric potential (=poten- tial=voltage). Electric potential is a scalar and can be represented by a single number at each point in space (and time). 1 Willem Einthoven began his investigation of the human heart in 1891. His invention of the string galvanometer in 1903 led directly to the modern EKG. For this work, he was awarded the Nobel Prize in Physiology or Medicine in 1924 [1]. 1
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Because potential, which is also known as voltage, is a scalar, it only has a magnitude, not a direc- tion. The potential difference Δ V between two points in space is easy to measure with a voltmeter. An equipotential line is, as the name suggests, a line made of points that all have the same potential. 2.2 Electric Field In textbooks, electric field is nearly always introduced before potential, and then the potential is derived from knowledge of the electric field properties. In the lab, the opposite is significantly easier. Electric fields are much more difficult to directly measure than voltages. If we consider a stationary positive charge Q , a positive test charge q 0 placed near Q will feel a force -→ F 0 repelling it away from Q . (We learned in last week’s lab that like charges repel each other.) The electric field -→ E is defined as the force per unit charge: -→ E = -→ F 0 / q 0 . From Mechanics, work is defined as W = -→ F · Δ -→ d , where Δ -→ d is the displacement of the charge. Above, we defined the voltage as the potential energy per unit charge. Work=potential energy, so we can mathematically define V = W / q 0 . Putting together the equations for voltage, electric field, and work, we can arrive at an equation relating the voltage to the electric field: Δ V = - -→ E · Δ -→ d (1)
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