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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- Spring '14
- Physics, Electric Potential, EKG Sensor
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