Lecture 8
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Transcript of Lecture 8
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Lecture 8
Electrical energy EMF Resistors in series and parallel Kirchoff’s laws
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Electrical Activity in the Heart Every action involving the body’s
muscles is initiated by electrical activity
Voltage pulses cause the heart to beat
These voltage pulses are large enough to be detected by equipment attached to the skin
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Operation of the Heart
The sinoatrial (SA) node initiates the heartbeat
The electrical impulses cause the right and left atrial muscles to contract
When the impulse reaches the atrioventricular (AV) node, the muscles of the atria begin to relax
The ventricles relax and the cycle repeats
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Fig. 17-13, p.583
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Fig. 17-14b, p.584
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Electrocardiogram (EKG) A normal EKG P occurs just before
the atria begin to contract
The QRS pulse occurs in the ventricles just before they contract
The T pulse occurs when the cells in the ventricles begin to recover
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Abnormal EKG, 1
The QRS portion is wider than normal
This indicates the possibility of an enlarged heart
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Abnormal EKG, 2
There is no constant relationship between P and QRS pulse
This suggests a blockage in the electrical conduction path between the SA and the AV nodes
This leads to inefficient heart pumping
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Abnormal EKG, 3
No P pulse and an irregular spacing between the QRS pulses
Symptomatic of irregular atrial contraction, called fibrillation
The atrial and ventricular contraction are irregular
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Implanted Cardioverter Defibrillator (ICD)
Devices that can monitor, record and logically process heart signals
Then supply different corrective signals to hearts that are not beating correctly
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Functions of an ICD Monitor artrial and ventricular
chambers Differentiate between arrhythmias
Store heart signals for read out by a physician
Easily reprogrammed by an external magnet
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More Functions of an ICD Perform signal analysis and
comparison Supply repetitive pacing signals to
speed up or show down a malfunctioning heart
Adjust the number of pacing pulses per minute to match patient’s activity
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Sources of emf The source that maintains the current in
a closed circuit is called a source of emf Any devices that increase the potential
energy of charges circulating in circuits are sources of emf
Examples include batteries and generators SI units are Volts
The emf is the work done per unit charge
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emf and Internal Resistance A real battery has
some internal resistance
Therefore, the terminal voltage is not equal to the emf
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More About Internal Resistance
The schematic shows the internal resistance, r
The terminal voltage is ΔV = Vb-Va
ΔV = ε – Ir For the entire circuit,
ε = IR + Ir
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Internal Resistance and emf, cont ε is equal to the terminal voltage
when the current is zero Also called the open-circuit voltage
R is called the load resistance The current depends on both the
resistance external to the battery and the internal resistance
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Internal Resistance and emf, final When R >> r, r can be ignored
Generally assumed in problems Power relationship
I = I2 R + I2 r When R >> r, most of the power delivered by the battery is transferred to the load resistor
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Resistors in Series When two or more resistors are
connected end-to-end, they are said to be in series
The current is the same in all resistors because any charge that flows through one resistor flows through the other
The sum of the potential differences across the resistors is equal to the total potential difference across the combination
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Resistors in Series, cont Potentials add
ΔV = IR1 + IR2 = I (R1+R2)
Consequence of Conservation of Energy
The equivalent resistance has the effect on the circuit as the original combination of resistors
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Equivalent Resistance – Series Req = R1 + R2 + R3 + … The equivalent resistance of a
series combination of resistors is the algebraic sum of the individual resistances and is always greater than any of the individual resistors
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Equivalent Resistance – Series: An Example
Four resistors are replaced with their equivalent resistance Demo1 Demo2
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Fig. 18-3, p.594
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Resistors in Parallel The potential difference across each
resistor is the same because each is connected directly across the battery terminals
The current, I, that enters a point must be equal to the total current leaving that point I = I1 + I2 The currents are generally not the same Consequence of Conservation of Charge
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Equivalent Resistance – Parallel, Example
Equivalent resistance replaces the two original resistances
Household circuits are wired so the electrical devices are connected in parallel
Circuit breakers may be used in series with other circuit elements for safety purposes
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Equivalent Resistance – Parallel
Equivalent Resistance
The inverse of the equivalent resistance of two or more resistors connected in parallel is the algebraic sum of the inverses of the individual resistance
The equivalent is always less than the smallest resistor in the group
Demo3
321eq R
1
R
1
R
1
R
1
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Fig. 18-9, p.599
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Problem-Solving Strategy, 1 Combine all resistors in series
They carry the same current The potential differences across them
are not the same The resistors add directly to give the
equivalent resistance of the series combination: Req = R1 + R2 + …
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321eq R
1
R
1
R
1
R
1
Problem-Solving Strategy, 2
Combine all resistors in parallel The potential differences across them
are the same The currents through them are not
the same The equivalent resistance of a parallel
combination is found through reciprocal addition:
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Problem-Solving Strategy, 3 A complicated circuit consisting of several
resistors and batteries can often be reduced to a simple circuit with only one resistor Replace any resistors in series or in parallel
using steps 1 or 2. Sketch the new circuit after these changes have
been made Continue to replace any series or parallel
combinations Continue until one equivalent resistance is
found
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Problem-Solving Strategy, 4 If the current in or the potential
difference across a resistor in the complicated circuit is to be identified, start with the final circuit found in step 3 and gradually work back through the circuits Use ΔV = I R and the procedures in
steps 1 and 2
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Equivalent Resistance – Complex Circuit
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Gustav Kirchhoff 1824 – 1887 Invented
spectroscopy with Robert Bunsen
Formulated rules about radiation
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Kirchhoff’s Rules There are ways in which resistors
can be connected so that the circuits formed cannot be reduced to a single equivalent resistor
Two rules, called Kirchhoff’s Rules can be used instead
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Statement of Kirchhoff’s Rules Junction Rule
The sum of the currents entering any junction must equal the sum of the currents leaving that junction
A statement of Conservation of Charge
Loop Rule The sum of the potential differences across
all the elements around any closed circuit loop must be zero
A statement of Conservation of Energy
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More About the Junction Rule I1 = I2 + I3 From
Conservation of Charge
Diagram b shows a mechanical analog