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Lecture 8

Lecture 8. Electrical energy EMF Resistors in series and parallel Kirchoff’s laws. Electrical Activity in the Heart. Every action involving the body’s muscles is initiated by electrical activity Voltage pulses cause the heart to beat

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Lecture 8

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  1. Lecture 8 • Electrical energy • EMF • Resistors in series and parallel • Kirchoff’s laws

  2. 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

  3. 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

  4. Fig. 17-13, p.583

  5. Fig. 17-14b, p.584

  6. 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

  7. Abnormal EKG, 1 • The QRS portion is wider than normal • This indicates the possibility of an enlarged heart

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

  14. emf and Internal Resistance • A real battery has some internal resistance • Therefore, the terminal voltage is not equal to the emf

  15. 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

  16. 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

  17. Internal Resistance and emf, final • When R >> r, r can be ignored • Generally assumed in problems • Power relationship • I e = I2 R + I2 r • When R >> r, most of the power delivered by the battery is transferred to the load resistor

  18. 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

  19. 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

  20. 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

  21. Equivalent Resistance – Series: An Example • Four resistors are replaced with their equivalent resistance Demo1Demo2

  22. Fig. 18-3, p.594

  23. 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

  24. 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

  25. 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

  26. Fig. 18-9, p.599

  27. 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 + …

  28. 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:

  29. 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

  30. 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

  31. Equivalent Resistance – Complex Circuit

  32. Gustav Kirchhoff • 1824 – 1887 • Invented spectroscopy with Robert Bunsen • Formulated rules about radiation

  33. 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

  34. 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

  35. More About the Junction Rule • I1 = I2 + I3 • From Conservation of Charge • Diagram b shows a mechanical analog

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