Chapter 17

1. Chapter 17 Current and Resistance

2. Example 17.5 – Page 582 • A circuit provides a maximum current of 20A at an operating voltage of 120V. • A) How many 75W bulbs can operate with this voltage source? • B) At $0.12 per kilowatt-hour, how much does it cost to operate these bulbs for 8h?

3. Example 17.6 – Page 583 • An electric heater is operated by applying a potential difference of 50V to a nichrome wire of total resistance 8Ω. • A) find the current carried by the wire and the power rating of the heater. • B) using this heater, how long would it take to heat 2500 moles of diatomic gas (ex. mixture of O2 and N2-air) from a chilly 100C to 250C? Take the molar specific heat at constant volume of air to be 5R/2.

4. Superconductors • A class of materials and compounds whose resistances fall to virtually zero below a certain temperature, TC • TC is called the critical temperature • The graph is the same as a normal metal above TC, but suddenly drops to zero at TC

5. Superconductors, cont • The value of TC is sensitive to • Chemical composition • Pressure • Crystalline structure • Once a current is set up in a superconductor, it persists without any applied voltage • Since R = 0

6. Superconductor Timeline • 1911 • Superconductivity discovered by H. Kamerlingh Onnes • 1986 • High temperature superconductivity discovered by Bednorz and Müller • Superconductivity near 30 K (~ -406F) • 1987 • Superconductivity at 96 K (~ - 287F) and 105 K (~ - 271F) • Current • More materials and more applications

7. 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

8. Operation of the Heart • The sinoatrial (SA) node initiates the heartbeat • The electrical impulses cause the right and left atria 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

9. 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

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

11. 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

12. 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

13. 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

14. 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

15. 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

16. Chapter 18 Direct Current Circuits

17. Simple Direct-Current Circuits • Use: • 2 rules known as Kirchhoff’s rules (from conservation of energy principle) • Law of conservation of charge Circuits are assumed to be in steady-state, which means that the currents are constant in magnitude and direction.

18. 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

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

20. More About Internal Resistance • The schematic shows the internal resistance, r • The terminal voltage is ΔV = Vb-Va • ΔV = ε – Ir • ‘ε’ is the emf of the source (battery), equal to terminal voltage when current is zero, or open-circuit voltage

21. Internal Resistance and emf, cont • For the entire circuit, ε = IR + Ir • R is called the load resistance • The current depends on both the resistance external to the battery and the internal resistance

22. 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

23. 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

24. 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

25. 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

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

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 + …