1. Current And Resistance
Chapter 17

2. Electric Current Current exists whenever charges of like signs move
17.1

3. Current is the rate of flow of charge

4. The unit for current is the Ampere (A).
1 Ampere = 1 Coulomb/Second

5. Conventional (positive) current flow is used.
Charge carriers may be positive or negative 75

6. Drift Speed
The speed of the charge carriers is an average speed called the drift speed (nd).
n is the number of charge carriers per unit volume
17.2

7. Current Produces Heat
Internal collisions between charge carriers produce heat.
17.4A

8. Current Flow Analogies
Water flow in a pipe
Marbles in a long pipe
Links in a bicycle chain
20-21

9. Resistance Resistance (R) is opposition to the flow of current
Measured in units called ohms
Symbol (W)
Occurs because of collisions between electrons

10. Ohm’s Law

11. Current And Voltage Measurements
Multimeters can measure voltage,current and resistance in an electrical circuit.

12. It is very important
to choose the correct
range and to connect
the multimeter leads
properly.

13. Ohmic and Non-Ohmic Resistance
Ohm’s Law
Ohmic resistance is constant over a wide range of voltages.
Non ohmic resistance varies with voltage
Semiconductor diodes
One way valves for electricity
17.5, 226, 175

14. Ohmic Resistance

15. Non-Ohmic Resistance

16. Resistors
A resistor is a device which has a specific amount of resistance
Circuit diagram symbol

17. Examples of Resistors

18. Resistor Color Code

19. Resistivity
Resistivity (r) is a constant of proportionality which affects the resistance of a specific material.
Units: (W.m)
Values are in Table 17.1 (Pg. 577)

20. Resistance and Resistivity

21. Applications Involving Resistance
The carbon microphone

22. The carbon microphone Sound waves cause a change in resistance.
Frequency response is very poor. (20 Hz to 4,000Hz)
The normal human hearing range is between 20 Hz and 20,000 Hz.

23. Temperature Variation Of Resistance
Usually, the resistivity of a metal increases with an increase in temperature.

24. Temperature Variation Of Resistance
Temperature coefficient of resistivity (a)
Values are in Table 17.1

25. Superconductors
The resistance of some metals and compounds falls to virtually zero below the critical temperature. (Tc)
Values are in Table 17.2 (Pg. 584)

26. Currents in a superconductor, could flow forever without an applied voltage.

27. The Meissner Effect

28. The goal is to produce high temperature superconductors
Mercury-oxide-based
The highest temperature so far is 150 K (-123 oC)

29. What is the ultimate goal?
Superconductivity at room temperature!

30. Applications of Superconductors
Microwave devices
Superconducting power lines
Mag-Lev trains
Motors

31. Cells and Batteries
A dry cell transforms chemical energy into electrical energy
Some of this energy becomes thermal energy
178, 179

33. Resistive Circuits A simple resistive circuit
Assume no loss of energy in the wires.
Joule heating occurs in the resistor.
17.11

34. Electrical Power
Electrical power is measured in Watts (W)

35. Electrical Energy
Electrical energy is measured in kilowatt hours (kWh)
Consumers Energy bill?
1 kWh = 3.60 x 106 J

36. A Kilowatt-Hour Meter

37. Electrical Activity In The Heart
Muscle action is initiated by electrical activity
Voltage pulses cause the heart to beat
These electrical waves travel through the body
Monitored by electrocardiographs
EKG (electrocardiogram)
177

38. If the atrial and ventricular contractions become irregular, we have fibrillation.
Defibrillators can be used to restore the normal heart rhythm
17.13

39. A Bad EKG

40. Cardiac Pacemakers
Cardiac pacemakers may be implanted under the skin and attached with a wire to the right ventricle.
A lithium battery is used to charge a capacitor. It discharges 60 times per minute to stimulate the heart beat. If the heart is beating normally, the capacitor is not allowed to charge.

41. Pacemaker Location

42. An Actual Pacemaker

43. Cardioverter Defibrillators
Implanted Cardioverter Defibrillators (ICDs) can monitor, record, process information, and send corrective signals to the heart.
The lithium batteries last 4 – 5 years.
Table 17.3 (Defibrillator Properties)

44. Electrical Activity In The Brain
The brain produces electrical waves
Monitored by electroencephalographs
EEG

45. An EEG

46. Questions
1, 3 - 5, 7, 8,
Pg. 587