Current Conservation of current Batteries Resistance and resistivity Simple circuits

1. Chapter 22 Current and Resistance • Current • Conservation of current • Batteries • Resistance and resistivity • Simple circuits Topics: Sample question: How can the measurement of an electric current passed through a person’s body allow a determination of the percentage body fat? Slide 22-1

2. Reading Quiz The charge carriers in metals are A. electrons B. positrons C. protons D. a mix of protons and electrons Slide 22-2

3. Reading Quiz 2. A battery is connected to a resistor. Increasing the resistance of the resistor will A. increase the current in the circuit. B. decrease the current in the circuit. C. not affect the current in the circuit. Slide 22-3

4. Reading Quiz 3. A battery is connected to a resistor. As charge flows, the chemical energy of the battery is dissipated as A. current B. voltage C. charge D. thermal energy Slide 22-4

5. Properties of a Current Slide 22-5

6. Definition of a Current Slide 22-6

7. Conservation of Current Slide 22-7

8. Rank the bulbs in the following circuit according to their brightness, from brightest to dimmest. The wires below carry currents as noted. Rate the currents IA, IB and IC: Slide 22-8

9. Batteries The potential difference between the terminals of a battery, often called the terminal voltage, is the battery’s emf. Wchem q ____ ∆Vbat = = Slide 22-9

10. Simple Circuits The current is determined by the potential difference and the resistance of the wire: ∆Vchem R _____ I = Slide 22-10

11. Resistivity The resistance of a wire depends on its dimensions and theresistivityof its material: Slide 22-11

12. Checking Understanding • A battery is connected to a wire, and makes a current in the wire. • Which of the following changes would increase the current? • Which would decrease the current? • Which would cause no change? • Increasing the length of the wire • Keeping the wire the same length, but making it thicker • Using a battery with a higher rated voltage • Making the wire into a coil, but keeping its dimensions the same • Increasing the temperature of the wire Slide 22-12

13. Measuring Light Intensity A battery is connected to a photoresistor. When light shines on this resistor, it increases the number of charge carriers that are free to move in the resistor. Now, the room lights are turned off, so less light falls on the photoresistor. How does this affect the current in the circuit? A. The current increases. B. The current decreases. C. The current is not affected. Slide 22-13

14. Measuring Body Fat • The resistivity of the body is a good measure of its overall composition. A measure of the resistance of the upper arm is a good way to estimate the percent fat in a person’s body. Let’s model a person’s upper arm as a cylinder of diameter 8.0 cm and length 20 cm. We can model the composition of the arm by assuming that the muscle, far, and nonconductive portions (the bone) form simple regions. This simple model actually works quite well. For a typical adult, the bone has a cross-sectional area of 1.0 cm2; to a good approximation, the balance of the arm is fatty tissue or muscle. • Assume that the arm has 30% fat and 70% muscle. What is the resistance of the arm? • Now, assume that the arm has only 10% muscle. What is the resistance now? • The measurement of the resistance of the arm is made by applying a voltage and measuring a current. Too much current can be uncomfortable and, as we will see, can be dangerous. Suppose we wish to limit the current to 1.0 mA. For each of the above cases, what is the maximum voltage that could be employed? Slide 22-14

15. Power in Circuits Slide 22-15

16. Checking Understanding • A resistor is connected to a 3.0 V battery; the power dissipated in the resistor is 1.0 W. The battery is now traded for a 6.0 V battery. The power dissipated by the resistor is now • 1.0 W • 2.0 W • 3.0 W • 4.0 W Slide 22-16

17. Electric Blankets • An electric blanket has a wire that runs through the interior. A current causes energy to be dissipated in the wire, warming the blanket. A new, low-voltage electric blanket is rated to be used at 18 V. It dissipates a power of 82 W. What is the resistance of the wire that runs through the blanket? • For the electric blanket of the above example, as the temperature of the wire increases, what happens to the resistance of the wire? How does this affect the current in the wire? The dissipated power? Slide 22-17

18. Additional Clicker Questions A set of lightbulbs have different rated voltage and power, as in the table below. Which one has the highest resistance? Bulb Rated voltage Rated power A 10 V 1 W B 8 V 1 W C 12 V 2 W D 6 V 2 W E 3 V 3 W Slide 22-18

19. Additional Clicker Questions 2. A set of lightbulbs have different rated voltage and power, as in the table below. Which one has lowest resistance? Bulb Rated voltage Rated power A 10 V 1 W B 8 V 1 W C 12 V 2 W D 6 V 2 W E 3 V 3 W Slide 22-19

20. Additional Clicker Questions 3. A battery makes a circuit with a lightbulb as shown. Two compasses are near the wires before and after the bulb in the circuit. Which compass experiences a larger deflection? • Compass 1 experiences a larger deflection. • Compass 2 experiences a larger deflection. C. Both compasses experience the same deflection. Slide 22-20

21. Additional Clicker Questions 4. In Trial 1, a battery is connected to a single lightbulb and the brightness noted. Now, in Trial 2, a second, identical, lightbulb is added. How does the brightness of these two bulbs compare to the brightness of the single bulb in Trial 1? • The brightness is greater. • The brightness is the same. • The brightness is less. Slide 22-21

22. Additional Examples • How much time does it take for 1.0 C to flow through each of the following circuit elements? • A 60 W reading light connected to 120 V. • A 60 W automobile headlamp connected to 12 V. Slide 22-22