Lecture 11—Ohm’s Law, Power

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Presentation transcript:

Lecture 11—Ohm’s Law, Power February 11

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

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

Power in Circuits Slide 22-15

A resistor is connected to a 3 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 W 2 W 3 W 4 W

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 A B C D E

Resistances RA = 100Ω RB= 64Ω RC = 72Ω RD= 18Ω RE= 3 Ω

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