2. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Voltage and Current, continued Potential difference is measured in volts. The potential difference between any two points, is the work that must be done against electric forces to move a unit charge from one point to the other. The volt, V, is equivalent to one joule per coulomb (1 J/C). Potential difference is often called voltage. Section 2 Current Chapter 16

3. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Potential Difference Section 2 Current Chapter 16

4. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Voltage Section 2 Current Chapter 16

5. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Current Voltage and Current, continued There is a voltage across the terminals of a battery. A cell is a device that is a source of electric current because of a potential difference, or voltage, between the terminals. One terminal is positive, and the other is negative. Batteries convert chemical energy into electrical energy. Chapter 16

6. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Battery Section 2 Current Chapter 16

7. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Electric Cell Section 2 Current Chapter 16

8. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Current Voltage and Current, continued A voltage sets charges in motion. Current is the rate that electric charges move through a conductor. The SI unit of current is the ampere, A. 1 amp = 1 C/s A battery is a direct current source because the charges always move from one terminal to the other in the same direction. Conventional current is defined as movement of positive charge. The direction of current in a wire is opposite the direction that electrons move in that wire. Chapter 16

9. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Comparing Direct and Alternating Current Section 2 Current Chapter 16

10. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Conventional Current Section 2 Current Chapter 16

11. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Current Electrical Resistance Resistance is the opposition posed by a material or a device to the flow of current. Resistance is caused by internal friction, which slows the movement of charges through a conducting material. Resistance can be calculated from current and voltage. Chapter 16 The SI unit of resistance is the ohm, Ω. 1 Ω = 1 V/A A resistor is a special type of conductor used to control current.

12. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Current Math Skills Resistance The headlights of a typical car are powered by a 12 V battery. What is the resistance of the headlights if they draw 3.0 A of current when turned on? 1.List the given and unknown values. Given: current, I = 3.0 A voltage, V = 12 V Unknown: resistance, R = ? Ω Chapter 16

13. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Current Math Skills, continued 2.Write the equation for resistance. R = 4.0  Chapter 16 3.Insert the known values into the equation, and solve.

14. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Current Electrical Resistance, continued Conductors have low resistances. Some materials become superconductors below a certain temperature. Certain metals and compounds have zero resistance when their temperature falls below a certain temperature called the critical temperature. Semiconductors are intermediate to conductors and insulators. The controlled addition of specific atoms of other materials as impurities dramatically increases a semiconductor’s ability to conduct electric charge. Chapter 16