1. Chapter 22 Current and Resistance

2. Current Conservation of current Batteries Resistance and resistivity Simple circuits Chapter 22 Current and Resistance 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

3. How can we control the flow of electric charges? Study the basic properties of current Understand how our investigations of static electricity relates to current.

4. Charged Parallel Plate Capacitor Connect the plates with a metal wire and the capacitor becomes discharged (neutral) The capacitor is discharged by a current in the connecting wire.

5. Properties of a Current Slide 22-8 Indicators of Current in the Wire Current is the motion of charges Charges flow not current Current is the flow

6. Charge Carriers Charge carriers are the charges that move in a current The charge carriers in metals are electrons.

7. Conduction Electrons in a Metal

8. Charge Carriers Insulator – does not have such free charges and cannot carry current Semiconductor – intermediate case – few charge carriers which can be either positive or negative ions Examples of ionic solutions as charge carriers – seawater, blood, intercellular fluids

9. Creating a Current We use an electric field to “push” on the electrons in in a conductor similarly to pushing a book on a table to give it kinetic energy When we apply an electric field the field exerts a force on the electrons and they begin to move.

10. Motion of electrons Kinetic energy of collisions in converted to thermal energy The metal gets warmer

11. Sustaining the current You must continue the push or the electrons will stop moving Maintain an electric field Drift Velocity - Electron’s average motion is opposite the field If the field goes to zero then the drift velocity goes to zero. A current is a motion of charges sustained by an internal electric field

12. Where does the electric field come from in a current carrying wire? A potential difference creates a current in the wire. Electric potential energy U elec is energy stored in a system of charged particles due to their electrical interactions. The energy transfer Δ U elec The potential is measured in volts

13. Creating Potential Electrons are bumped and pushed along The E field has some strength in N/C This is similar to the strength of a gravitational field in N/kg. (9.8N/kg) Think about gravitational potential energy being mgh and electrical potential energy being qEd

14. Creating Potential U elec = qEd energy stored in a particle U g = mgh energy stored by height E d = J/C =ΔV energy independent of charge gh =J/kg We care about the change in potential Protons go from high to low potential Electrons got from low to high potential

15. Creating Potential In reality the electrons are moving A battery adds energy Think about an energy elevator. A 1.5 V battery is like a charge escalator The battery reenergizes the particle The volt is the unit of potential Voltage is the difference in potential A particle uses all of its potential in a circuit

16. Simple Circuits The current is determined by the potential difference and the resistance of the wire: Slide 22-13 I = ∆ V chem R _____

17. Batteries The potential difference between the terminals of a battery, often called the terminal voltage, is the battery’s emf. Slide 22-12 ∆ V bat = =  W chem q ____

18. Difference in Potential creates current

19. Capacitor The potential difference across the capacitor depends on the capacitor’s charge. The potential difference (and the current) steadily falls as the capacitor discharges When the capacitor is fully discharged there is no longer a potential difference and the current ceases

20. Conservation of Current Add a lightbulb between two capacitors How does the current at A compare to current at B/ They are the same

21. Conservation of Current The number of electrons is not changed by the lightbulb The lightbulb cannot destroy electrons The lightbulb can’t store electrons It does use energy

22. Law of Conservation of Current The current is the same at all points in a current carrying wire

23. Definition of a Current Slide 22-9

24. Current Current is a rate The current direction in a wire is from higher potential to lower potential (in the direction of the electric field) Unit is ampere 1ampere = 1 coulomb per second = 1C/s

25. Conservation of Current Slide 22-10