Devil physics The baddest class on campus IB Physics

1. Devil physicsThe baddest class on campusIB Physics

2. Tsokos Lesson 5-4:Electric current and electric resistance

3. Reading Activity Questions?

4. Objectives • By the end of this class you should be able to: • State the definition of electric current, • State the definition of electric resistance, • Appreciate that metallic conductors at constant temperature satisfy Ohm’s Law,

5. Objectives • By the end of this class you should be able to: • Appreciate that the potential drops as one moves across a resistor in the direction of the current • Understand that a resistor dissipates power,

6. Assessment Statements • IB Assessment Statements for Topic 5.1, Electric Current and Resistance. • Define electric current. • Define resistance. • Apply the equation for resistance in the form, R = ρL/A, where ρ is the resistivity of the material of the resistor. • State Ohm’s law.

7. Assessment Statements • IB Assessment Statements for Topic 5.1, Electric Current and Resistance. • Compare Ohmic and non-Ohmic behavior. • Derive and apply expressions for electrical power dissipation in resistors. • Solve problems involving potential difference, current and resistance.

8. Video: Electric Current

9. Electric Current • Electric current is the amount of charge that moves through the cross-sectional area of a wire per unit time • The unit for current is the ampere (A) and is equal to 1C/s

10. Electric Current • EXAMPLE: Light falling on a metallic surface causes the surface to emit 2.2x1015 electrons per second. What is the current leaving the surface?

11. Electric Current • EXAMPLE: Light falling on a metallic surface causes the surface to emit 2.2x1015 electrons per second. What is the current leaving the surface? • ANSWER:

12. Electric Current • In an uncharged conductor, the electrons move randomly at speeds on the order of 105 m/s • The presence of an electric field in a conductor causes the electrons to accelerate in a direction opposite to the electric field. • This ordering of the electron motion is what causes current.

13. Electric Current • As the electrons move, they collide with atoms of the material and impart some of their energy to those atoms • This causes the atoms to increase the amplitude of their vibrations about their equilibrium position • These increased vibrations show up as heat • This is how we get toast

14. Electric Current • After the collision, the electrons are again accelerated by the electric field • The graph below represents this pattern. The dotted line represents the average, or drift velocity of the electron

15. Electric Current • For a typical metal, the drift velocity is on the order of 6x10-4 m/s • With this velocity, how long should it take for the lights to come on when you flip the switch?

16. Electric Current • For a typical metal, the drift velocity is on the order of 6x10-4 m/s • With this velocity, how long should it take for the lights to come on when you flip the switch? • So why do the lights come on instantaneously?

17. Electric Current • When an electric field is applied, every free electron in the conductor is energized – like the difference between opening a valve at the end of a pipe that is full of water versus opening a valve at the beginning of a pipe that is empty.

18. Electric Current • By convention, the direction of current is the opposite direction of the flow of electrons • Current flows from positive to negative • Electrons move from negative to positive

19. Electric Current • Special Cases: • When a conductor is heated, it emits electrons through a process called thermionic emission which creates a current, or increases conductivity • When light hits a metallic surface, electrons are emitted which creates a current – photoelectric effect

20. Electric Resistance and Ohm’s Law • Electric resistance of a conductor is defined as the potential difference across its ends, divided by the current flowing through it: • The unit for resistance is the Ohm (𝛀) and is equal to 1 V/A

21. Electric Resistance and Ohm’s Law • When the temperature of a conductor is kept constant, current is proportional to the potential difference across it • Ohm’s Law • This implies resistance is constant

22. Electric Resistance and Ohm’s Law • A conductor with zero electric resistance is known as a perfect conductor • Current can flow without a potential difference • Superconductors can achieve zero resistance at very low temperatures (critical temperature) and are thus perfect conductors

23. Electric Resistance and Ohm’s Law • For metals, increase in temperature results in increased resistance • Assuming the conductor is kept at constant temperature, three factors affect resistance: • properties of the material • length • cross-sectional area.

24. Electric Resistance and Ohm’s Law • Electric resistance of a wire (at constant temperature) is proportional to its length (L) and inversely proportional to the cross-sectional area • Resistance increases with length • Resistance decreases with cross-sectional area

25. Electric Resistance and Ohm’s Law • Electric resistance of a wire (at constant temperature) is proportional to its length (L) and inversely proportional to the cross-sectional area

26. Potential Drop • This derivation tells us that for current to flow through a resistor, there must be a potential difference (V) across the resistor. • In a circuit, the voltage is said to “drop” across the resistor • If we assume the resistance of the conductor is negligible, the voltage and potential difference will be the same

27. Potential Drop • Example: 3A 10Ω A B 30Ω

28. Electric Power • We have seen that it requires work to move a charge across a potential • A current is a movement of charge so work is being done, and it is movement per unit time • Where there is work per unit time, there is POWER

29. Electric Power • This power is translated into mechanical work or thermal energy • We can re-write the formula for power for devices that obey Ohm’s law (ohmic behavior)

30. Electric Power • This power is translated into mechanical work or thermal energy • We can re-write the formula for power for devices that obey Ohm’s law (ohmic behavior)

31. Electric Power • Electrical devices are normally rated in watts (power) and volts (potential) • A light bulb rated as 60W at 220V (normal household voltage) means it will dissipate 60 watts of energy when a potential of 220 V is applied across it

32. Objectives • By the end of this class you should be able to: • State the definition of electric current, • State the definition of electric resistance, • Appreciate that metallic conductors at constant temperature satisfy Ohm’s Law,

33. Objectives • By the end of this class you should be able to: • Appreciate that the potential drops as one moves across a resistor in the direction of the current • Understand that a resistor dissipates power,

34. Assessment Statements • IB Assessment Statements for Topic 5.1, Electric Current and Resistance. • Define electric current. • Define resistance. • Apply the equation for resistance in the form, R = ρL/A, where ρ is the resistivity of the material of the resistor. • State Ohm’s law.

35. Assessment Statements • IB Assessment Statements for Topic 5.1, Electric Current and Resistance. • Compare Ohmic and non-ohmic behavior. • Derive and apply expressions for electrical power dissipation in resistors. • Solve problems involving potential difference, current and resistance.

36. Questions?

37. Homework #1-20