# DEVIL PHYSICS THE BADDEST CLASS ON CAMPUS IB PHYSICS

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1 DEL PHYSCS THE BADDEST CLASS ON CAMPUS B PHYSCS

2 TSOKOS LESSON 5-4: ELECTRC CURRENT AND ELECTRC RESSTANCE

4 Objectives By the end of this class you should be able to: Q State the definition of electric current, t State the definition of electric resistance, R 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, P

6 Assessment Statements B 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 B 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 ideo: 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 Q t 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 2.2x10 15 electrons per second. What is the current leaving the surface? Q t

11 Electric Current EXAMPLE: Light falling on a metallic surface causes the surface to emit 2.2x10 15 electrons per second. What is the current leaving the surface? ANSWER: Q t 2.2x10 15 electrons sec ond x1.6 x10 19 Coulombs electron 3.5x10 4 C s A

12 Electric Current n an uncharged conductor, the electrons move randomly at speeds on the order of 10 5 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. Q t

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 Q t

14 Electric Current After the collision, the electrons are again accelerated by the electric field Q t 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? Q t

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? 1 7m s hrs 6x10 4 So why do the lights come on instantaneously? Q t 11,667 3,15min

17 Electric Current Q 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. t

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

19 Electric Current Special Cases: Q 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 t

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: R The unit for resistance is the Ohm (Ω) and is equal to 1 /A

21 Electric Resistance and Ohm s Law R 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 R 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. R

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

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

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

27 Potential Drop R Example: 3A 10Ω R 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 Work P Time W Q P Q t P t Q

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) P R R P R R P R 2 2

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) P R R P R R P R 2 2

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

32 Objectives By the end of this class you should be able to: Q State the definition of electric current, t State the definition of electric resistance, R 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, P

34 Assessment Statements B 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 B 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 QUESTONS?

37 Homework #1-20

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