PHYS 272 Matter and Interactions II Electric And

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PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ PHYSICS 272 Electric & Magnetic Interactions Lecture 9 Electric Potentials [EMI 17. 1 -17. 3] Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ Potential Energy To understand the dynamics of moving objects consider: forces, momenta, work, energy Introduced the concept of electric field E to deal with forces Introduce electric potential – to consider work and energy Electric potential: electric potential energy per unit charge Practical importance: • Reason about energy without having to worry about the details of some particular distribution of charges • Batteries: provide fixed potential difference • Electric Potential is easily measurable in electronics (voltage) • Predict possible pattern of E field • Potential is a scalar (number), simpler than vector (eg. E-field) Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ Physics 172 Physics 272 Gravitational potential Electric potential Gravitational potential energy Electric potential is defined as electric potential energy per unit charge. Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ 0 (“closed” system; no external force) Because electric forces are conservative. (We neglect the small gravitational force. ) Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ Generally Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ 20 20 Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ If the proton reaches the left side It will not reach. Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ At the turning point Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ Electric Potential Electric potential electric potential energy per unit charge Units: J/C = V (Volt) Volts per meter = Newtons per Coulomb Alessandro Volta (1745 - 1827) Electric potential – often called potential Electric potential difference – often called voltage Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ Potential energy is ability to do work i f E Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ Sign of potential difference Path going in the direction of : Potential is decreasing (∆V < 0) Path going opposite to : Potential is increasing (∆V > 0) Path perpendicular to : Potential does not change (∆V = 0) Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ Example: Different Paths near Point Charge 2. Special case i. A: AB: BC: + Cf: Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ Example: Different Paths near Point Charge 3. Arbitrary path + Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ V at Infinity (often define as 0) r , V=0 Positive charge Negative charge Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ Clicker Question 1 A proton is free to move from right to left in the diagram shown. There are no other forces acting on the proton. As the proton moves from right to left, its potential energy: A) B) C) D) Is constant during the motion Decreases Increases Not enough information Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ Clicker Question 1 A proton is free to move from right to left in the diagram shown. There are no other forces acting on the proton. As the proton moves from right to left, its potential energy: A) B) C) D) Is constant during the motion Decreases Increases Not enough information B V 1 < V 2 Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ Clicker Question 2 A system consists of a proton inside of a capacitor. The proton moves from left to right as shown at a constant speed due to the action of an external agent. Which of the following statements are true? A) The proton’s potential energy is unchanged and the external agent does no work on the system. B) The proton’s potential energy decreases and the external agent does work W > 0 on the system. C) The proton’s potential energy decreases and the external agent does work W < 0 on the system. D) The proton’s potential energy increases and the external agent does work W < 0 on the system. E) The proton’s potential energy increases and the external agent does work W > 0 on the system. Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ Clicker Question 2 A system consists of a proton inside of a capacitor. The proton moves from left to right as shown at a constant speed due to the action of an external agent. Which of the following statements are true? A) The proton’s potential energy is unchanged and the external agent does no work on the system. B) The proton’s potential energy decreases and the external agent does work W > 0 on the system. C) The proton’s potential energy decreases and the external agent does work W < 0 on the system. D) The proton’s potential energy increases and the external agent does work W < 0 on the system. E) The proton’s potential energy increases and the external agent does work W > 0 on the system. V 1 < V 2 E. Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ For a conductor in equilibrium: The potential difference is zero between any two locations inside. Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ Two adjacent regions with different fields C In general, From A to C, ∆V 1 = -E 1 x(x. C - x. A) From C to B, ∆V 2 = -E 2 x(x. B - x. C) From A to B, ∆V = ∆V 1 + ∆V 2 = Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ Example: x Q > 0, ∆V < 0 If x. B → , VB → 0 Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ Recall Physics I: The potential energy differences depend only on the initial and final states of a system and are independent of path. Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics.

PHYS 272: Matter and Interactions II -- Electric And Magnetic Interactions http: //www. physics. purdue. edu/academic_programs/courses/phys 272/ Example: Different Paths near Point Charge 1. Along straight radial path: rf ri +q Fall 2010 Prof. Yong Chen (yongchen@purdue. edu) Prof. Michael Manfra (mmanfra@purdue. edu) Lecture 7