Chapter 16 Electric Energy and Capacitance 16 1 Potential difference and electric potential Potential to do a lot of damage Potential energy He is full of potential at this moment Has to do with gravity a force Has to do with height or distance in the direction of the force Recall Work from PHY101 Potential energy is always related to a force In the presence of a force gravity or electrical force potential energy increases when an object is moved in the opposite direction of the force A B A A B EPEA EPEB B EPEA You don t always have to think of potential energy as something related to up and down EPEB An object by itself moves from a higher potential energy position to a lower energy position Use this to determine which point has the higher potential energy Someone has to do some work to move an object to a higher potential energy position Quantitatively speaking A A B PE PEfinal PEinitial EB EA F x F parallel to x A B B EPE EPEB EPEA EPE EPEB EPEA F x Eq x F x Eq x F parallel to x mgh The electric potential difference F antiparallel to x q A Duracell B How much is the potential energy Depends on how heavy he is AA What is the electrical potential energy of q We don t really know q or a device with flow of q and it depends on q The electric potential difference How high is the person Convention The place near the plate with positive charges has the higher potential A x B EPE Eq x EPE Vq For a positive charge the higher the potential the higher the potential energy like a mass with respect to height For a negative charge the higher the potential the more negative or lower the potential energy like a negative mass with respective to height Another way of saying it A positive charge always wants go to a lower potential A negative charge always wants to go to a higher potential Just like a negative mass will fly up rather than coming down any time you see a negative charge imagine what would happen to a negative mass 1 2 3 Example A charge of 2C has potential energies 2J and 5J when it is located at A and B 1 m apart respectively What is the potential difference VA VB Which plate has positive charges How much energy does it take to move a 5C charge from B to A What is the electrical field in the area between the two plates A Energy is energy regardless what the source is chemical electric nuclear Energy conservation B A unit for energy 1 eV energy needed to move an electron over 1 V 1 6 x 10 19 C x 1 V 1 6 x 10 19 J Applications Batteries medical devices Everything electric or electronic living things etc Pacemaker etc Example an old fashioned TV has 25 kV to accelerate the electrons from the back to your screen Assume that electrons start from rest what is their speed when they hit the screen e 1 6 x 10 19C me 9 1 x 10 31 kg Start with force 16 2 The Electric Potential Difference Created by a Point Charge For HW energy needed to bring charges together from infinity Force field EPE V Field vector potential scalar environment associated with a charge or charges Describe the surrounding of a charge starts with ONE charge For one charge E kq r2 V kq r Force vector EPE scalar interaction between two charges Describe the interaction between two charges at least ONE PAIR of charges For one pair of charges F kq1q2 r2 EPE kq1q2 r Our own potential energy is from the interaction between us and the Earth Pay attention to the directions and magnitudes for vectors but signs for scalars Considering things are all invisible to our eyes we have to think through the process you are not alone 16 3 Potentials and charged conductors How much work would it take you to assemble 3 charges from infinity q 1 C q 2 C q 3 C 1 2 3 5m q1 q3 5m 5m q2 The whole surface of a conductor has the same potential Potential remains the same when extension cords conductors are used 16 6 7 Capacitance parallel plate capacitor 16 8 Combinations of capacitors Circuits the stuff electronic gadgets are made of First thing wires q CV C capacitance Unit Farad F Dielectrics are not covered Things in series V1 V2 Vbattery Some electrical thing light bulb capacitors TVs Capacitors in Parallel Both ends connected together by wire Same voltage V1 V2 Veq Share Charge Qeq Q1 Q2 Add Areas Ceq C1 C2 Things in parallel V1 V2 Vbattery 15 V 15 V C1 C2 10 V 10 V 15 V Ceq 10 V Capacitors in Series Connected end to end with NO other exits Same Charge Q1 Q2 Qeq Share Voltage V1 V2 Veq Q Q Q Q Vtotal C1 V1 Q C1 Example V2 Q Q C2 Ceq C2 1 Q Q Q 1 Q Ceq C1 C2 C1 C2 Example What is the equivalent C Q 1 1 1 Ceq C1 C2