Electric Potential Energy II-Classical Physics-Handouts, Lecture notes of Classical Physics

Download Electric Potential Energy II-Classical Physics-Handouts and more Lecture notes Classical Physics in PDF only on Docsity! PHYSICS –PHY101 VU © Copyright Virtual University of Pakistan 73 V I I R Summary of Lecture 26 – ELECTRIC POTENTIAL ENERGY 1. Electric current is the flow of electrical charge. If a small amount of charge flows in time , then the current is . If the current is constant in time, then in time , the curren dq dqdq i t dt = t that flows is . The unit of charge is ampere, which is define as: 1 coulomb 1 ampere second A car's battery supplies upto 50 amperes w q i t= × = 3 hen starting the car, but often we need to deal with smaller values: 1 milliampere 1 ma 10 1 mi A−= = 6 9 12 croampere 1 10 1 nanoampere 1 n 10 1 picoampere 1 p 10 2. The direction of current A A A A A A μ − − − = = = = = = flow is the direction in which positive charges move. However, in a typical wire, the positive charges are fixed to the atoms and it is really the negative charges (electrons) that move. In that case the direction of current flow is reversed. 3. Current flows because something forces it around a circuit. That "something" is EMF, electromotive force. But remember that we are using bad terminology and that EMF is not a force - it is actually the difference in electric potentials between two parts of a circuit. So, in the figure below, is the EMF which causes current to flow in the resistor. How much current? Gener a bV V V= − ally, the larger is , the more current will flow and we expect . In general this relation will not be completely accurate but when it holds, we say Ohm's Law applies: . Here, i V I V V VI R R I ∝ = = s called the resistance. + − conventional current flow electron flow device docsity.com PHYSICS –PHY101 VU © Copyright Virtual University of Pakistan 74 1R 2R 1V 2V a a b b1R 2R eqR 1i 2i 3i 4. Be careful in understanding Ohm's Law. In general the current may depend upon the applied voltage in a complicated way. Another way of saying this is that the resistance may depend upon the current. Example: when current passes through a resistor, it gets hot and its resistance increases. Only when the graph of current versus voltage is a straight line does Ohm's Law hold. Else, we can only define the "incremental resistance". 1 2 3 5. Charge is always conserved, and therefore current is conserved as well. This means that when a current splits into two currents the sum remains constant, . i i i= + 1 1 2 2 1 2 1 2 1 2 6. When resistors are put in series with each other, the same current flows through both. So, and . The total potential drop across the pair is ( ). . So resieq V iR V iR V V V i R R R R R = = = + = + ⇒ = + stors in series add up. 1 2 1 2 1 2 1 2 1 2 7. Resistors can also be put in parallel. This means that the same voltage is across both. So the currents are , . Since it follows that 1 1 1 eq eq V VV i i R R V V Vi i i i R R R R R R = = = + = = + ⇒ = + 1 2 1 2 or . This makes sense: with two possible paths the current will find less resistance than if only one was present. 8. When current flows in a circuit work is done. Suppose a sma eq R RR R R = + ll amount of charge is moved through a potential difference V. Then the work done is . Hence dq dW Vdq V idt= = ( )voltsV ( )AmpsI obeys Ohm's Law does not obey Ohm's Law iR V Δ = Δ docsity.com