PHYS 1444 – Section 004Lecture #9Electric Current and ResistanceThe Electric BatteryHow does a battery work?How does a battery work?Electric CurrentExample 25 – 1Direction of the Electric CurrentOhm’s Law: Resistance and ResistorsOhm’s Law: ResistanceExample 25 – 3Ohm’s Law: ResistorsOhm’s Law: Resistor ValuesResistivityExample 25 – 4Example 25 – 5PHYS 1444 – Section 004Lecture #9Mon day, Feb. 19 2007Dr. Andrew BrandtMonday, Feb. 19, 2007 1PHYS 1444-004, Spring 2007Dr. Andrew Brandt• Electric Current and Resistance• The Battery• Ohm’s Law: Resistors•Resistivity• Electric PowerHW4 due Fri 2/23 at 8 pm;HW5 will be due Fri Mar. 2;Test Ch 21-25 Mon Mar. 5Electric Current and Resistance• So far we have been studying electrostatics– The charges have not been in motion• Now we will learn the dynamics of electricity• What is the electric current?– A flow of electric charge– Examples of things that use electric current?• In an electrostatic situation, there is no electric field inside a conductor but when there is current, there is a field inside a conductor– Electric field is needed to keep charges movingMonday, Feb. 19, 2007 2PHYS 1444-004, Spring 2007Dr. Andrew BrandtMonday, Feb. 19, 2007 3PHYS 1444-004, Spring 2007Dr. Andrew BrandtThe Electric Battery• What is a battery?– A device that produces electrical energy from the stored chemical energy and produces electricity.• Electric battery was invented by Volta in 1790s in Italy– It was made of disks of zinc and silver based on his research that certain combinations of materials produce a greater electromotive force (emf), or potential, than others• Simplest batteries contain two plates made of dissimilar metals, electrodes– Electrodes are immersed in a solution, electrolyte– This unit is called a cell and many of these form a battery• Zinc and Iron in the figure are called terminalsHow does a battery work?• One of the electrodes in the figure is zinc and the other carbon• The acid electrolyte reacts with the zinc electrode and dissolves it.• Each zinc atom leaves two electrons on the electrode and enters into the solution as a positive ion Î the zinc electrode acquires negative charge and the electrolyte becomes positively charged• The carbon electrode picks up the positive charge• Since the two terminals are oppositely charged, there is a potential difference between themMonday, Feb. 19, 2007 4PHYS 1444-004, Spring 2007Dr. Andrew BrandtMonday, Feb. 19, 2007 5PHYS 1444-004, Spring 2007Dr. Andrew BrandtHow does a battery work?• When the terminals are not connected, a certain amount of zinc is dissolved into the solution establishing an equilibriumcondition.• How is a particular equilibrium potential maintained?– If the terminals are not connected:• the zinc electrode becomes negatively charged up to the equilibrium pint• zinc ions then recombine with the electrons in the zinc electrode• Why does battery go dead? – When the terminals are connected, the negative charges will flowaway from the zinc electrode– More zinc atoms dissolve into the electrolyte to produce more charge– One or more electrodes get used up stopping the flow of chargeMonday, Feb. 19, 2007 6PHYS 1444-004, Spring 2007Dr. Andrew BrandtElectric Current• When a circuit is powered by a battery (or a source of emf), charge can flow through the circuit.• Electric Current: Any flow of charge– Current can flow whenever there is potential difference between the ends of a conductor (or when the two ends have opposite charges)– Electric current in a wire can be defined as the net amount of charge that passes through the wire’s full cross section at any point per unit time (just like the flow of water through a pipe)– Average current is defined as:– The instantaneous current is:– What kind of a quantity is the current?IQt=∆∆IdQ dt=Unit of current?C/s1A=1C/sIn a single circuit, conservation of electric charge guarantees that the current at one point of the circuit is the same as any other point on the circuit.ScalarExample 25 – 1 Current is the flow of charge: A steady current of 2.5A flows in a wire for 4.0min. (a) How much charge passed by any point in the circuit? (b) How many electrons would this be? Current is total amount charge flow through a circuit in a given time. So from we obtain QIt∆=∆QIt∆=∆=2.5 4.0 60 600C××=The total number of electrons passed through the circuit is Qe∆=21196003.8 101.6 10CelectronsC−=××eN =Monday, Feb. 19, 2007 7PHYS 1444-004, Spring 2007Dr. Andrew BrandtDirection of the Electric Current• What do conductors have in abundance?– Free electrons• What happens if a continuous loop of conducting wire is connected to the terminals of a battery?– Electrons start flowing continuously through the wire as soon asboth the terminals are connected to the wire. How?• The potential difference between the battery terminals sets up an electric field inside the wire and in the direction parallel to it• Free electrons in the conducting wire get attracted to the positive terminal• The electrons leaving negative terminal flow through the wire and arrive at the positive terminal– Electrons flow from negative to positive terminal– Due to historical convention, the direction of the current is opposite to the direction of flow of electrons Î Conventional CurrentMonday, Feb. 19, 2007 8PHYS 1444-004, Spring 2007Dr. Andrew BrandtOhm’s Law: Resistance and Resistors• What do we need to produce electric current?– Potential difference• Georg S. Ohm experimentally established that the current is proportional to the potential difference ( )– If we connect a wire to a 12V battery, the current flowing through the wire is twice that of 6V, three times that of 4V and four times that of 3V battery.– What happens if we reverse the sign of the voltage?• It changes the direction of the current flow• Does not change the magnitude of the current– Just as in water flow case, if the height difference is large the flow rate is large Î If the potential difference is large, the current is large.IV∝Monday, Feb. 19, 2007 9PHYS 1444-004, Spring 2007Dr. Andrew BrandtMonday, Feb. 19, 2007 10PHYS 1444-004, Spring 2007Dr. Andrew BrandtOhm’s Law: Resistance• The exact amount of current flow in a wire depends on– The voltage– The resistance of the wire to the flow of
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