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Current– Flow of Electric Charge Electric Current (Symbol I) Electrical Resistance (Symbol R)

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L 26 Electricity and Magnetism [3] Current– flow of electric charge If I connect a battery to the ends of the Electric circuits copper bar the electrons in the copper will what conducts electricity be pulled toward the positive side of the what doesn’t conduct electricity battery and will flow around and around. Current voltage and resistance Î this is called current –flow of charge Ohm’s Law Heat in a resistor – power loss copper Making simple circuit connections An electric circuit! Duracell + Potential difference or Voltage Electric current (symbol I) (symbol V) • Electric current is the flow of electric • Voltage is what causes charge to move in charge q (Coulombs) a conductor • It plays a role similar to pressure in a pipe; q to get water to flow there must be a pressure difference between the ends, this • It is the amount of charge q that passes a pressure difference is produced by a pump given point in a wire in a time t, I = q / t • A battery is like a pump for charge, it • Current is measured in amperes provides the energy for pushing the charges around a circuit • 1 ampere (A) = 1 C / 1 s Practical considerations: voltage and current are not the same thing Electrical resistance (symbol R) • You can have voltage, but without a path • Galileo told us that no force is required to keep (circuit) there is no current. something moving with constant velocity • So, why is it necessary to keep pushing the An charges to keep them moving in a wire? WHITE WIRE electrical • As they move through the wire, the electrons NEUTRAL outlet collide with the atoms, so there is a type of friction involved; in this case a force is required BLACK WIRE • This continuous obstruction to the motion of the HOT GREEN WIRE electrons is called electrical resistance Æ R GROUND 1 Electrons pass through an obstacle Direction of current flow course in a conductor atoms R resistor An electric circuit! Duracell + electron path The resistance (R) is a measure of the degree to The electrons go one way but the current which the conductor impedes the flow of current. goes the other way by convention Resistance is measured in Ohms (Ω) (this is due to Ben Franklin’s choice!) Current, Voltage and Resistance Examples OHM’S LAW • Ohm’s law is a simple • (1) If a 3 volt flashlight bulb has a relation between these Resistance R resistance of 9 ohms, how much current three important circuit will it draw parameters • I = V / R = 3 V / 9 Ω = 1/3 Amps • Ohm’s law: Current I • I = Voltage/ Resistance = V / R • (2) If a light bulb draws 2 A of current Battery voltage V • V in volts, R in ohms, I when connected to a 120 volt circuit, what in amps is the resistance of the light bulb? •V = I R other forms • R = V / I = 120 V / 2 A = 60 Ω •R = V / I of Ohm’s Law Can be a light bulb, or a cell phone or a radio Heat produced in a resistor Heat produced in a resistor •Power Î P = I ×V or I2 × R • As we have seen before, friction causes heat • The collisions between the electrons and the • Power is measured in Watts = amps × volts atoms in a conductor produce heat Æ wires get • All wire is rated for the maximum current that warm when they carry large currents Æ in an it can handle based on how hot it can get electric stove this heat is used to cook food • To carry more current you need wire of a • The amount of energy converted to heat per larger diameter Æ this is called the wire second is called the power loss in a resistor gauge, the lower the gauge the more current • If the resistor has a voltage V across it and carries a current I the power dissipated is given it can carry by Æ Power P = I × V or I2 × R • Using extension cords can be dangerous! 2 example extension cords and power strips • How much current is drawn by a 60 Watt • extension cords are rated for maximum light bulb connected to a 120 V power current Æ you must check that whatever is line? plugged into it will not draw more current • Solution: P = 60 W = I x V = I x 120 than the cord can handle safely. so I = 0.5 Amps (A) • power strips are also rated for maximum • What is the resistance of the bulb? current Æ since they have multiple inputs you must check that the total current • Solution: V = I RÆ 120 V = ½ A x R drawn by everything on it does not exceed so R = 240 Ω, or R = V/I the current rating Simple direct current (DC) electric circuits Exercise: given a battery, some wire and a light bulb, connect them so that the bulb is on. The battery polarity +/- does not matter, Either way the bulb Will be on. 1.5 V Electric circuits - key points What is DC? • a circuit must provide a closed path for the current to circulate around • With DC or direct current the current • when the electrons pass through the light bulb they loose always flows in the same direction some of their energy Æ the conductor (resistor) heats up • this is the type of current you get when • we refer to conductors as resistors because they impede (resist) the flow of current. you use a battery as the voltage source. • the battery is like a pump that re-energizes them each • the direction of the current depends on time they pass through it how you connect the battery • the current flows in the direction that is opposite to the direction that the electrons travel • the electricity that you get from the power • Ohm’s law is the relation between current, voltage nad company is not DC it is AC (alternating). resistance: V = I R 3 connecting batteries dueling batteries Æ do’s and don’ts Do not don’t connect a wire from the + side to the – side, do this this shorts out the battery and will make it get hot and will shorten its lifetime. + + Duracell Do not Duracell do this + Duracell The batteries are trying to push currents in opposite directions Æ they are working against each other. This does not work. Proper connections Batteries in parallel This connection still Duracell Connecting two 1.5 volt batteries gives 1.5 volts but + gives like this gives 3.0 volts. since there are 2 Duracell batteries it will provide + electrical current for a longer time Duracell Duracell + + 1.5 V D Cell Longer lasting power series and parallel combination 1.5 V + 1.5 V + Series connection [ –+ –+] gives 3.0 V 1.5 V + 1.5 V + Parallel connection [ –+ ] [ –+ ] provides 3.0 V This connection provides 3.0 volts and will provide power for a longer amount of time 4.
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