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CU-Boulder PHYS 1010 - Lecture Notes

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11/1/2011 1 Lecture 21 : Electric circuits Reminders: HW 9 due Monday 7th at midnight Office hours next week: Tues 9.15am, 7pm (G1B31) Start making a 3rd formula card Email me with review topics for Tuesdays lecture Electric circuits Circuits so far Wires: Make complete circuit necessary for steady flow of electrons Usually have negligible (zero) resistance Battery: Maintains a voltage difference V between terminal Provides each electron with eV of EPE to spend in circuit Provides push for electrons around circuit (bigger V, bigger push) Bulb: Filament is a high resistance wire KE of electrons converted into heat via collisions Electronman rules for analyzing circuits a) No electron deaths/births b) No passing of electrons c) Electrons have energy (high at start, low at end) d) Different conducting materials have different resistances e Think like an electron! a. Light will not light up, No current will flow b. Light will light up, Current will flow c. Light will barely light up, Current will flow d. Light will not light up, Current will flow e. Light will light up, No current will flow. a. Light will not light up, No current will flow b. Light will light up, Current will flow c. Light will barely light up, Current will flow d. Light will not light up, Current will flow e. Light will light up, No current will flow. Voltage (difference) (V) a) Across battery: Measure of EPE given to each e- as it passes through battery. EPE given = eV. Related to pushing force on electrons in circuit b) Across a resistor (wire, filament etc): Measure of EPE lost by each e- as it passes through. EPE lost = eV. Unless told otherwise voltage difference across connecting wire = 0. Units: Volts (V) Ohm’s Law: V = IR Circuit language Resistance (R) of a circuit element is measure of how hard it is for electrons to pass through. Units: Ohms (W) Current (I) : charge per second flowing past a point in the circuit (= electrons per second × charge on electron) Units : Amps (1 A = 1 C/s) Voltage dropped across component Current through component Resistance of component Note: All quantities specific to one component. Don’t mix and match! Resistance is measure of how hard it is for electrons to pass through an object … or how much stuff they will run into. What if increase resistance (R) of filament… add more stuff for e- to hit… a. Rate at which electrons pass through filament stays the same b. Rate at which electrons pass through filament decreases c. Rate at which electrons pass through filament increases V What if increase voltage difference (V) across battery? a. Rate at which electrons pass through filament stays the same b. Rate at which electrons pass through filament decreases c. Rate at which electrons pass through filament increases V11/1/2011 2 6V If the battery on the left has a voltage (difference) of 6V and it is pushing a current of 1.5 A through the bulb, what is the resistance of the bulb? a) 9 W b) 6 W c) 4 W d) 1.5 W e) 0 W 1.5A Electrical Power P = IV Electrical power dissipated (used up) in component Current through component Voltage dropped across component Don’t mix and match! What is the electrical power used up by each component in circuit? Also Ohm’s Law: V = IR Substitute into power law to get different forms: P = V2/R - I = V/R - Useful if you know V and R but not I (parallel circuits) P = I2R - V = IR - Useful if you know I and R but not V (series circuits) P = IV - Useful if you know I and V but not R Power question I have a 60W bulb plugged into the mains. Assume that the mains supply is like a 120V battery What current flows through the bulb? a) 120A b) 60A c) 0.5A d) 2A e) 7200A 120V 60W Power question I have a 60W bulb plugged into the mains. Assume that the mains supply is like a 120V battery What current flows through the bulb? What is the resistance of the bulb filament? a) 240 W b) 2 W c) 0.5 W d) 30 W e) Can’t determine 120V 60W Batteries in series Batteries provide a voltage difference between their terminals If each battery below is an identical 3V battery, what is the total voltage across the following arrangement? V? a) 3V b) 0V c) 9V d) 6V e) Other case 1 case 2 Compare the brightness of the bulbs in case 1 and case 2. All bulbs and batteries are identical a. 2 twice as bright as 1 b. 2 same brightness but runs twice as long c. 2 much more than twice as bright as 1 d. 2 produces no light Batteries in series (nose to tail) V V V R R11/1/2011 3 1.5 V + Bulbs in series and parallel Given 2 bulbs and a battery, there are 2 different ways that you can connect the circuit 1.5 V + Series Parallel 1.5 V + Compare the brightness of identical bulbs. How does bulb 1 compare to bulb 2? a. brighter, b. dimmer, c. same 1 2 Bulbs in series + Electrons marching around in a line, pushing each other through. Each electron passes through both bulbs. Same number of electrons passing by per second at any point on the circuit. So same current! + - e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e Remember rules: - No Passing - No electron man deaths or births on route Large R Large R Electron man picture of bulb problem case 1 + All bulbs(R) and batteries (V) are identical. How does current flow through circuit in case 1 compare to case 2? a. Current in case 2 is 1/4 of current in case 1 b. Current in case 2 is 1/2 of current in case 1 c. Current in case 2 is same as current in case 1 d. Current in case 2 is 2 times current in case 1 e. Current in case 2 is 4 times current in case 1 case 2 Series circuit question - current + ?.?? V + 120 V + How big is voltage drop (difference) across lefthand bulb (both bulbs identical)? a. 60 V, b. 120 V, c. 240 V, d. 0 V Series circuit question - voltage + • Electrons marching around in a line, pushing each other through. • Each electron passes through both bulbs. • Bulbs like 2 identical mudpits. Electron loses half energy ( voltage) in each. + - e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e Remember rules: - No Passing - No electron man deaths or births on route Large R Large R Electron man picture of bulb …


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