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Physics 106 Lesson #11 Series & Parallel Circuits I Dr. Andrew Tomasch 2405 Randall Lab [email protected] Review: Cells, Batteries & Current • When connected to a circuit, the potential difference between the cell or battery terminals creates an electric force on the charges in the conductor causing them to move and establishing an electric current • The conventional current is from regions of higher potential to regions of lower potential, positive to negative Conventional current Review: Good and Bad Wiring • Good: • Bad: Broken! • Ugly: Short! No device! = Review: Resistors Resistors are used in appliances to convert electrical energy into thermal energy (heat) or light Toaster Space Heater Light Bulb When an extension cord is used with a space heater, the cord must have a resistance that is sufficiently small to prevent overheating of the cord Stove Heating Element Review: Series Resistors • For resistors R1 & R2 connected in series (sequentially), the current i passing through each resistor must be the same • The voltages across R1 & R2 must add up to V (Loop Rule) V + Review: Parallel Resistors • Resistors R1 & R2 connected in parallel have the same potential difference (voltage) V across them • Charge must be conserved, so the currents I1 & I2 flowing through the two resistors must add up to the total current I leaving the battery (continuity for electric current!) V + I I1 I 2 Review: Power • Power has a precise definition in physics: Power is the rate at which work is done or how much work is done per unit of time W P t Units: J/s Watt (W) James Watt invented the steam engine In an electric circuit power is the product of current and voltage: Power I V Concept Test #1 The label on a car battery proudly claims that the battery will provide “250 Ampere hours”. This rating describes the amount of what quantity that can be drawn from the battery? 1) 2) 3) 4) 5) Charge Current Voltage Power Energy current =charge/time current × time = charge 1 Ampere-hour = 1 Coulomb/second x 3600 s/hr = 3600 Coulombs of charge Concept Test #2 How much energy is stored in a 2 V battery with a capacity of 1 Ampere-hour? 1) 2) 3) 4) 5) 3,600 Joules 1,800 Joules 7,200 Joules 10,000 Joules 5,000 Joules Energy = charge (C) x potential (V) 1 Ampere-hour = 1 Coulomb/second x 3600 s/hr = 3600 Coulombs of charge 3600 Coulombs x 2 V = 7,200 Joules of energy Concept Test #3 What is the average power delivered when I discharge an 8.4 V battery with a capacity of 2 A-hr in 10 minutes? 1) 3,600 Watts 2) 1,800 Watts 3) 7,200 Watts 4) 100 Watts Power = energy/time = charge (C) x potential (V)/time 2 Ampere-hour = 2 Coulomb/second x 3600 s/hr = 7,200 Coulombs of charge 7,200 Coulombs x 8.4 V = 60,480 Joules of energy 10 min = 600 seconds 60,480 Joules/600 seconds ≈ 100 Watts of power