PHY213_Chapter17_Sec1to4_6-Chapter18_Sec1to3_5

PHY213_Chapter17_Sec1to4_6-Chapter18_Sec1to3_5 - Chapter 17...

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Chapter 17 / 18 Current and Resistance / Direct Current Circuits
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General Physics Current, Resistance, and Power Ch 17, Secs 1–4, 6 DC Circuits and RC Circuits Ch 18, Secs 1–3, 5
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General Physics Electric Current Whenever electric charges of like signs move, an electric current is said to exist The current is the rate at which the charge flows through this surface • Look at the charges flowing perpendicularly to a surface of area A The SI unit of current is Ampere (A) • 1 A = 1 C/s Q I t
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General Physics Electric Current, cont The direction of the current is the direction positive charge would flow • This is known as conventional current direction In a common conductor, such as copper, the current is due to the motion of the negatively charged electrons It is common to refer to a moving charge as a mobile charge carrier • A charge carrier can be positive or negative
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General Physics Current and Drift Speed Charged particles move through a conductor of cross- sectional area A n is the number of charge carriers per unit volume (charge carrier density) n A Δx is the total number of charge carriers The total charge is the number of carriers times the charge per carrier, q • ΔQ = (n A Δx) q
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General Physics Current and Drift Speed, cont The drift speed, v d , is the speed at which the carriers move • v d = Δx/Δt Rewritten: ΔQ = (n A v d Δt) q So, current, I = ΔQ/Δt = nqv d A If the conductor is isolated, the electrons undergo random motion When an electric field is set up in the conductor, it creates an electric force on the electrons and hence a current
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General Physics Charge Carrier Motion in a Conductor The zig-zag black line represents the motion of charge carrier in a conductor • The net drift speed is small The sharp changes in direction are due to collisions The net motion of electrons is opposite the direction of the electric field Active Figure: Electron Drift in a Conductor
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General Physics Electrons in a Circuit The drift speed is much smaller than the average speed between collisions When a circuit is completed, the electric field travels with a speed close to the speed of light Although the drift speed is on the order of 10 -4 m/s the effect of the electric field is felt on the order of 10 8 m/s
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General Physics Meters in a Circuit – Ammeter and Voltmeter An ammeter is used to measure current • In line with the bulb, all the charge passing through the bulb also must pass through the meter A voltmeter is used to measure voltage (potential difference) • Connects to the two ends of the bulb
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General Physics Resistance In a conductor, the voltage applied across the ends of the conductor is proportional to the current through the conductor The constant of proportionality is the resistance of the conductor V R I
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Physics Resistance, cont Units of resistance are ohms (Ω) • 1 Ω = 1 V / A Resistance in a circuit arises due to
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PHY213_Chapter17_Sec1to4_6-Chapter18_Sec1to3_5 - Chapter 17...

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