PHYS
S13Phys2BaLec28A

# R time constant τ make presents the capacitor source

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( R ) Time constant τ “MAKE presents: The Capacitor” (Source: makemagazine)

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RC Circuits Example The circuit has a capacitance of C = 0.3 μ F, a total resistance of 20k Ω and a battery “emf” of 12V. What is the maximum charge it can acquire? What time does it take to reach 99% of this value? Answer First, we need to identify which variables we need. We want Q f and time, t, for a given Q.
RC Circuits Answer Perform a Kirchhoff loop over the circuit to find Q f . When the capacitor is fully charged, the current, I, will become zero. This is the maximum charge the capacitor can hold.

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RC Circuits Answer The charge at any time is given by: We can eliminate an exponent with natural log: <- to increase, increase either R or C.
The time constant for the discharge of this capacitor after the switch is closed is: A) 5s. B) 2s. C) 4s. D) 1s. E) The capacitor doesn’t discharge because the resistors cancel each other. Clicker Question 28A-1

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History of Magnets Magnets were discovered by early man. Asia Minor has region known as Magnesia where rocks would attract each other. Early researchers found that if you put a magnet on a string it would want to face a certain direction (i.e. towards the North Pole). Thus, magnets were used as compasses. Later on it was discovered that magnets will only attract iron or other very select metals (cobalt, nickel, etc.).
History of Magnets Researchers also found that magnets “always” came with two sides a north pole and a south pole.

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