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EF152 – Physics for Engineers Summer, 2012 Recitation 4.6 Review 1. Three point charges are arranged along the x axis. Charge q1=+3.00μC is at the origin, and charge q2= (-5.00) μC is at x=0.20m. Charge q3= (-8.00) μC. Where is charge q3 located if the net force on q1 is 7.00N in the (-x) direction? [-0.144 m] 2. A proton is placed in a uniform electric field of 2.75x103 N/C. Calculate: a. The magnitude of the electric force felt by the proton. b. The proton’s acceleration c. The proton’s speed and position after 1.00 μs in the field, assuming it starts form rest. [a. 4.41x10-16 N; b. 2.63x1011 m/s2; c. 2.63x105 m/s, 0.132m] 3. A small particle has a charge of (-5.00) μC and a mass of 2.00x10-4 kg. It moves from point A, where the electric potential is VA=+200V, to point B where the electric potential is VB=+800V. The electric force is the only force acting on the particle. The particle has a speed of 5.00m/s at point A. What is its speed at point B? [7.42 m/s] 4. In the figure to the right, C1=6.00μF, C2=3.00μF, C3=5.00μF. After the charges on the capacitors have reached their final values, the charge on C2 is 40.00μC. a. What are the charges on capacitors C1 and C3? b. What is the applied voltage Vab? [a. Q1=80μC, Q3=120μC; b. 37.3 V] 5. In household wiring, 12 gauge copper wire (2.05 mm diameter) is often used. Find the resistance of a 24.0 m (78.7 ft) length of this wire. [0.125Ω] 6. Each resistor in the circuit represents a light bulb with a resistance of 4.5Ω. The voltage source is 9.0V. Which bulb or bulbs glows the brightest? [P1=10.1 W] Now bulb R4 is removed from the circuit. Does bulb 1 glow brighter or duller with R4 removed? [P1=8.0 W] 7. Two hikers are reading a compass under an overhead transmission line that is 5.50 m above the ground and carries a current of 800 A in a horizontal direction from north to south. Find the magnetic field at a point on the ground directly underneath the conductor. Considering that the magnitude of the earth’s field is on the order of 0.5x10-4 T, is the current really a problem? [B=2.91x10-5T] EF152 – Physics for Engineers Constants Summer, 2012 Capacitance me=9.109x10 -31 kg mp=1.673x10 -27 kg R-C circuit time constant Magnetism Capacitance – parallel plates -27 mn=1.675x10 kg k = 9x10 9 ε0 = 8.854 ·10 Capacitance – energy storage Magnetic force on moving charged particle Capacitors in series Magnetic field of a moving charged particle -12 e = -1.6022 ·10 -19 C (electron charge) -19 J (electron volt) eV = 1.6022·10 Coulomb’s Law (electrostatic force) Capacitors in parallel Magnetic Field of a straight wire Resistors in series Electric Field – point charge - unit vector from charge to a location Magnetic field outside two parallel conductors with current in opposite directions Resistors in parallel Current Potential energy - point charges Voltage, Resistance, Current, Power Potential V=I·R P=I·V d – distance from the center line of the wire pair to the axis of one wire. x – distance from the center line of the wire pair to the point of interest. I – current in the wires Magnetic field along the axis of a looped wire Resistance Potential of a set of point charges Kirchhoff’s Rules Potential difference between two parallel plates for a closed loop for a junction a – radius of the loop x – distance from the center of the loop to point of interest on the axis. I – current in the wires N – number of loops