Physics 123 Exam #3 Fall 2002
Do any THREE of the following four questions. X-OUT the question you OMIT.

1. An 23.0 gram particle with a charge of 42.0 mC is moving in a constant horizontal magnetic field equal to 61.0 G, pointing due east. At one moment the particle is observed to moving horizontally at 810 cm/s at an angle of 51.0o east of due north.

(A) What is the magnitude & direction of the magnetic force acting on the charged particle at that moment?
Ans:{1.31 mN, Down}

(B) What is the radius of the helix along which the charged particle spirals?
Ans:{458 m}

(C) Sketch the particles motion and determine how fast is the particle moving along the direction of the magnetic field?
Ans:{6.29 m/s}
2. Three horizontal wires run from south to north and carry current of I1 = 10.0A, I 2 = 15.0A, and I3 = 20.0A towards the north. Wire I1 is to the west and is separated from the middle wire I2 by 11.0 cm. Wire I3 is to the east and is 28.0 cm from the middle wire I2.

(A) What is the magnitude and direction of the magnetic field that wire I1 creates at the location of wire I3 ?
Ans:{5.13 mT, Down}

(B) What magnitude and direction of the force that wire I2's magnetic field exerts on a 3.50 m long segment of wire I1?
Ans:{955 mN, East}

(C) What is magnitude and direction of net force that wires I1's and I3's magnetic fields exert on a 7.80 m long segment of the middle wire I2?
Ans:{456 mN, West}
3. A stationary, horizontal loop has a radius of 150 cm, 59,000 turns, and a total resistance of 2.80 Ω. A vertical magnetic field varies with time according to B(t) = B o(1 + t/τ) with Bo = 44.0 mT and τ = 360 ms.
(A) At what moment will the flux through a single loop be equal to 415 mT·m2 ?
Ans:{120 ms}

(B) What is the magnitude and direction (as viewed from above) of the induced current flowing in the loop ?
Ans:{18.2 kA, Clockwise}

(C) What is the magnitude and direction of the secondary magnetic field at the center of the loop at time t = 0 ?
Ans:{450 T, Down}
4. A long horizontal wire carries a current of 28.0 A, flowing from west to east. A 19.0 cm long, horizontal, conducting rod with negligible resistance moves north at a constant speed of 31.0 m/s along two frictionless, conducting rails with a resistance of 3.70 Ω from point 1 around the loop to point 2. When the rod is 58.0 cm from the wire:


(A) What is the magnitude and direction of the magnetic field that the rod experiences ?
Ans:{9.66 mT, Up}

(B) What is the magnitude of the potential across the end of the rod? Which end is at a higher potential ?
Ans:{56.9 mV, 2 – the east end}

(C) What force is needed to pull the rod north at a constant speed?
Ans:{28.2 pN}

Physics 123 Exam #3 Spring 2002

1. A horizontal wire carries a current of 210 A from South to North. A horizontal metal conductor oriented parallel to the wire is 77.0 m long and is moving to the East at a constant speed of 170 m/s. When the metal conductor is 2.20 cm from the current caring wire:

(A) What is the magnitude and direction of the magnetic field at the location of the metal conductor ?
Ans:{1.91 mT, Down}

(B) What is the magnitude and direction of the force that the magnetic field exerts on an electron ( e = 1.602x10-19 C) in the conductor ?
Ans:{5.20x10-20 N, South}

(C) What is the potential voltage across the conductor and which end would have the highest potential ?
Ans:{25.0 V, North or towards 1}
2. A circular, vertical coil lying in North-South plane has a radius of 55.0 cm, a resistance of 3.90 Ω, and is composed of 1200 turns. A changing magnetic field produces a constant current of 45.0 mA flowing in a counterclockwise direction when view from the East. If the magnetic field is horizontal and pointing due East.
(A) What is the magnitude and direction of the secondary magnetic field created by the induced current at the center of the coil? Ans:{61.7 mT, East}

(B) What is the strength of the electric field in any one of the coils of the loop? Ans:{42.3mV/m}

(C) What is the rate at which the magnetic field is changing per second ? Is it increasing or decreasing each second?
Ans:{-154 mT/s, decreasing}
3. Three horizontal wires run in a North-South direction and carry currents of I1 = 20.0A, I2 = ? and I3 = 25.0A. The current in wires I1 and I3 flows North. Wires I1 and I2 are separated by 5.00 cm while wires I2 and I3 are separated by 8.00 cm. Wire I1 is East of wire I2 while I2 is East of wire I3 as shown.

(A) What is magnitude and direction of the magnetic field that wire I1 created at the location of wire I3 ?
Ans{30.8 mT, Up}

(B) What is the magnitude and direction of the net magnetic field at the location of the center-wire I2 ?
Ans:{17.5 mT, Up}

(C) If the force on a 3.80 m long section of the center-wire I2 is 3.50 mN to the West, what is the magnitude and direction of the current in the center-wire I2 ?
Ans:{52.6 A, South}
4. A thin vertical slab with a resistance of 1.70 Ω is placed in series with 95.0 V battery. The slab’s dimensions are 34.0 cm tall, 16.0 cm wide, and 8.60 mm thick. An 81.0 T magnetic field passes through the slab at an angle of 38.0o above due East. If a potential of 78.0 mV builds up across the slab:
(A) What is the net magnitude and direction of the magnetic force on the whole slab ?
Ans:{1.21 kN, North}

(B) What the magnitude and direction of the electric field in the slab in the vertical direction ?
Ans:{279 V/m, Up}

(C) What is the magnitude and direction of the electric field in the North-South direction in the slab ?
Ans:{15.6 mV/m, North]

(D) After equilibrium is established, what is the drift velocity of the conduction charges through the slab?
Ans:{0.245 mm/s}