PRS24: 4/08/04 Bulbs and Batteries An ideal battery is hooked to a light bulb with wires. A second identical light bulb is ;connected in parallel to the first light bulb. After the second light bulb is connected, the current from the battery compared to when only one bulb was connected. 1. 2. 3. 4. Is Higher Is Lower Is The Same Don’t know PRS24: 4/08/04 Bulbs and Batteries (1) More current flows from the battery There are several ways to see this: (A) The equivalent resistance of the two light bulbs in parallel is half that of one of the bulbs, and since the resistance is lower the current is higher, for a given voltage. (B) The battery must keep two resistances at the same potential Æ I doubles. PRS24: 4/08/04 Bulbs and Batteries An ideal battery is hooked to a light bulb with wires. A second identical light bulb is connected in parallel to the first light bulb. After the second light bulb is connected, the power output from the battery (compared to when only one bulb was connected) 1. 2. 3. 4. 5. 6. Is four times higher Is twice as high Is the same Is half as much Is one quarter as much Don’t know PRS24: 4/08/04 Bulbs and Batteries (2) Twice as much The current from the battery must double (it must raise two light bulbs to the same voltage difference) and P=IV PRS24: 4/08/04 Bulbs and Batteries An ideal battery is hooked to a light bulb with wires. A second identical light bulb is connected in series with the first light bulb. After the second light bulb is connected, the light from the first bulb (compared to when only one bulb was connected) 1. is four times as bright 2. is twice as bright 3. is the same 4. is half as bright 5. is one quarter as bright PRS24: 4/08/04 Bulbs and Batteries (5) The light is ¼ as bright The resistance in the circuit doubled so the current is cut in half. This means that the power delivered by the battery is half what it was. But that power is further divided between two bulbs now. Alternatively, 2 P=IR PRS24: 4/08/04 RC Circuit An uncharged capacitor is connected to a dc voltage source via a switch. A resistor is placed in series with the capacitor. The switch is initially open. At t = 0, the switch is closed. A very long time after the switch is closed, the current in the circuit is 1. nearly zero 2. at a maximum and decreasing 3. nearly constant but non-zero PRS24: 4/08/04 RC Circuit (1) After a long time the current is 0 Eventually the capacitor gets “full” – the voltage increase provided by the battery is equal to the voltage drop across the capacitor. The voltage drop across the resistor at this point is 0 – no current is flowing. PRS24: 4/08/04 RC Circuit S IC IR ε R C R Consider the above circuit, with an initially uncharged capacitor and two identical resistors. At the instant the switch is closed: 1. IR = IC = 0 2. IR = IC = ε/R 3. IR = ε/2R; IC = 0 4. IR = 0; IC = ε/R 5. IR = ε/2R; IC = ε/R PRS24: 4/08/04 RC Circuit S ε R C R (4) IR = 0; IC = ε/R Initially there is no charge on the capacitor and hence no voltage drop across it – it looks like a short. Thus all current will flow through it rather than through the bottom resistor. So the circuit looks like: S ε R PRS24: 4/08/04 Force on Charged Particle What direction is the force on a positive charge when entering a uniform B field in the direction indicated? q 1) 2) 3) 4) 5) 6) 7) v up down left right into page out of page there is no net force B PRS24: 4/08/04 Force on Charged Particle (6) Force is out of the page q v B PRS24: 4/08/04 Force on Charged Particle What direction is the force on a negative charge when entering a uniform B field in the direction indicated? B 1) 2) 3) 4) 5) 6) 7) v up down left right into page out of page there is no net force -q PRS24: 4/08/04 Force on Charged Particle (2) Force on the negative charge is down B v -q PRS24: 4/08/04 Rail Gun A bar is free to slide on two parallel rails. A current I flows through the bar in the direction shown. An external magnetic field points out of the page. The bar in the center of the figure will: I I 1) move left 2) move right 3) stay in place PRS24: 4/08/04 Rail Gun (2) The rail will move to the right I I G G G FB = Il × B , and up cross out is right PRS24: 4/08/04 Dipole in Field The coil above will rotate 1. clockwise 2. counterclockwise 3. stay in the orientation shown because the total force is zero PRS24: 4/08/04 Dipole in Field Answer: 1. The coil above will rotate clockwise because the I ds x B forces shown produce a torque r x F into the page. This implies clockwise rotation. PRS24: 4/08/04 A Helmholtz coil is hooked up with current running parallel in both coils. A magnetic dipole is placed along the z-axis at the point z = 0 with the magnetic moment pointing in the +x direction. Which of the following statements is true? z z=0 1. Force & torque on the dipole are zero 2. Force on the dipole is zero and torque on the dipole is non-zero 3. Force & torque on the dipole are nonzero 4. The force on the dipole is non-zero and the torque on the dipole is zero PRS24: 4/08/04 Helmholtz Coil (2) Zero force, non-zero torque In a Helmholtz coil with parallel currents, the field is uniform at the center. There will be a torque to align the dipole with the field (along the zaxis) but no force, since the field has no gradient. PRS24: 4/08/04 Iron Filings Above is a iron filings representation of the magnetic field created by two loops of current. Which is true? 1. Currents are parallel (bigger in top); loops attracted 2. Currents are parallel (bigger in bottom); loops repelled 3. Currents are anti-parallel (bigger in bottom); loops attracted 4. Currents are anti-parallel (bigger in top); loops repelled 5. None of the above PRS24: 4/08/04 Iron Filings (5) None of the above Loops are repelling so currents must be anti-parallel. But the field zero is above the top loop, so the field from the bottom is stronger, so the current must be bigger in the bottom loop. So: Currents are anti-parallel (bigger in bottom); loops repelled PRS24: 4/08/04 Dipole in Field The current carrying coil above will move 1. upwards 2. downwards 3. stay where it is because the total force is zero PRS24: 4/08/04 Dipole in Field Answer: 2. The coil above will move downward because the I ds x B forces shown produce a net force downward PRS24: 4/08/04 The current-carrying coil above will move 1. upwards 2. downwards 3. stay where it is because the total force is zero PRS24: 4/08/04 Answer: 2. The coil above will move downward because the I ds x B forces shown produce a net force downward PRS24: 4/08/04 Free dipoles attract because: 1. The force between dipoles is always attractive independent of orientation. 2. A dipole will always move towards stronger field, independent of orientation. 3. The torque on the dipole aligns it with the local field and the dipole will then move toward stronger field strength. PRS24: 4/08/04 Answer: 3. Free dipoles attract because the torque on a dipole aligns the dipole with the local field and the dipole then moves toward stronger field strength— that is closer to another dipole. If the dipole were anti-aligned with the local field then it would move toward regions of weaker field strength. PRS24: 4/08/04 Bent Wire ĵ k̂ î The magnetic field at point P 1. points towards the +x direction 2. points towards the +y direction 3. points towards the +z direction 4. points towards the -x direction 5. points towards the -y direction 6. points towards the -z direction 7. points nowhere because it is zero PRS24: 4/08/04 Bent Wire (6) B is in the –z direction ĵ The vertical line k̂ î segment contributes nothing to the field at P (it is parallel to the displacement). The horizontal segment makes a field into the page.

PRS24: 4/08/04, Answer Key

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