Phys 102 Lecture 12 Currents & magnetic fields

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1 Phys 102 Lecture 12 Currents & magnetic fields 1

2 Today we will... Learn how magnetic fields are created by currents Use specific examples Long straight wire Current loop Solenoid Apply these concepts Electromagnets & MR Phys. 102, Lecture 12, Slide 2

3 Currents generate fields A long straight wire carrying current generates a field wire μ 0 2πr Current Distance from wire wire 7 μ0 4π 10 T m A Permeability of free space (similar to ε 0 for electricity) DEMO Side view r wire Right-hand rule for wire: Thumb along Curl fingers along Top view is to r Phys. 102, Lecture 12, Slide 3

4 Superposition principle Total field due to several charges = sum of individual fields tot Ex: what is the field at point P due to 1, 2, and 3? 1 tot P tot 2 3 Same approach as for E fields tot Order does not matter! Phys. 102, Lecture 12, Slide 4

5 Calculation: 2 wires A long straight wires 1 carries current 1 = 0.1 A out of the page. What must be the direction and magnitude of the current 2 in wire 2 such that there is no net field at point P? 1 2? P 2 cm 4 cm Phys. 102, Lecture 12, Slide 5

6 ACT: CheckPoint 1.1 Two long wires carry the same current in opposite directions P What is the direction of the total field above and midway between the two wires at point P? A. Left. Right C. Up D. Down E. Zero Phys. 102, Lecture 12, Slide 6

7 Calculation: field from 2 wires Calculate the magnitude of the total field from the 2 wires at P P 2 cm = 0.1A = 0.1A y cm x Phys. 102, Lecture 12, Slide 7

8 ACT: Current loop A loop of wire carries current as shown. n what direction is the field at the center of the loop? A. Left. Right C. Up D. Down E. Zero Phys. 102, Lecture 12, Slide 8

9 Current loops & magnetic dipoles Recall Lect. 11: A current loop behaves like a magnetic dipole Generates the same field N S = N S μ Phys. 102, Lecture 12, Slide 9

10 ACT: Many current loops Which configuration of two loops generates a larger field at point P midway between the loops? P P A. Left. Right C. Same Phys. 102, Lecture 12, Slide 10

11 Solenoid A solenoid is a long coil consisting of N turns of wire field inside solenoid sol μ n 0 Number of turns of wire per length (in m) N/L Current Side view N L Note there no dependence on r. field inside solenoid is uniform Right-hand rule for loop(s): Curl fingers along Thumb along Top view Phys. 102, Lecture 12, Slide 11

12 Electromagnets Solenoids are a way to make powerful magnets that can be turned on and off! spring solenoid input piston output flow line seal Junkyard magnet Solenoid valve MR magnet Phys. 102, Lecture 12, Slide 12

13 Calculation: MR magnet How many turns of wire are needed to generate a 1.5 T MR magnet? L = 2 m d = 0.6 m How much wire does that correspond to? Phys. 102, Lecture 12, Slide 13

14 Magnetic field recap fields exert forces on moving charges v + F qv sin θ Moving charges generate fields F L sin θ v, F wire μ 0 2πr N L sol μ n 0 Phys. 102, Lecture 12, Slide 14

15 ACT: CheckPoint 3.1 A long straight wire is carrying current to the right. A distance r from the wire are 2 charges +q with speed v v (a) q + v (b) + r r q Compare the magnitude of magnetic force on q for (a) vs. (b) A. (a) has the larger force. (b) has the larger force C. force is the same for (a) and (b) Phys. 102, Lecture 12, Slide 15

16 CheckPoint 3.1 A long straight wire is carrying current to the right. A distance r from the wire are 2 charges +q with speed v v (a) q + v (b) + r r q Compare the direction of magnetic force on q for (a) vs. (b) Phys. 102, Lecture 12, Slide 16

17 ACT: Force between wires Current-carrying wires generate fields, fields exert force on current-carrying wires. So, wires must exert forces on each other! The two wires 1 & 2 carry current in the same direction. n which direction does the force on wire 2 point? 1 2 A. Toward wire 1. Away from wire 1 C. The force is zero Phys. 102, Lecture 12, Slide 17

18 Force between wires Wires generate fields, fields exert force on wires. Therefore, wires exert forces on each other r r 1 2 μ 2πr sin μ 2πr F L θ sin F L θ DEMO Phys. 102, Lecture 12, Slide 18

19 ACT: Force between wires The two wires 1 & 2 carry current in perpendicular directions. n which direction does the force on wire 2 point? 1 2 A. Toward wire 1. Away from wire 1 C. The force is zero Phys. 102, Lecture 12, Slide 19

20 Summary of today s lecture fields are generated by currents Long straight wire Current loop Solenoid Current carrying wires exert forces on each other Likes attract, opposites repel Don t confuse different RHRs! Phys. 102, Lecture 12, Slide 20

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