Magnetic Fields Generated by Currents in Physics 102 Lecture
Explore how magnetic fields are produced by electric currents through examples like long straight wires, current loops, and solenoids. Understand the application of these principles in electromagnets and MRI technology in Physics 102 Lecture 12.
Download Presentation
Please find below an Image/Link to download the presentation.
The content on the website is provided AS IS for your information and personal use only. It may not be sold, licensed, or shared on other websites without obtaining consent from the author. If you encounter any issues during the download, it is possible that the publisher has removed the file from their server.
You are allowed to download the files provided on this website for personal or commercial use, subject to the condition that they are used lawfully. All files are the property of their respective owners.
The content on the website is provided AS IS for your information and personal use only. It may not be sold, licensed, or shared on other websites without obtaining consent from the author.
E N D
Presentation Transcript
Phys 102 Lecture 12 Currents & magnetic fields 1
Today we will... Learn how magnetic fields are created by currents Use specific examples Long straight wire Current loop Solenoid Apply these concepts Electromagnets & MRI Phys. 102, Lecture 12, Slide 2
Currents generate B fields A long straight wire carrying current I generates a B field I B I Current = 0 r wire B Bwire 2 Distance from wire 7 = T m A 4 10 I 0 Side view Permeability of free space (similar to 0 for electricity) DEMO Bwire r Right-hand rule for wire: I Thumb along I Curl fingers along B B is to r Top view Phys. 102, Lecture 12, Slide 3
Superposition principle Total B field due to several charges = sum of individual B fields tot= B B Ex: what is the B field at point P due to I1, I2, and I3? B 1 B P tot B 3 B 1 B 2 I1 B B B 2 3 tot I3 Order does not matter! I2 Same approach as for E fields tot= B + + B B B Phys. 102, Lecture 12, Slide 4 1 2 3
Calculation: 2 wires A long straight wires 1 carries current I1 = 0.1 A out of the page. What must be the direction and magnitude of the current I2 in wire 2 such that there is no net B field at point P? I1 I2 P ? 2 cm 4 cm Phys. 102, Lecture 12, Slide 5
ACT: CheckPoint 1.1 Two long wires carry the same current I in opposite directions P I I What is the direction of the total B field above and midway between the two wires at point P? A. Left B. Right C. Up D. Down E. Zero Phys. 102, Lecture 12, Slide 6
Calculation: B field from 2 wires Calculate the magnitude of the total B field from the 2 wires at P P 2 cm I = 0.1A I = 0.1A 1 2 y 2 cm x Phys. 102, Lecture 12, Slide 7
ACT: Current loop A loop of wire carries current as shown. In what direction is the B field at the center of the loop? I A. Left B. Right C. Up D. Down E. Zero Phys. 102, Lecture 12, Slide 8
Current loops & magnetic dipoles Recall Lect. 11: A current loop behaves like a magnetic dipole Generates the same B field N N = S S I Phys. 102, Lecture 12, Slide 9
ACT: Many current loops Which configuration of two loops generates a larger B field at point P midway between the loops? I I P P I I A. Left B. Right C. Same Phys. 102, Lecture 12, Slide 10
Solenoid B I A solenoid is a long coil consisting of N turns of wire = B nI Current 0 sol L N B field inside solenoid Side view Number of turns of wire per length (in m) N/L I Note there no dependence on r. B field inside solenoid is uniform B Right-hand rule for loop(s): I Curl fingers along I Thumb along B Top view Phys. 102, Lecture 12, Slide 11
Electromagnets Solenoids are a way to make powerful magnets that can be turned on and off! spring solenoid piston input output flow line seal Junkyard magnet Solenoid valve MRI magnet Phys. 102, Lecture 12, Slide 12
Calculation: MRI magnet How many turns of wire are needed to generate a 1.5 T MRI magnet? L = 2 m d = 0.6 m How much wire does that correspond to? Phys. 102, Lecture 12, Slide 13
Magnetic field recap B fields exert forces on moving charges v, I v I = = sin sin F qvB F ILB F B + B B B Moving charges generate B fields I I B L N = B nI I 0 sol = 0 r wire B I 2 Phys. 102, Lecture 12, Slide 14 I
ACT: CheckPoint 3.1 A long straight wire is carrying current I to the right. A distance r from the wire are 2 charges +q with speed v v q q + (a) (b) v + r r I Compare the magnitude of magnetic force on q for (a) vs. (b) A. (a) has the larger force B. (b) has the larger force C. force is the same for (a) and (b) Phys. 102, Lecture 12, Slide 15
CheckPoint 3.1 A long straight wire is carrying current I to the right. A distance r from the wire are 2 charges +q with speed v v q q + (a) (b) v + r r I Compare the direction of magnetic force on q for (a) vs. (b) Phys. 102, Lecture 12, Slide 16
ACT: Force between wires Current-carrying wires generate B fields, B 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. In which direction does the force on wire 2 point? I2 I1 A. Toward wire 1 B. Away from wire 1 C. The force is zero Phys. 102, Lecture 12, Slide 17
Force between wires Wires generate B fields, B fields exert force on wires. Therefore, wires exert forces on each other I2 I1 I1 I2 r r I = = 0 1 2 r I r 1sin F I LB B 21 2 1 = = 0 2 2 2sin F I LB B 12 1 2 DEMO Phys. 102, Lecture 12, Slide 18
ACT: Force between wires The two wires 1 & 2 carry current in perpendicular directions. In which direction does the force on wire 2 point? I2 I1 A. Toward wire 1 B. Away from wire 1 C. The force is zero Phys. 102, Lecture 12, Slide 19
Summary of todays lecture B 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
