Physics 2112 Unit 15 Today’s Concept: Ampere’s Law B d I o enclosed Unit 15, Slide 1 Ampere’s Law We know for an infinite current carrying wire o I B 2R But what is 2R? Circumference of circle! B 2R o I B d I o Electricity & Magnetism Lecture 15, Slide 2 Ampere’s Law Any closed loop B d I o ENCL Current enclosed by that closed loop Electricity & Magnetism Lecture 15, Slide 3 Checkpoint 1A Two loops are placed near identical current carrying wires as shown in Case 1 and Case 2 below. For which loop is ∫B·dl greater? A. Case 1 B. Case 2 C. The integral is the same for both Electricity & Magnetism Lecture 15, Slide 4 Checkpoint 1B Two loops are placed near identical current carrying wires as shown in Case 1 and Case 2 below. For which loop is ∫B·dl greater? A. Case 1 B. Case 2 C. The integral is the same for both Electricity & Magnetism Lecture 15, Slide 5 Checkpoint 1C Two loops are placed near current carrying wires as shown in Case 1 and Case 2 below. In both cases the direction of the current in the two wires are opposite to each other. For which loop is ∫B·dl greater? A. Case 1 B. Case 2 C. The integral is the same for both Electricity & Magnetism Lecture 15, Slide 6 Example 15.1 (B field from a thick wire) A wire with a radius of r=1cm has a uniform current of 1A flowing through it. What is the B field 1 meter from the center of the wire? What is the B field 0.5cm from the center of the wire? Unit 15, Slide 7 Example 15.1 (graph) X X X X X X X X X A wire with a radius of r=1cm has a uniform current of 1A flowing through it. X X X X X X X X X B r Unit 15, Slide 8 CheckPoint 2A An infinitelyX long hollow conducting tube carries current I in the direction shown. What is the direction of the magnetic field inside the tube? A. clockwise B. counterclockwise C. radially inward to the center D. radially outward from the center E. the magnetic field is zero Electricity & Magnetism Lecture 15, Slide 9 Why is that? X X Electricity & Magnetism Lecture 15, Slide 10 Example 15.3 (Pipe of current) y An infinitely long cylindrical shell carries a uniformly distributed current of 5A out of the screen. The inner radius is a=4cm and outer radius=8cm a I x b What is B at r = 2cm? What is B at r = 6cm? What is B at r = 16cm? Electricity & Magnetism Lecture 15, Slide 11 Example 15.3 (Pipe of current) Conceptual Analysis Complete cylindrical symmetry (can only depend on r) can use Ampere’s law to calculate B Strategic Analysis Calculate B for the three regions separately: 1) r < a 2) a < r < b 3) r > b B d o I enc For circular path concentric with shell. B d o I enc Electricity & Magnetism Lecture 15, Slide 12 Example 15.2 (Two thick wires) A X D B C Two thick cables both of radius R and length L carry a current, I, side-by-side as shown to the left. The left cable has the current into the screen and the right cable has the current out of the screen. What is the magnetic field at points A, B, C and D? - A is at the center of the left cable - B is a distance R/2 from the center of the left cable - C is at the point where the cables touch. - D is the bottom of the left cable Unit 15, Slide 13 The Plan A X B C What is the magnetic field at points A, B, C and D? D Conceptual Analysis use Ampere’s law to calculate B from each cable separately Strategic Analysis Note direction of B from each cable at each point. Add B like vectors Unit 15, Slide 14 Sign of the dot product I into screen B d I enc Electricity & Magnetism Lecture 15, Slide 15 Ampere’s Law B d o Ienc dl B dl B dl B Electricity & Magnetism Lecture 15, Slide 16 Sign of the dot product B d o Ienc dl dl B B B dl Electricity & Magnetism Lecture 15, Slide 17 Sign of the dot product B d o Ienc dl B B dl B dl Electricity & Magnetism Lecture 15, Slide 18 B Field in Solenoid ~cancel out X X . X X . . . add up Unit 15, Slide 19 B Field in Solenoid X X . X X . . X . X . X X . . . L B d o I enc 0 0 0 Bx * L o * n * L * I Bx o nI n = turns per unit length (Ideal Solenoid L>>>r) Unit 15, Slide 20 Solenoid Bar Magnet Unit 15, Slide 21 CheckPoint 2B A current carrying wire is wrapped around cardboard tube as shown below. In which direction does the magnetic field point inside the tube? A. B. C. D. E. F. left right up down out of the screen into the screen Electricity & Magnetism Lecture 15, Slide 22 Example 15.3 (B from Solenoid) A long thin solenoid consists of 500 turns of wire carrying 1.5A. It has a length of 250cm and a radius of 1cm. What is the magnetic field in the exact center of the coil? What is the contribution to this field from the 250th coil (the center coil)? Unit 15, Slide 23 Make Sense? X X . X X . . X . X . X X . . . Note: • B250/Btot = 0.024 • So closest coil contributes over 10 tens more than average coil. Make sense? (1/500 = 0.002) Unit 15, Slide 24 When does “Loop” “Solenoid”? Electricity & Magnetism Lecture 15, Slide 25 A hint….. B d A 0 surface B d l I o ENCL loop Qenc E d A 0 o surface E d l 0 loop Unit 15, Slide 26