Elements of Physics II. Agenda for Today

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1 Physics 132: Lecture e 18 Elements of Physics II Agenda for Today Magnets and the Magnetic Field Magnetic fields caused by charged particles B-field from a current-carrying wire Magnetic fields and forces B-fields Magnetic forces on moving particles Physics 201: Lecture 1, Pg 1

2 Example: In the following figure, what is the value of the potential at points a and b? Physics 201: Lecture 1, Pg 2

3 Magnets and Magnetic Forces Magnet: An object consisting of two poles called North and South Poles of a magnet are the ends where objects are most strongly attracted Magnets will apply a force on other magnets Like poles repel each other and unlike poles attract each other Magnetic poles cannot be isolated If a permanent magnetic is cut in half repeatedly, you will still have a north and a south pole This differs from electric charges There is some theoretical basis for monopoles, but none have been detected Physics 201: Lecture 1, Pg 3

4 Magnetism Physics 201: Lecture 1, Slide Pg

5 B-field Similar to electric charges a magnet will create a magnetic field in the area around it Due to convention we will call the magnetic field a B-field The force magnets apply on each other can be described as an interaction between a magnet and another magnet s B-field Physics 201: Lecture 1, Pg 5

6 B-field lines We will also draw magnetic field lines (1) The direction of a magnetic field ed is tangent to a magnetic field line at any point (2) The number of lines per area is proportional to the magnitude of the B-field Physics 201: Lecture 1, Pg 6

7 Mapping Magnetic Field Physics 201: Lecture 1, Pg 7

8 B-Field Lines Physics 201: Lecture 1, Pg 8

9 Field Lines of Bar Magnet S N Magnetic field lines don t start or stop. There are no magnetic charges (monopoles) Physics 201: Lecture 1, Pg 9

10 Clicker Question 1: Physics 201: Lecture 1, Pg 10

11 B-field of Current-Carrying Wire In 1819 Hans Christian Oersted discovered that an electric current in a wire causes a compass to turn. Physics 201: Lecture 1, Pg 11

12 Notation Magnetism requires a three-dimensional perspective, but two-dimensional figures are easier to draw. We will use the following notation: Physics 201: Lecture 1, Pg 12

13 B-Field from wire Magnetic field lines are imaginary lines drawn through a region of space so that: t A tangent to a field line is in the direction of the magnetic field. The field lines are closer together th where the magnetic field strength is larger. Physics 201: Lecture 1, Pg 13

14 RHR Physics 201: Lecture 1, Pg 14

15 The Source of the Magnetic Field: Moving Charges The magnetic field of a charged particle q moving with velocity v is given by the Biot-Savart law: Physics 201: Lecture 1, Pg 15

16 The Magnetic Field The constant 0 in the Biot-Savart law is called the permeability constant: 0 = T m/a = T m/a The SI unit of magnetic field strength is the tesla, abbreviated as T: 1 tesla = 1 T = 1 N/A m Physics 201: Lecture 1, Pg 16

17 Magnetic Field of a Moving Positive Charge The right-hand hand rule for finding the direction of B-field due to a moving gpositive charge is similar to the rule used for a current carrying wire. Note that t the component of parallel to the line of motion is zero. Physics 201: Lecture 1, Pg 17

18 Currents Create B-Fields Magnitude: I B 0 I 2 r Tm / A 7 B r Current I OUT r = distance from wire Direction of B-field from current carrying wire: Curly right hand rule Lines of B Thumb along current I, direction of B along curled fingers. Physics 201: Lecture 1, Pg 18

19 Clicker Question 2: What is the current direction in the wire of the figure? A. Upward. B. Downward. Physics 201: Lecture 1, Pg 19

20 Clicker Question 3: A long, straight wire extends into and out of the screen. The current in the wire is A. Into the screen. B. Out of the screen. C. There is no current in the wire. D. Not enough info to tell the direction. Physics 201: Lecture 1, Pg 20

21 Clicker Question 4: What is the direction of the magnetic field at the position of the dot? A. Into the screen. B. Out of the screen. C. Up. D. Down. E. Left. - Physics 201: Lecture 1, Pg 21

Clicker Question 5: The wires are located at (-2,0) meters

What is B y, the y component of the magnetic field at the

22 Clicker Question 5: The wires are located at (-2,0) meters and (2,0) meters. The former carries 3 A coming out from the sheet of the paper, and the latter 3 A going into it. What is B y, the y component of the magnetic field at the origin? (a) B =-6-7 y 10 T (b) B y = T (c) B y = 0 T (d) B y = T (e) B y = T Physics 201: Lecture 1, Pg 22

23 Clicker Question 6: Two long wires carry equal but opposite current. What is the direction of the magnetic field above and midway between the two wires carrying current (point x)? A. Left B. Right C. Up D. Down E. Zero X x Physics 201: Lecture 1, Pg 23

(c) At which point is the total magnetic field the strongest?

24 Clicker Question 7: A current-carrying carrying wire passes through a uniform external magnetic field, as shown above. (c) At which point is the total magnetic field the strongest? (a) A (b) B (c) C (d) D (e) The total magnetic field strength is uniform throughout. Physics 201: Lecture 1, Pg 24

25 Clicker Question 8: The magnetic field at point P is zero. What are the magnitude and direction of the current in the lower wire? A. 10 A to the right. B. 5 A to the right. C. 2.5 A to the right. D. 10 A to the left. E. 5 A to the left. Physics 201: Lecture 1, Pg 25

What is the direction of the magnetic field at A?

26 Clicker Question 9: A square loop of wire carries a current I. Current is flowing in a counterclockwise direction as shown. What is the direction of the magnetic field at A? (a) (b) (c) into the page out of the page the B-field is zero Physics 201: Lecture 1, Pg 26

Physics 202: Lecture 8, Pg 1

Physics 202: Lecture 8, Pg 1 Physics 132: Lecture e 18 Elements of Physics II Agenda for Today Magnets and the Magnetic Field Magnetic fields caused by charged particles B-field from a current-carrying carrying wire Magnetic fields