Physics 10. Lecture 24A. "When you feel bad about yourself you reverse your magnet and repel people." --S A Grafio

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1 Physics 10 Lecture 24A "When you feel bad about yourself you reverse your magnet and repel people." --S A Grafio

2 History of Magnets Magnets were discovered by early man. Asia Minor has region known as Magnesia where rocks would attract each other. Early researchers found that if you put a magnet on a string it would want to face a certain direction (i.e. towards the North Pole). Thus, magnets were used as compasses. Later on it was discovered that magnets will only attract iron or other very select metals (cobalt, nickel, etc.).

But north poles repel north poles and south poles repel south poles.

3 History of Magnets Researchers also found that magnets always came with two sides a north pole and a south pole. North poles attract south poles. But north poles repel north poles and south poles repel south poles. With the aid of compasses, magnetic field lines were mapped for simple shapes of magnets. It was defined that magnetic field lines exited the north end and entered the south end.

4 History of Magnets Magnetic field lines are actually easier to visualize than electric field lines. Just put some iron filings next to a magnet and they will align themselves with the magnetic field.

5 History of Magnets But it was later discovered that inside the physical magnet the magnetic field lines actually moved from south to north. This means that magnetic field lines (unlike electric field lines) formed an endless loop. If a particle were to follow a magnetic field line it would eventually come back to the same location.

6 History of Magnets Many similarities between magnetism (MG) and electrostatics (ES) were found: (ES): Two types of charges, + and. (MG): Two types of poles (north and south). (ES): Like charges repel, opposite charges attract. (MG): Like poles repel, opposite poles attract. (ES): Can move objects at a distance. (MG): Can also move objects at a distance.

7 History of Magnets There are also many differences between magnetism (MG) and electrostatics (ES): (ES): Can separate a + away from a. (MG): Can t isolate a north pole from a south pole. S N S N S N S N S N (ES): Field lines always start on + and end on -. (MG): Field lines have no end; endless loops.

8 History of Magnets In 1820, it was discovered that electric current produced a magnetic field. That led to an interesting question: What did moving electrical charges have to do with rocks in Magnesia? Everything as it turns out, since all magnetism is fundamentally caused by moving charges.

The iron atoms act like tiny magnets that appear to have north and south poles.

9 History of Magnets In iron, for example, the magnetic properties are caused by moving electrons. The moving electrons act as current, creating magnetic fields. The iron atoms act like tiny magnets that appear to have north and south poles. There are not really north and south poles there, only locations that appear to act that way. Poles are a man-made creation to explain magnetism.

10 Magnetic Fields The proper way to handle magnetic forces is to concentrate on magnetic fields. A magnetic field is located in a region of space surrounding a moving charge. This charge will also have an electric field surrounding it. A magnetic field is a vector quantity given by: The SI unit of the magnetic field, B is the Tesla. Earth s magnetic field is: 0.5x10 4 Teslas.

11 Magnetic Fields We will treat magnetic forces and fields how we treated electric forces and fields. We will use a two-step process: step (i): Moving charges, I, creates a B field. step (ii): Moving charge, q, experiences magnetic force from the field. exerts force on creates Moving charges (current I) [Amps] via RHR1 B field [Teslas] via RHR2 other moving charges q [Coul]

12 Magnetic Fields Note that a moving charge will not create a magnetic field, B, that will exert a force on itself. That violates Newton s Third Law (you need two objects to have a force). Let s examine the simplest case of a long, straight wire with current I. This current will create a magnetic field, B, surrounding it.

13 Magnetic Fields The direction of the magnetic field from the wire will be given by Right Hand Rule 1. Grasp the wire in your right hand. Point your thumb in the direction of the current I (positive charge flow). Your fingers will curl in the direction of the magnetic field.

14 Step 1 Many times you will be asked for the magnetic field at a given point. I Here you will just apply RHR1, but take the tangent of the curve at the given point for your answer. For example, if we were standing below the wire we would observe the magnetic field moving to the right. But if we were standing above the wire we would observe the magnetic field moving to the left.

Step 2 Then to find the direction of the magnetic force on a moving charge, q, due to magnetic field, B use RHR2. Note: this works for a positive charge, q, only.

15 Step 2 Then to find the direction of the magnetic force on a moving charge, q, due to magnetic field, B use RHR2. Note: this works for a positive charge, q, only. So, if the moving charge that is experiencing the force is a negative, just change the direction of the resulting magnetic force by 180 o. Basically, just flip the final force direction. Note: A charged particle at rest will not interact with a magnetic field.

Force on a Wire If the current in a wire is moving downward and the magnetic field is into the board apply Right Hand Rule 2: Your positive current velocity points down

16 Force on a Wire If the current in a wire is moving downward and the magnetic field is into the board apply Right Hand Rule 2: Your positive current velocity points down (thumb) Your magnetic field points into the board (forefinger). Middle finger then points to the right. Force is to the right since current is defined as positive charge moving.

17 Force on a Beam of Charges If a moving charged particle (either positive or negative) is placed in an external magnetic field it will feel a force. The force that this charged particle feels will be perpendicular to both the magnetic field and the particle s velocity. As in the picture, via RHR2 the electron would feel an upwards force. If it were a proton the force would instead be down.

The current in wire 1 moves up and the current in wire 2 moves up, as well.

18 Clicker Question 24A-1 Two current-carrying wires are exactly parallel to one another and both carry 1 Amp of current. The current in wire 1 moves up and the current in wire 2 moves up, as well. What is the direction of the magnetic field caused by wire 1 at the location of wire 2? A) Into the page. B) Out of the page. C) Up. D) To the left. E) To the right. Wire 1 I I Wire 2

19 For Next Time (FNT) Start reading Chapter 25. Start the homework for Chapters 23 and 24.

Elements of Physics II. Agenda for Today

Elements of Physics II. Agenda for Today 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