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Magnetism Magnets have 2 poles (north and south) Unlike poles attract Like poles repel Magnets create a MAGNETIC FIELD around them Magnetic Field A bar magnet has a magnetic field around it. This field is 3D in nature and often represented by lines LEAVING north and ENTERING south To define a magnetic field you need to understand the MAGNITUDE and DIRECTION We sometimes call the magnetic field a B-Field as the letter “B” is the SYMBOL for a magnetic field with the TESLA (T) as the unit. Magnetic Dipoles Electric Dipole & ε field Electric dipole consists of two equal but opposite charges separated by some distance such as in a polar molecule. Every magnet is a magnetic dipole. A bar magnet is a simple example. Note how the ε field due an electric dipole is just like the magnetic field (B field) of a bar magnet. Field lines emanate from the + or N pole and reenter the - or S pole. Although they look the same, they are different kinds of fields. ε fields affect any charge in the vicinity B field only affects moving charges. Magnetic Dipole & B field Magnetic Monopoles? Unlike electricity, there are no magnetic monopoles. If a bar magnet is split in two, it will create a new N&S pole at the split. Magnetic Dipole Electric monopole & ε field Lodestones were the first magnets discovered(?) by the Greeks. Lodestones consisted of iron oxide and was able to exert forces of attraction on small iron objects. Magnets are now mainly made of iron, nickel, colbalt, and gadolinium which are part of the Ferromagnetic materials. Ferromagnetism actually means the ability for certain materials to be magnetic or magnetized. Four of the most common permanent magnets are: Neodymium (NdFeB:neodymium-iron-boron) Samarium-Cobalt (SmCo) Alnicos (aluminum-nickel-cobalt) Ceramic/Ferrite unmagnetized state If a ferromagnetic object is placed in a magnetic field, some of the domains rotate to align themselves with the external magnetic field. Earth’s Magnetic Field Force Due to Magnetic Field The force exerted on a charged particle by a magnetic field is given by the vector cross product: F = force (N) q = charge on the particle (C) v = velocity of the particle relative to field (m/s) B = magnetic field strength measured in Teslas (T) We'll get to Telsas soon What is a Vector Cross Product? Let v1 = x1 i+y1 j+z1 k and v2 = x2 i+y2 j+z2 k. By definition, the cross product of these vectors is given by the following determinant. = (y1 z2 - y2 z1) i + (z1 x2 - x1 z2) j + (x1 y2 - x2 y1) k The cross product of two vectors is a vector itself that is to each of the original vectors. i, j, and k are the unit vectors pointing, along the positive x, y, and z axes, respectively. Right Hand Rule For vector cross products A quick way to determine the direction of a vector cross product is to use the right hand rule. To find vector a b, place the knife edge of your right hand (pinky side) along vector a and curl your hand toward vector b, making a fist. Your thumb then points in the direction of vector a b It can be proven that the magnitude of vector a is given by: |a b b| = ab·sinθ Where θ is the angle between vector a and vector b. θ Force Due to Magnetic Field You will use Units: 1 N = 1C(m/s)·(T) A magnetic field of one tesla is very powerful magnetic field. Sometimes it may be convenient to use the gauss, which is equal to 1/10,000 of a tesla. Earth’s magnetic field, at the surface, varies but has the strength of about one gauss. Magnetic Force on a moving charge If a MOVING CHARGE moves into a magnetic field it will experience a MAGNETIC FORCE. This deflection is 3D in nature. The conditions for the force are: Must have a magnetic field present Charge must be moving Charge must be positive or negative Charge must be moving PERPENDICULAR to the field. Magnetic Force on a moving charge perpendicular vectors This force is always perpendicular to both the magnetic field and velocity Direction of the magnetic force? Right Hand Rule #1 (RHR#1) RHR#1 determines the directions of magnetic force, velocity of the charge, and the magnetic field. Given any two of theses, the third can be found. Using your right-hand: point your index finger in the direction of the charge's velocity, v; point your middle finger in the direction of the magnetic field, B. Your thumb now points in the direction of the magnetic force, Fmagnetic. Use your left-hand if you have an electron Determine the direction of the unknown variable for a proton moving in the field using the coordinate axis given B = -x v = +y F= Determine the direction of the unknown variable for a proton moving in the field using the coordinate axis given B = +z v = +x F= Determine the direction of the unknown variable for a proton moving in the field using the coordinate axis given B = -z v = +y F= Determine the direction of the unknown variable for an electron moving in the field using the coordinate axis given B = +x v = +y F= Determine the direction of the unknown variable for an electron moving in the field using the coordinate axis given B= v = -x F = +y Determine the direction of the unknown variable for an electron moving in the field using the coordinate axis given B = +z v= F = +y Electromagnetism While demonstrating to students that the current passing through a wire produces heat, Danish professor Hans Christian Ørsted (1777–1851) noticed that the needle of a nearby compass deflected each time the circuit was switched on. Right hand rule #2 If your right thumb is pointing in the direction of conventional current, and you curl your fingers forward, your curled fingers point in the direction of the magnetic field lines. Magnetic Field of a Loop If you make a circular loop from a straight wire and run a current through the wire, the magnetic field will circle around each segment of the loop. The field lines inside the loop create a stronger magnetic field than those on the outside because they are closer together. Right-Hand Rule #2 for a Solenoid If you coil the fingers of your right hand around a solenoid in the direction of the conventional current, your thumb points in the direction of the magnetic field lines in the centre of the coil. Applying a current through a solenoid as described above causes the solenoid to become an electromagnet. Stronger electromagnets can be made using a solenoid with a magnetic material, such as iron, nickel, or cobalt, within the coil. Junk yard Scrap metal Speakers Door bell Do the questions on page 80, #1-9