Magnetism Slide 3 / 49 Slide 4 / 49 Magnets Magnetic Poles Since - PDF document

Slide 1 / 49 Slide 2 / 49 Magnetic Material Very few materials exhibit strong magnetism. These materials are called ferromagnetic. Examples include iron, cobalt, nickel, and gadolinium. Magnetism Slide 3 / 49 Slide 4 / 49 Magnets Magnetic Poles Since magnets have two poles they are said to be dipoles. Magnets have two No magnet with a monopole has ever been found, ends – poles – called therefore, when a magnet is cut in half, the two resulting north and south. magnets both have two poles. Like poles repel; unlike poles attract. This attraction or repulsion is the magnetic force. These are examples of bar magnets. Slide 5 / 49 Slide 6 / 49 Magnetic Fields The Earth's Magnetic Field Magnetic fields can be visualized using magnetic field The Earth’s magnetic field is similar to that of a bar magnet. lines, which are always closed loops. Note two things: · the Earth’s “North Magnetic fields are always Pole” is really a south drawn coming out of the magnetic pole as the north pole and going into north ends of magnets the south pole. are attracted to it The more lines per unit · the Earth's poles are area, the stronger the field. not located along the rotation axis

Slide 7 / 49 Slide 8 / 49 Definition of B Uniform Magnetic Fields A uniform magnetic field is constant in magnitude and direction. The magnetic field is often expressed as B. The field is a vector and has both magnitude and direction. How can we create a uniform magnetic field? Often the magnetic field will be referred to as a "B-field". Aligning the opposite The unit of B is the tesla, T. poles of two bar magnets will create a field which is 1 T = 1 N almost uniform. A m Which areas in the Another unit sometimes used: the gauss (G). diagram are non-uniform? 1 G = 10 -4 T To gain perspective, the weak magnetic field of the Earth at its surface is around 0.5 x 10 -4 T or simply 0.5 G. Slide 9 / 49 Slide 10 / 49 Electric Currents Produce Magnetic Fields Electric Currents Produce Magnetic Fields Experiment shows that an electric current produces a magnetic field. The direction of the field is given by a right-hand rule. First, orient your right hand thumb in the direction of the current... Then wrap your fingers in the direction of the B Field. Slide 11 / 49 Slide 12 / 49 Direction of Magnetic Fields Magnetic Fields Picture the field line like an arrow. The head of the arrow is the direction of the field. Because we need three dimensions to describe magnetic field and our paper is essentially two dimensional, we need to represent the third If the magnetic field is into the page, you will see the tail of the dimension somehow. arrow. We have left / right : Up / down : If the magnetic field is out of the page, you will see the front of the arrow. What is the third dimension?

Slide 13 / 49 Slide 14 / 49 Which diagram correctly shows 1 2 Which diagram correctly the magnetic field (red) around a shows the magnetic field current carrying wire (blue)? (red) around a current carrying wire (blue)? C A . . . . . . . . . . . A . . . . . . . . . . . C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B D B D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slide 15 / 49 Slide 16 / 49 4 Which diagram correctly 3 Which diagram correctly shows the magnetic field shows the magnetic field inside and outside a (black) around a current current carrying loop of carrying wire (red)? wire? . . . . . . . . . . . x x x x x x x x x A C . . . . . . . A C x x x x x x x x B D . . . . . x x x x x x x x x . . . . . x x x . . . . . . . . . . . . . . . . . . . . . . x x x x . . . . . . . x x . . . . . . . . . . . . . . . . . . . . . . x x x x x x x x . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x x x x x x x x x . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x x x x x x x x x B D . . . . . . . x x x x x x . . . . . . . . . . . . . . . . . . . x x x x . . . . . . . . . . . . . . . x x x x . . . . . . . . . . . x x x x x x . . . . . . . . . . . x x x x x x x x x Slide 17 / 49 Slide 18 / 49 6 Which diagram correctly 5 Which diagram correctly shows the magnetic field shows the magnetic field around a current carrying around a current carrying wire? wire? . . . . . . . . . . . A B A . . . . . . . . . . . B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E . . . . . . . . . . . . . . . . . . . . . . C D C D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Slide 19 / 49 Slide 20 / 49 Force on an Electric Current in a Magnetic Force on an Electric Current in a Magnetic Field; Definition of B Field; Definition of B A magnet exerts a force on a current-carrying wire. The force on the wire depends on the current, The direction of the force is given by another different the length of the wire, the magnetic field, and right-hand rule, we will call this the right-arm rule to its orientation. avoid confusion. F B = I L B sin # This equation defines the magnetic field, B. I is the current L is the length of wire B is the magnetic field Slide 21 / 49 Slide 22 / 49 Force on an Electric Current in a Magnetic 7 A wire carries a current of 2 A in a direction Field; Definition of B perpendicular to a 0.3 T magnetic field. What is the magnitude of the magnetic force acting on the 0.5 m long wire? As you can see from the equation, the magnetic force depends on the angle the 0.8 N A magnetic field makes with the current. 0.5 N B F B = I L B sin # 0.3 N C 0.1 N The force is the greatest when the magnetic field is D perpendicular the the current and zero when it is 1.23 N E parallel to the current. Slide 23 / 49 Slide 24 / 49 8 A uniform magnetic field exerts a maximum force 9 A 0.05 N force acts on a 10 cm wire as a result of it of 20 mN on a 0.25 m long wire, carrying a current being located in a 0.3 T, perpendicularly oriented, of 2 A. What is the strength of the magnetic field? magnetic field. What is the electric current through the wire? A 0.1 T 1.67 A A 0.2 T B 1.25 A B 0.3 T C C 2.13 A 0.4 T D 3.95 A D 0.5 T E 3.32 A E

Slide 25 / 49 Slide 26 / 49 Force on an Electric Current in a Magnetic What is the direction of the force 10 Field; Definition of B on the current carrying wire (green) in the magnetic field (red)? To make sure we have the right direction for B, we use the right-arm rule: Orient your arm in the direction of the current. Rotate your wrist until your thumb is in the direction of the force. Bend your fingers 90 o for the direction of the G zero D A B C E F magnetic field. All three vectors are now perpendicular Slide 27 / 49 Slide 28 / 49 What is the direction of the force What is the direction of the force 12 11 on the current carrying wire on the current carrying wire (green) in the magnetic field (red)? (green) in the magnetic field (red)? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G zero G zero D A B C D E F A B C E F Slide 29 / 49 Slide 30 / 49 What is the direction of the force What is the direction of the force 13 14 on the current carrying wire on the current carrying wire (green) in the magnetic field (red)? (green) in the magnetic field (red)? G zero G zero D F A B C E A B C D E F