Magnetism Slide 2 / 49 Magnetic Material Very few materials - PowerPoint PPT Presentation

Slide 1 / 49 Magnetism

Slide 2 / 49 Magnetic Material Very few materials exhibit strong magnetism. These materials are called ferromagnetic. Examples include iron, cobalt, nickel, and gadolinium.

Slide 3 / 49 Magnets Magnets have two ends – poles – called north and south. Like poles repel; unlike poles attract. This attraction or repulsion is the magnetic force. These are examples of bar magnets.

Slide 4 / 49 Magnetic Poles Since magnets have two poles they are said to be dipoles. No magnet with a monopole has ever been found, therefore, when a magnet is cut in half, the two resulting magnets both have two poles.

Slide 5 / 49 Magnetic Fields Magnetic fields can be visualized using magnetic field lines, which are always closed loops. Magnetic fields are always drawn coming out of the north pole and going into the south pole. The more lines per unit area, the stronger the field.

Slide 6 / 49 The Earth's Magnetic Field The Earth’s magnetic field is similar to that of a bar magnet. Note two things: · the Earth’s “North Pole” is really a south magnetic pole as the north ends of magnets are attracted to it · the Earth's poles are not located along the rotation axis

Slide 7 / 49 Uniform Magnetic Fields A uniform magnetic field is constant in magnitude and direction. How can we create a uniform magnetic field? Aligning the opposite poles of two bar magnets will create a field which is almost uniform. Which areas in the diagram are non-uniform?

Slide 8 / 49 Definition of B The magnetic field is often expressed as B. The field is a vector and has both magnitude and direction. Often the magnetic field will be referred to as a "B-field". The unit of B is the tesla, T. 1 T = 1 N A m Another unit sometimes used: the gauss (G). 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 Electric Currents Produce Magnetic Fields Experiment shows that an electric current produces a magnetic field.

Slide 10 / 49 Electric Currents Produce Magnetic Fields 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 Direction of Magnetic Fields Because we need three dimensions to describe magnetic field and our paper is essentially two dimensional, we need to represent the third dimension somehow. We have left / right : Up / down : What is the third dimension?

Slide 12 / 49 Magnetic Fields Picture the field line like an arrow. The head of the arrow is the direction of the field. If the magnetic field is into the page, you will see the tail of the arrow. If the magnetic field is out of the page, you will see the front of the arrow.

Slide 13 / 49 Which diagram correctly shows 1 the magnetic field (red) around a current carrying wire (blue)? . . . . . . . . . . . A . . . . . . . . . . . C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Slide 14 / 49 2 Which diagram correctly shows the magnetic field (red) around a current carrying wire (blue)? C A . . . . . . . . . . . D B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D B C A carrying wire (red)? (black) around a current shows the magnetic field Which diagram correctly 3 Slide 15 / 49

Slide 16 / 49 4 Which diagram correctly shows the magnetic field inside and outside a current carrying loop of wire? . . . . . . . . . . . A x x x x x x x x x C . . . . . . . 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 x x x x x x x x D B . . . . . . . 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 5 Which diagram correctly shows the magnetic field around a current carrying wire? A B . . . . . . . . . . . . . . . . . . . . . . E . . . . . . . . . . . C D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Slide 18 / 49 6 Which diagram correctly shows the magnetic field around a current carrying wire? . . . . . . . . . . . B A . . . . . . . . . . . . . . . . . . . . . . C D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Slide 19 / 49 Force on an Electric Current in a Magnetic Field; Definition of B A magnet exerts a force on a current-carrying wire. The direction of the force is given by another different right-hand rule, we will call this the right-arm rule to avoid confusion.

Slide 20 / 49 Force on an Electric Current in a Magnetic Field; Definition of B The force on the wire depends on the current, the length of the wire, the magnetic field, and its orientation. F B = I L B sin θ 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 Force on an Electric Current in a Magnetic Field; Definition of B As you can see from the equation, the magnetic force depends on the angle the magnetic field makes with the current. F B = I L B sin θ The force is the greatest when the magnetic field is perpendicular the the current and zero when it is parallel to the current.

Slide 22 / 49 7 A wire carries a current of 2 A in a direction perpendicular to a 0.3 T magnetic field. What is the magnitude of the magnetic force acting on the 0.5 m long wire? 0.8 N A 0.5 N B 0.3 N C 0.1 N D 1.23 N E

Slide 23 / 49 8 A uniform magnetic field exerts a maximum force of 20 mN on a 0.25 m long wire, carrying a current of 2 A. What is the strength of the magnetic field? 0.1 T A 0.2 T B 0.3 T C 0.4 T D 0.5 T E

Slide 24 / 49 9 A 0.05 N force acts on a 10 cm wire as a result of it being located in a 0.3 T, perpendicularly oriented, magnetic field. What is the electric current through the wire? 1.67 A A 1.25 A B 2.13 A C 3.95 A D 3.32 A E

Slide 25 / 49 Force on an Electric Current in a Magnetic Field; Definition of B 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 magnetic field. All three vectors are now perpendicular

Slide 26 / 49 What is the direction of the force 10 on the current carrying wire (green) in the magnetic field (red)? G zero D F A B C E

Slide 27 / 49 What is the direction of the force 11 on the current carrying wire (green) in the magnetic field (red)? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G zero D F A B C E

Slide 28 / 49 What is the direction of the force 12 on the current carrying wire (green) in the magnetic field (red)? G zero D F A B C E

Slide 29 / 49 What is the direction of the force 13 on the current carrying wire (green) in the magnetic field (red)? G zero D F A B C E

Slide 30 / 49 What is the direction of the force 14 on the current carrying wire (green) in the magnetic field (red)? G zero D F A B C E

Slide 31 / 49 Force on Electric Charge Moving in a Magnetic Field The force on a moving charge is related to the force on a current: F = qvB sin θ Once again, the direction is given by a right-arm rule.

Slide 32 / 49 Force on a Moving Charge v (velocity) B v (velocity)

Slide 33 / 49 Force on a Moving Charge F F v (velocity) B v (velocity) F For a negative charge, negate the force.