Electricity and Magnetism: Understanding Waves, Charges, and Magnetic Fields - Prof. Brian, Study notes of Physics

Download Electricity and Magnetism: Understanding Waves, Charges, and Magnetic Fields - Prof. Brian and more Study notes Physics in PDF only on Docsity! Electricity and Magnetism Waves Wave – a disturbance that travels through a medium in such a way that energy travels through the medium but matter does not. All waves (sound waves, waves on a string, seismic waves, electromagnetic waves) have a vibrating object as their source. the compression and expansion of air produced sound waves a vibrating atom produces electromagnetic waves shaking the end of a string produces a wave Two types of waves: transverse waves - the medium is disturbed perpendicular to the direction of wave motion. example: waves on guitar string http://en.wikipedia.org/wiki/Transverse_wave longitudinal waves – medium is disturbed parallel to the direction of wave motion. example: sound waves http://en.wikipedia.org/wiki/Longitudinal_wave Water waves are a combination: http://www.kettering.edu/~drussell/Demos/waves/w avemotion.html Amplitude – (A)The maximum distance of the object of the object from the equilibrium position. In the absence of friction, the object oscillates between – A and + A. period – (T) the time it takes for the object to move through one complete cycle. For example if you pulled back a pendulum. The time it takes to swing to the other side and back. frequency (f) the number of complete vibrations per unit time. The reciprocal of the period. (f = 1/T) Waves have alternating maxima and minima separated by nodes. At a node there is no displacement. In a transverse wave there are crests and troughs. In a longitudinal wave there are regions of high density and low density. Interference of waves When two traveling waves meet, they pass through each other without changing. When they overlap, they add up. This is a superposition principle. You just have to add up the displacements of the individual waves point by point. see figure 8.8 When add up in phase (crest overlap with the other crest), they add up constructively. The resultant is larger in amplitude. When waves are out of phase, they add up destructively. The resultant is diminished. It is possible for two waves to cancel each other out. http://www.colorado.edu/physics/2000/applets/fo urier.html http://id.mind.net/~zona/mstm/physics/waves/inte rference/intrfrnc.html Reflection Reflection: When a wave encounters a change in medium the wave reflects, bounces back. Examples: string tied to a pole, light hitting a mirror, ocean wave reaching the shore. http://www.kettering.edu/~drussell/Demos/reflect/reflec t.html Refraction when a wave encounters a change in medium it also can get bet (refracted). This happens when the speed of the wave changes. examples: ocean waves when the depth of the water changes. light going from air to water. This is why a stick appears bent at the surface o the water http://upload.wikimedia.org/wikipedia/commons/b/b 9/Refraction-with-soda-straw.jpg SI unit of charge is the Coulomb (C) Positive charges are carried by protons. The protons are stuck in the nucleus of the atoms in material. Negative charges are carried by electrons. The electrons orbit around the nucleus. It is easy for atoms to share or pass around electrons. protons have charge of 1.6 x 10-19 C electrons have charge of - 1.6 x 10-19 C same magnitude of charge Proton has mass of about 1836 x mass of electron Objects typically are neutral. They have equal numbers of positive and negative charges. Objects are charged when there is a net positive charge (lost electrons) or a net negative charge (gained electrons) When a charge is built up, it is because electrons were transferred from one object to another. How charges interact The electric force is an example of a ‘field force’. Suppose charges A and B interact. Charge A produces an electric field. The electric field exerts the force on Charge B. (resourcefulphysics org)  Magnetic field lines Magnetic field lines go from the north pole of the magnet to the south pole. Then they follow the magnet back to the north pole. Magnetic field lines form closed loops. Earth’s Magnetic Field Due to the rotation of the Earth and the iron core inside it, the Earth has a magnetic field. The field is similar to that of a bar magnet. The north pole of a compass needle is a magnetic north pole. It is attracted to the geographic North Pole, which is a magnetic south pole (opposite magnetic poles attract). There is evidence that sometimes, for unknown reasons that the magnetic field of the Earth reverses itself. When certain magnetic rocks harden, they will align themselves with the magnetic field of the Earth. Sample show that rocks of different ages are aligned in different directions. http://science.nasa.gov/headlines/y2003/29dec _magneticfield.htm Magnetic force Magnetic field exert forces only on moving charges. Also the charge cannot be moving parallel or anti-parallel to the magnetic field. There has to be some angle in between the direction of motion and the field.