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PHYA10H3 (30)
Lecture 1

# PHYA10H3 Lecture 1: EM(1)

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Department
Physics and Astrophysics
Course
PHYA10H3
Professor
Benj Hellie
Semester
Fall

Description
Electricity and Magnetism Experiments reveal a property of matter called electric charge that can result in the attraction or repulsion of objects. There are two types of elec tric charge, positive and negative. Most things around us are neutral. They contain an equal number of positive and negative charges. Rubbing one substance against another can transfer charges such that one object acquires a net or total positive charge and the other acquires a net or total negative charge. Two objects that both possess net positive or net negative charge repel one another. There is an attraction between two objects if one has net positive and the other net negative charge. The simplest expression for the 2 electric force between objects with net charges Q and q separa ted by a distance R is F=kQq/R , where k is a constant. Charles Coulomb (1736-1806) proposed this law and it has the same form as Newton’s Law of Gravitation. The net charge can be a positive or a negative number. Michael Faraday (1791-1867) envisioned that the electric force between charged objects is mediated by an electric field. The electric field is a quantity defined at each point in space. It can be used to determine the force F on an object wi th charge q according to F=qE, where E is the electric field at the position of the object. One way of representing an electric field is to draw field lines. For example, the electric field lines surrounding an isolated object with net positive or negative charge are and Some objects are called permanent magnets. They are able to attract or repel other materials even though they are electrically neutral. They have a north (N) and a south (S) pole. It is seen that if the north poles or the south poles of two magnets are brought clos e to one another, they repel. The magnets exhibit an attraction if the north pole of one and th e south pole of the other are in close proximity. The magnetic force between these objects is due to a magnetic field. As with electric fields, magnetic fields can be represented by field lines. Here are some of the field lines for a bar magnet. S N Suppose an object with ch arge q and velocity v is exposed to a magnetic field B. The magnetic force on the object due to B depends on the product of q, v, B, and the directions of v and B. The magnetic force on a charged object is more compli cated than the electric force on a charged object due to its dependence on the directions of v and B. Materials are made of atoms. The evidence for this statement will be examined later. Atoms consist of nuclei surrounded by el ectrons. The nuclei contain protons and neutrons. An electron, proton, and neutron possess one negative, one pos itive, and zero units of electric charge, respectively. Applying an electric field to a wire made of copper results in some of the electrons in the wire travelling on average in one direction. The motion of charged objects constitutes an electric current. Many experiments have uncovere d connections between electri c and magnetic phenomena. An electric current in a copper wire causes magnet s located near the wire to deflect from their original orientations when the electric current is zero. This shows that electric currents or moving charged objects produce magnetic fields. electric current As a bar magnet is moved towards a closed loop of copper wire, an electric current is generated in the wire. If the bar magnet is held fixed, then the electric current is zero. This demonstrates that a changing magnetic field creates an electric field. A changing magnetic field means that the magnitude and/or the directi
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