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Transcript
Bell Ringer Proton and Electron A proton and an electron are held 1) proton apart a distance of 1 m and then 2) electron released. Which particle has the 3) both the same larger acceleration at any one moment? p e Bell Ringer Proton and Electron A proton and an electron are held 1) proton apart a distance of 1 m and then 2) electron released. Which particle has the 3) both the same larger acceleration at any one moment? p The two particles feel the same force. Since F = ma, the particle with the smaller mass will have the larger acceleration. This would be the electron. e Q1Q 2 Fk r2 Electric Charge and Electric Field © 2014 Pearson Education, Inc. The Electric Field • According to Michael Faraday, an electric field extends outward from every charge and permeates all of space. • If a second charge (Q2) is placed near the first charge, it feels a force exerted by the electric field that is there. • The electric field at point P is considered to interact directly with charge Q2 to produce the force on Q2. © 2014 Pearson Education, Inc. The Electric Field • The electric field surrounding a charge can be determined by measuring the force on a small positive test charge which is at rest. • Test charge: A so small charge that the force it exerts does not significantly affect the charges that create the field. The electric field is the force on a small charge, divided by the charge: © 2014 Pearson Education, Inc. The Electric Field • The electric field at any point in space is a vector whose direction is the direction of the force on a tiny positive test charge at that point. • The magnitude is the force per unit charge. • SI units N/C • If q is positive, 𝑭 and 𝑬 point in the same direction. If q is negative, 𝑭 and 𝑬 point in opposite directions. © 2014 Pearson Education, Inc. The Electric Field For a point charge: The Electric Field Problem solving in electrostatics: electric forces and electric fields 1. Draw a diagram; show all charges, with signs, and electric fields and forces with directions 2. Calculate forces using Coulomb’s law 3. Add forces vectorially to get result © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Electric Field Lines • Since the electric field is a vector, it is sometimes referred to as a vector field • The electric field can be represented by field lines. These lines start on a positive charge and end on a negative charge. © 2014 Pearson Education, Inc. Electric Field Lines • The number of field lines starting (ending) on a positive (negative) charge is proportional to the magnitude of the charge. • The electric field is stronger where the field lines are closer together. • The direction of the electric field at any point is tangent to the field line at that point. Electric dipole: two equal charges, opposite in sign: © 2014 Pearson Education, Inc. Electric Field Lines Electric field lines for two equal positive charges • Unequal charges –Q, and +2Q. • Note that twice as many lines leave +2Q as enter –Q (number of lines is proportional to magnitude of Q). © 2014 Pearson Education, Inc. Electric Field Lines The electric field between two closely spaced, oppositely charged parallel plates is constant. © 2014 Pearson Education, Inc. Electric Field Lines Summary of field lines: 1. Field lines indicate the direction of the field; the field is tangent to the line. 2. The magnitude of the field is proportional to the density of the lines. 3. Field lines start on positive charges and end on negative charges; the number is proportional to the magnitude of the charge. © 2014 Pearson Education, Inc.