HR22 - Chapter 22 Electric Fields (22-1) In this chapter we...

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Chapter 22 Electric Fields In this chapter we will introduce the concept of an electric field . As long as charges are stationary Coulomb’s law describes adequately the forces among charges. If the charges are not stationary we must use an alternative approach by introducing the notion of an electric field In connection with the electric field, the following topics will be covered: -Calculate the electric field generated by a point charge . -Using the principle of superposition determine the electric field created by a collection of point charges as well as continuous charge distributions. -Assuming that the electric field at a point P is known, calculate the electric force on any charge placed at P -Define the notion of an electric dipole . Determine the net force , and the net torque , exerted on an electric dipole by a uniform electric field, as well as the dipole potential energy . (22-1)
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In chapter 21 we discussed Coulomb’s law that gives the force between two point charges. The law is written in such as way as to imply that q 2 acts on at a distance r instantaneously ( action at a distance ) Electric interactions propagate in empty space with a large but finite speed ( c = 3 × 10 8 m/s). In order to take into account correctly the finite speed at which these interactions propagate we have to abandon the action at a distance point of view and still be able to explain how does q 1 know about the presence of q 2 . The solution is to introduce the new concept of an electric field vector as follows: Point charge q 1 does not exert a force directly on q 2 . Instead, q 1 creates in its vicinity an electric field that exerts a force on q 2 according to the scheme: 12 2 1 4 o qq F r πε = →→ generates electric field exert charg s a f orce e on G G G 1 2 F E q E q (22-2)
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Ole Roemer 1644-1710 Hippolyte Fizeau 1819-1896
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Consider the positively charged rod shown in the figure For every point P in the vicinity of the rod we define the electric field vector as follows: We place E Definition of the electric field vector 1. G a small test charge at point P. We measure the electrostatic force exerted on by the charged rod. We define the electric field vector at point P a s : o o o F E q q Fq E = 2. 3 po SI U . sitive G G G G From the definition it follows that is parallel to We assume that the test charge is small enough so that its presence at point P does not affect the charge distributi on on t o EF q nits : N/ No : C te GG he rod and thus alters the electric field vector we are trying to determine. E G o F E q = G G (22-3)
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q q o r P E G 2 1 4 o q E r πε = Consider the positive charge shown in the figure. At point P a distance from we place the test charge . The force exerted on by is equal to: o oo q r qq Electric field generated by a point charge 2 22 1 4 11 44 The magnitude of is a number In terms of direction, points radially as shown in then figure.
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HR22 - Chapter 22 Electric Fields (22-1) In this chapter we...

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