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Section 23.4 : The Electric Field

Learn about the electric force as a field force, its effects on objects, and the concept of an electric field. Understand the directions of electric fields and calculate forces using Coulomb's Law.

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Section 23.4 : The Electric Field

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  1. Section 23.4: The Electric Field

  2. Section 23.4: The Electric Field Introduction • The Electric Force is a Field Force. • Field forces can act through all space. • Their effects are produced even with • no physical contact between objects. • Faraday developed the concept of a • field in terms electric fields.

  3. The Electric Field • The Electric Field is defined as the force on a • small positive charge q, (a “Test Charge”) divided • by the magnitude of the charge:  • The direction of the • Electric Field due to • a positive charge +Q • is radially outward • from the charge.

  4. An Electric Fieldsurrounds every charge. • An Electric Fieldexists • in the region of space • around a charged object. • This charged object is the • Source Charge. • When another charged • object, the • Test Charge, • enters this electric field, • an electric force acts on it.

  5. The Electric Field due to a single point chargeQ is: In terms of ε0,the permittivity of free space:

  6. The direction of E is that of the force on a positive test charge. • We can also say that an electric field exists at a • point if a test charge at that point experiences an • electric force. • The SI units of E • are N/C (or V/m).

  7. The Force on a positivepoint • chargeq in an electric field E: Electric Field E Force F on a positive point charge q. • This is valid for a point • charge only. If q is negative, • the Force F & the Electric • Field E point in the opposite • direction than when qis • positive. Force F on a negative point charge -q.

  8. Electric Field, Vector Form Remember that Coulomb’s Law, between the source and test charges can be expressed as Then, the electric field will be

  9. More On Electric Field Direction • If a Charge q is Positive, the Force is • Directed Away fromq. • (Fig. a) • The Electric Field Direction is • Also Away from a Positive Source Charge. • (Fig. b)

  10. If a Charge q is Negative, the Force is • Directed Towardsq. • (Fig. c) • The Electric Field Direction is • Also Towards a Negative Source Charge. • (Fig. b)

  11. Electric Fields from Multiple Charges • At any point P, the total electric • field E due to a group of source • charges qiequals • The Vector Sumof the electric fields of all of the charges:

  12. Problem Solving in Electrostatics: Electric Forces & Electric Fields Sketcha Diagramshowing all charges, with signs, & electric fields & forces with directions. CalculateForces using Coulomb’s Law. Add ForcesVectorially to get the result. CheckYour Answer!

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