Electric Fields PDF

Electric Fields Goals Describe an electric field. Calculate the electric field due to the presence of a collection of charges. Visualize and interpret electric fields DEFINITION OF ELECTRIC FIELD A charged body produces an electric field in the space around it. DEFINITION OF...

Electric Fields Goals Describe an electric field. Calculate the electric field due to the presence of a collection of charges. Visualize and interpret electric fields DEFINITION OF ELECTRIC FIELD A charged body produces an electric field in the space around it. DEFINITION OF ELECTRIC FIELD An electric field is an imaginary space surrounding a charged particle where any charge placed in the region experiences an electric force. A small test charge q0 can be used to find out if an electric field is present. DEFINITION OF ELECTRIC FIELD The electric field that exists at a point is the electrostatic force experienced by a small test charge placed at that point divided by the charge itself: SI Units of Electric Field: newton per coulomb (N/C) DEFINITION OF ELECTRIC FIELD – E fields are VECTOR fields – and solutions to problems require magnitude, direction, and units. DEFINITION OF ELECTRIC FIELD – E fields are VECTOR fields – and solutions to problems require magnitude, direction, and units. The direction of E is the same as the direction of F when the test charge used is positive. If the test charge is negative, the direction of E is opposite The direction of F. DEFINITION OF ELECTRIC FIELD The positive charge experiences a force which is the vector sum of the forces exerted by the charges on the rod and the two spheres. This test charge should have a small magnitude so it doesn’t affect the other charge. It is the surrounding charges that create the electric field at a given point. The total electric field at a point is the vector sum of the fields due to all the charges present. Example The Electric Field of a Point Charge The isolated point charge of q=+15μC is in a vacuum. The test charge is 0.20m to the right and has a charge qo=0.8μC. Determine the electric field at point P.   F E qo q1 q2 F k r2  q qo F k r2  8.99 10 9   N m 2 C 2 0.80 10 6 C 15 10 6 C  2.7 N 0.20m 2 F 2.7 N 6 E  3.4 10 NC qo 0.80 10 -6 C  F q qo 1 E k 2 qo r qo The electric field does not depend on the test charge. q Point charge q: E k 2 r Example An Electric Field Leads to a Force The charges on the two metal spheres and the ebonite rod create an electric field at the spot indicated. The field has a magnitude of 2.0 N/C. Determine the force on the charges in (a) and (b) (a) F  qo E 2.0 N C 18.0 10  8 C  36 10  8 N (b) F  qo E 2.0 N C 24.0 10 8 C  48 10  8 N  Electric field of a point charge – E fields from positive charges point AWAY from the charge – E fields point in the direction a POSITIVE test charge would move! Electric Field Lines Electric field lines or lines of force provide a map of the electric field in the space surrounding electric charges. – E fields point TOWARDS a negative charge – E fields point in the direction a POSITIVE test charge would move! Electric Field Lines Electric field lines are always directed away from positive charges and toward negative charges. Electric Field Lines The number of lines leaving a positive charge or entering a negative charge is proportional to the magnitude of the charge. Electric Field Lines Summary The electric field at a point is the electrostatic force experienced by a small test charge divided by the charge itself. Electric field is a vector quantity and is in the same direction as the force for a positive charge. For a point charge, electric field is equal to q E k 2 r The direction of E is radially outward for a positive charge, and radially inward for a negative charge. References: Cutnell, Johnson, Young, & Staedler (2012).Cutnell & Johnson Physics, 10 th ed. John Wiley & Sons, Inc. Walker (2014). Halliday & Resnick Fundamentals of Physics 10th ed. Wiley & Sons,Inc. Young & Freedman (2016). University Physics 14th ed. Pearson Education, Inc/