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Transcript
Electric Field Force at a Distance Electric charges act on each other at a distance. + • Empty space between charges • Net force on a charge - kQ2 F- F- 2 r+ + kQ F+ F+ 2 r+ + 2 FE F+ + F- Force has to act on another charge. • Specific charge for a specific value of force • Value needed for force magnitude Test Charge Introduce an arbitrary positive q + - q test charge of magnitude q. The force can be calculated from the charges using Coulomb’s law. The test charge can be moved to other points in space to find the force. Field Strength The electric field is the electric q force per unit test charge. • Measured in N/C + - q The field has a value at every FE E q point in space. • Independent of force value • Parallel to force direction Point Charge The field of a point charge is just like the vector force that would act at a point near the charge. • Magnitude comes from Coulomb’s law kQ E 2 r Superposition Two point charges of +10. nC are separated by 8.0 mm. Find the electric field at a point 4.0 mm perpendicular to the midpoint of the charges. E2 q1 The electric fields of point charges add together. • Vector addition The x-components cancel out. The y-components are equal. E y1 + E y 2 2 E1 kq sin 45 r2 r 2 x2 + y2 q2 • k = 8.99 x 109 Nm2/C2 • q = 10 x 10-9 C • x = y = 4.0 x 10-3 m Answer: E = 4.0 x 106 N/C Screened Charge Two oppositely charged point Only the x components matter. charges of 10. nC are separated by 2.0 mm. Find the electric field at a point 20.0 mm from the midpoint along the line of the charges on the positive side. - kq E x1 r + d 2 Ex 2 • • • • q1 q2 E1 E2 + kq r - d 2 k = 8.99 x 109 Nm2/C2 q = 10 x 10-9 C r = 20.0 x 10-3 m d = 1.0 x 10-3 m Answer: E = 4.5 x 104 N/C Environmental Fields Electric fields exist from charges in the environment. Distance affects the field strength from point sources. Typical field strengths in N/C • Radio waves: 0.1 • Living room: 3 • 30 cm from a stereo: 90 • Fair weather sky: 150 • Shower (water on): 800 • Thunderstorm: 10,000 • Cell membrane: 10,000,000 next