05_02_FR_Efields_V

05_02_FR_Efields_V - Today5/2 o Questions? o E-Fields and...

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Today5/2 o Questions? o E-Fields and Potential Difference (review)
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Many Charge Example E = kQ s /r 2 +q o -3q o +4q o -2q o r o What is the force on +4q o ? (direction also) Find the field at +4q o due to the “other” charges. These are the “source” charges. E +q = kQ s /r 2 = k(q o )/(r o ) 2 = E o Direction? Away from + so right E -2q = kQ s /r 2 = k(2q o )/( 2r o ) 2 = E o = k2q o /2(r o ) 2 = k(q o )/(r o ) 2 E -3q = kQ s /r 2 = k(3q o )/(r o ) 2 = 3E o = 3kq o /(r o ) 2
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Many Charge Example E = kQ s /r 2 +q o -3q o +4q o -2q o r o What is the force on +4q o ? (direction also) E +q = E o E -2q = E o E -3q = 3E o Now add E-Field vectors E x = E +q,x + E -2q,x E x = E o - E o sin45 E x = 0.29E o E y = E -2q,y + E -3q,y E y = E o sin45 + 3E o E y = 3.7E o θ = tan -1 (0.29/3.7) = 4.5° E total = 3.7 2 + 0.29 2 E o = 3.71E o F on +4q = (4q o )3.71E o = 14.84q o E o F on +4q = 14.84kq o 2 /r o 2 same direction as E total
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What direction would the force point on a small positive charge at 1,2,3? Compare the size of the force exerted on a small charge at a. 1 and 2 b. 2 and 3 The test charge is doubled when at 2 a. how does the force change?
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05_02_FR_Efields_V - Today5/2 o Questions? o E-Fields and...

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