lecture6

Ke q1q2 r12final r12initial physics

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Unformatted text preview: the test charge. D)  A) and C) are true E)  B) and C) are true Physics 2B - Summer Session 2 - C. Palmer August 12, 2013 24 +y   Uniform electric field   POSITIVELY charged object in the field   Moved from 1 to 2   What the sign of the change in potential energy? +Q +Q 1 2 +x A)  Positive B)  Negative Physics 2B - Summer Session 2 - C. Palmer August 12, 2013 25 +y   Uniform electric field   POSITIVELY charged object in the field   Moved from 1 to 2   What the sign of the change in potential energy? +Q +Q 1 2 +x A)  Positive B)  Negative Physics 2B - Summer Session 2 - C. Palmer August 12, 2013 26 +y   Uniform electric field   NEGATIVELY charged object in the field   Moved from 1 to 2   What the sign of the change in potential energy? - Q - Q 1 2 +x A)  Positive B)  Negative Physics 2B - Summer Session 2 - C. Palmer August 12, 2013 27 +y   Uniform electric field   NEGATIVELY charged object in the field   Moved from 1 to 2   What the sign of the change in potential energy? - Q - Q 1 2 +x A)  Positive B)  Negative   If the charge sign is <0, the opposite is true!   Sign dependent! Physics 2B - Summer Session 2 - C. Palmer August 12, 2013 28   Electric potential (AKA Voltage, V) is independent of the test charge:   Units: Volts U Elec V= q0 Joules N*m [ V] = C = C Physics 2B - Summer Session 2 - C. Palmer August 12, 2013 29   More useful quantity is the electric potential difference (ΔV): ΔVAB ΔU Elec = VB − VA = q0   For a point (or spherical) charges: ΔVAB ⎛ 1 1⎞ ΔU Elec = = ke q ⎜ − ⎟ q0 ⎝ rB rA ⎠ Physics 2B - Summer Session 2 - C. Palmer August 12, 2013 30 +y   Uniform electric field   NEGATIVE charge   Moved from 1 to 2 - Q - Q 1 2 +x   Which is true? A)  Both the electric potential and electric potential energy increased. B)  Both the electric potential and electric potential energy decreased. C)  The electric potential increased and the electric potential energy decreased. D)  The electric potential decreased and the electric potential energy increased. Physics 2B - Summer Session 2 - C. Palmer ΔVAB August 12, 2013 ΔU Elec = q0 31 +y   Uniform electric field   NEGATIVE charge   Moved from 1 to 2 - Q - Q 1 2 +x   Which is true? D) The electric potential decreased and the electric potential energy increased. ΔVAB The voltage difference tells you if the charge is moving WITH the field. The electric force is pointing to the left and so it takes work to move the charge to 2. Physics 2B - Summer Session 2 - C. Palmer August 12, 2013 ΔU Elec = q0 32   It is customary for the potential at infinity to be zero.   For an single charge, qi, which is at ri, the voltage at r is: ei i i   For a collection of charges, the total voltage is the sum of the parts: kq V (r ) = r −r ke qi V (r ) = ∑ i r −r i Physics 2B - Summer Session 2 - C. Palmer August 12, 2013 33   Electric potential energy difference ΔU = − ∫ final initial final F ⋅ ds = − q0 ∫ E ⋅ ds initial   Electric potential difference final ΔU ΔV = = −∫ E ⋅ ds initial q0   For conservative forces (like electromagnetic forces) these integrals are independent of the path taken/chosen. Physics 2B - Summer Session 2 - C. Palmer August 12, 2013 34...
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This note was uploaded on 09/10/2013 for the course PHYS 2B 2b taught by Professor Hirsch during the Summer '10 term at UCSD.

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