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Shouldn't k appear in the expression on the right side? How did you get the power of -4? Did you take into account that the charges are in micro Coulombs? Otherwise, your approach looks right.Uf = k(7*(-5) + 7(-4) + (-5)*(-4))/0.1 = -4.3*10^-4
I aciddently left out that i multiplied by 10^-6 for the product of q's.The provided answer choices seem out of line for the given problem statement, but your calculated answer is also rather suspect. How did you determine the order of magnitude of the results? What value did you use for ##k##?
I get a different result on the order of a few Joules. Maybe check your arithmetic?I aciddently left out that i multiplied by 10^-6 for the product of q's.
But the answer is right.
Thanks, i entered the E-6 wrong! answer is -150e-12I get a different result on the order of a few Joules. Maybe check your arithmetic?
Hi, gneill. Apparently they don't want you to substitue a value for ##k##. Thus,##U_o = 8.988 \times 10^9~\frac{V~m}{C}\left( \frac{7\times10^-6~C \cdot (-4\times 10^-6~C)}{0.1~m} \right)##
Hmm. Okay, I wasn't expecting that. When I see kJ I immediately think kilo-Joules. It seems to me a bit odd to expect students to know that they need not invoke the relevant constant values.Hi, gneill. Apparently they don't want you to substitue a value for ##k##. Thus,
##U_o = k \left( \frac{7\times10^-6~C \cdot (-4\times 10^-6~C)}{0.1~m} \right) = k~ \left(-280 \times10^{-12} \, C^2/m \right) = -280 \times10^{-12}~ k~ J##.
Here, the ##k## is Coulomb's constant (even in the final expression). The units for ##k## have been absorbed into ##J## in the last step. This is an awkward way to express the answer, but I guess they didn't want the student to bother with looking up the value of ##k##.
Yes, it threw me off at first. In the problem statement, it says, "answer in terms of k = 1/(4πε0)." It could have been clearer as what was meant here.Hmm. Okay, I wasn't expecting that. When I see kJ I immediately think kilo-Joules. It seems to me a bit odd to expect students to know that they need not invoke the relevant constant values.