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Unformatted text preview: SOLUTIONS WM1 : Q21-9) Gravitation is always attractive as masses are always >0. The Coulomb force can be attractive (for oppositely charged objects, + and -) or repulsive (for equal signs, + and + or and -). Compare the size of the two forces for two unit charges: A mass of 1 kg and an electric charge of 1C, both separated by a distance of r=1m: N m C C C Nm r qQ k F N m kg kg kg Nm r mM G F e G 9 2 2 2 9 2 11 2 2 2 11 2 10 . 9 ) 1 ( 1 1 / 10 . 9 10 67 . 6 ) 1 ( 1 1 / 10 67 . 6 = = = = = = The electric force is much larger. Keep in mind, however, that while every day objects weight on the order of a couple kg, they rarely carry as much charge as 1 C (typical values are 10-6 C). Q21-10) Gravitational forces are small unless the masses are large (see above example). Electric forces are small unless the charges are large. Most objects we encounter in every day life are neutral or just weekly charged. Q21-18) Positive charges follow the field lines (from + to -) and are repelled by the two charges in the Figure. Thus, A and B will experience a force along the field lines and away from the charges. C will feel no net force as it sits symmetrically between the two charges. The strongest field is closest to the charge (Coulomb force!), so: A, B, C. Also note that the density of field lines is higher near A than it is near B again an indication that the field is stronger near A....
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