This document contains detailed notes on Coulomb’s law, Newton’s law of universal gravitation and electric fields. Detailed explanations and examples are given on the following:
Laws and definitions
Formulae and units
Theory questions
Staggered and Multiple choice question examples
Fi...
, Electrostatics
Coulombs law
Coulomb's law: Two point charges in free space or air exert forces on each other. The force is
directly proportional to the product of the charges and inversely proportional to the square of
the distance between the charges
Formula: F: force of attraction between q1 & q2 (unit: N)
9 -2
F = k.q1.q2 k: coulomb’s constant, 9x10 N.m2.C
q: magnitude of charge (unit: C) NB! Don’t add positive/negative
r 2
r: distance between the charges (unit: m)
Other formulae:
Substitute charge magnitude only. Include signs! Used to determine
Direction determined by nature of charge (repel, attract). Qnew = (Q1+Q2)/2 new charges after contact
-19 How many electrons
Both objects experience the same force (Newton’s 3rd Law). Ne- = Q/qe qe: 1,6x10 C present on each sphere
Example Unit change Micro- (µC): 9x10
-6
-9
Staggered question
-2
Centi- (Cc): 9x10 Nano- (nC): 9x10
-3 -12
Milli- (Mc): 9x10 Pico- (pC): 9x10
Sphere X carries a charge of -3nC & sphere Y
carries a charge of +4nC. After contact, they Multiple choice question
separate to 48mm. Calculate the repulsive Two charges experience a force F when held
force experienced by X after contact with Y. a distance r apart. How would the force be
35°
affected if one charge is doubled, the other
Y
-
48mm - X charge is tripled and the distance is halved.
F = k.q1.q2 F= (1)(1)(1) = 1N
r
To determine q1 & 2
-3 + (+4) /2 = +0,5nC q2 after contact
2 (1)
F = k.q1.q2 Swap all values with 1 to be able to compare!
r 2
F= (1)(2)(3)
9 2
= (9x10 )(0,5x10-9)(0,5x10-9 ) (0,5)
-3 2 Convert to m!
(48x10 ) = 24N
-7
= 9,77x10 N repulsion, X moved to the right Answer - 1:24
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