Electrostatic forces between macroscopic objects.

In chemistry and physics courses we learn that atoms are electrically neutral because the number of electrons equals the number of protons (atomic number Z Z ) in an atom. In addition, we know that the charges of the proton and electron are equal in magnitude. As a result, macroscopic objects do not experience considerable electrostatic interactions. Have you ever considered what would happen if the magnitudes of the charges of the electron and proton were slightly different? Suppose that the charge of the proton differs in 1 % 1\% from the charge of the electron. That is, Q ( proton ) = Q ( electron ) ( 1 + 1 / 100 ) . Q(\mbox{proton})=|Q(\mbox{electron})|(1+1/100). Therefore the net electric charge on an atom with atomic number Z Z is Q N e t = Q ( proton ) Q ( electron ) = 0.01 Z Q ( electron ) Q_{Net}=Q(\mbox{proton})-|Q(\mbox{electron})|=0.01Z|Q(\mbox{electron})| .

In this case, what would be the magnitude of the force of interaction in Newtons between two small 1-gram copper balls separated by one meter? There are N = 9.5 × 1 0 21 N=9.5 \times 10^{21} atoms in one gram of copper and its atomic number is Z = 29 Z=29 . Hint: the answer's big... real big.

Details and assumptions

  • Q ( electron ) = e = 1.6 × 1 0 19 C |Q(\text{electron})|=e=1.6 \times 10^{-19}~\mbox{C}
  • k = 9 × 1 0 9 N m 2 / C 2 k=9\times 10^{9}~\mbox{N m}^{2}/\mbox{C}^{2}
  • Since the balls are small and spherical you may treat them as point charges and apply Coulomb's law.


The answer is 1.75E+15.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

6 solutions

Discussions for this problem are now closed

Jordan Osborn
Jul 30, 2013

Charge on each gram of copper is Q_{total} Q t o t a l = Q n e t N Q_{total} = Q_{net}N Q n e t = 0.01 29 1.6 1 0 19 C = 4.64 1 0 20 C Q_{net} = 0.01 * 29 * 1.6 * 10^{-19} C = 4.64 * 10^{-20} C Q t o t a l = 4.64 1 0 20 C 9.5 1 0 21 = 440.8 C Q_{total} = 4.64 * 10^{-20} C * 9.5 * 10^{21} = 440.8 C F = k ( Q t o t a l ) 2 r 2 F = \frac {k(Q_{total})^2} {r^{2}} F = 9 1 0 9 N m 2 C 2 ( 440.8 C ) 2 ( 1 m ) 2 F = \frac {9 * 10^{9} N m^{2} C^{-2} * (440.8 C)^2} {(1m)^{2}} F = 1.749 1 0 15 N F = 1.749 * 10^{15} N

Maedhros 777
Jul 29, 2013

The net charge on one atom of copper will be 0.01 Z Q ( e l e c t r o n ) = 0.01 ( 29 ) ( 1.6 1 0 19 ) = 4.64 1 0 20 C 0.01Z|Q(electron)| = 0.01(29)(1.6 * 10^{-19}) = 4.64 * 10^{-20} C .

Since there are N = 9.5 1 0 21 N = 9.5 * 10^{21} atoms in one gram of copper, each ball has a net charge of ( 9.5 1 0 21 ) ( 4.64 1 0 20 ) = 440.8 C (9.5 * 10^{21})(4.64 * 10^{-20}) = 440.8 C .

Now Coulomb's law may be applied: F = k q 1 q 2 / r 2 = ( 9 1 0 9 ) ( 440.8 ) ( 440.8 ) / ( 1 2 ) = 1.75 1 0 15 N F = k q_{1}q_{2} / r^{2} = (9 * 10^{9})(440.8)(440.8) / (1^{2}) = 1.75 * 10^{15} N .

The number of atoms in 1 1 g g of copper = N = N and each copper atom has Z Z electrons each with 0.01 e 0.01e extra charge, therefore, the total extra charge Q Q of 1 g 1\space g of copper is given by:

Q = 0.01 e N Z = 0.01 × 1.6 × 1 0 19 × 9.5 × 1 0 21 × 29 = 440.8 C Q = 0.01eNZ = 0.01 \times 1.6\times 10^{-19}\times 9.5\times 10^{21}\times 29 = 440.8\space C

By Coulomb's Law, the force F F between two 1 1 g g of copper separated by a distance r = 1 r=1 m m apart is:

F = k Q 2 r 2 = 9 × 1 0 9 × 440. 8 2 1 = 1.75 × 1 0 15 N F = k\frac{Q^2}{r^2}=9\times 10^9\times \frac{440.8^2}{1}=\boxed {1.75\times 10^{15}}\space N

First, we determine the net electric charge on a copper atom. Qnet= 0.01Z|Q(electron)|=0.01×29×1.6×10-19=46.4×10-21 coulombs. We multiply the number of charge with the number of atoms in a 1-gram copper ball. 9.5×1021×Qnet=440.8 coulombs. Then, we use F=kq1q2(r^-2). (note: q1=q2 since the balls are identical). F=(9E+9)(440.8)(440.8)(1^-2). Thus we get 1.75E+15 newtons.

Jiunn Shan Tan
Jul 31, 2013

1g = 9.5E21 atoms or electron

1 atom charge is 0.01ZQ = 0.01 x 29 x 1.6E-19 = 4.64E-20 = a

Total charge for 1g , T = 9.5E21 x a = 440.8

R = 1

F= (Q1 x Q2) / (4πε R^2 ) = (Q1 x Q2) / (KR^2)

Lk Tan
Jul 28, 2013

The overall charge on 1 atom of copper is given by 0.01Z|Q(electron)|, which is 0.01 \times 29 \times 1.6 \times 10^{-19}. This is multiplied by the number of atoms, 9.5 \times 10^{21}, giving us the charge of 1 ball. Coulomb's law states that the electrostatic force experienced by 2 charged bodies is \frac {Q 1 \times Q 2}{4 \times \pi \times \varepsilon_0 \times r^2}. putting in the values gives us \frac {(9.5 \times 10^{21} \times 0.01(29) \times 1.6 \times 10^{-19})^2}{4 \times 3.1415 \times \8.854 \times 10^{-12} \times 1}=1.75 \times 10^{15}

Moderator note:

A very nice solution.

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...