Download 3 – More Electric Fields Questions

Grade 11 Physics – More Electric Fields
1. A charged rod attracts bits of dry cork dust which, after touching the rod, often jump
violently away from it. Explain why this occurs.
2. The Earth has an excess of 600 000 electrons on each square centimeter of surface area.
Calculate the number of coulombs of excess charge on each square kilometer of surface.
3. As you walk across a carpet, you might pick up 1 x 10-10C of negative charge. How
many additional electrons have you acquired?
4. An electric force of 2.4N [down] is exerted on a -1.8µC charge. What is the magnitude
and direction of the electric field at this point?
5. Humid air becomes ionized (the molecules become charged) in an electric field of
3.0 x 106N/C. What is the magnitude of the electric force on an electron in this field?
6. Why can electric field lines never cross?
7. If a uranium atom was exposed to an extremely high temperature and as a result all its
electrons were stripped away, draw the electric field around this uranium nucleus.
8. A -0.5C charge experiences a force of 0.2N [left]. What is the magnitude and direction of
the electric field at that point?
9. Calculate the magnitude and direction of the force exerted on a +3.0C charge in an
electric field of 12N/C [up].
10. In an experiment, a charge of +20mC feels a force of 0.03N [right]. If a different charge
was placed in this exact spot and feels a force of 0.15N [left], what is the size of this
second charge? Is it positive or negative?
11. A photocopier uses an electric field to attract charged ink particles to paper. If it exerts a
force of 3 x 10-12N [South] on a proton, calculate the electric field in N/C and N/ec.
12. When in an electric field of 1.10 x 1018N/C [East] a charge had a force of 0.900N [West]
on it.
a. Calculate the charge on this object.
b. How many electrons has it lost or gained?
K. Byrnes
PHY30S
Grade 11 Physics – More Electric Fields Answers
1. The attraction happens because the electrons or protons (depending on the charge on the
rod) in the cork were first attracted to the rod. Once contact occurs, the cork becomes
charged by conduction, and becomes charged in the same way as the rod, so it in now
repelled.
2. 1km = 100cm, so 1km2 = 1 x 1010cm2. Since there are 600 000 electrons on each square
cm, this would give 6 x 1015 electrons on a square km. This would mean the total charge
would be 9.6 x 10-4C.
3. 625 000 000 electrons
4. 1.33 x 106 N/C [up]
5. An electron has a charge of 1.6 x 10-19C, so the force would be 4.8 x 10-13N
6. If field lines were to cross, a test charge placed there would experience two forces in two
directions at the same time. We know that when multiple forces act on a single object,
those forces combine and the resulting motion will be determined by the resultant force,
one total force on that object (if lines cross, that would mean this wouldn’t be
happening).
7. If it lost all electrons, it would be positively charged, and the electric field would look
like the following:
8. 0.4N/C [right]
9. 36N [up]
10. -0.1C
11. 3 x 10-12 N/ec [South] and 1.9 x 107N/C [South]
12. (a) -8.18 x 10-19C
(b) 5.11 electrons, which is impossible, so 5 electrons. It gained 5 electrons, since it is
negatively charged.
K. Byrnes
PHY30S
K. Byrnes
PHY30S