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Review
There are FOUR quantities to know:
Two vectors:
FORCE, ELECTRIC FIELD STRENGTH
Two scalars:
Energy, Potential (Voltage)
FORCE is a push or a pull. A force is ALWAYS between two objects.
FIELD STRENGTH is a measure of how strong a force field is at a point in space. The field has been
created by an object.
GRAVITATIONAL FIELD STRENGTH measures the strength of a gravitational field near a massive object
(such as the Earth or the Sun).
ELECTRICAL FIELD STRENGTH measures the strength of an electrical field near a charged object (such as
a proton or an electron).
POTENTIAL ENERGY is a measure of how much work can be done by an object. Conversely, it is also the
amount of work someone must have done to give the object its energy.
ELECTRICAL POTENTIAL ENERGY is a measure of how much work can be done by a system of charges (at
least two). A lone charge sitting in the middle of deep space has no electrical P.E.
ELECTRICAL POTENTIAL (voltage) is a measure of how much work can be done by a +1 Coulomb charge.
ELECTRICAL POTENTIAL DIFFERENCE (voltage) is a measure of how much work can be done by a +1
Coulomb charge if it moves from A to B.
Electric Potential and Electric Potential Difference is roughly analogous to height and height difference
in a gravitational system. The higher the hill, the more work can be done by any mass at the top of the
hill. The total energy of the system depends on the mass (charge) you place at the top.
To understand voltage, here’s an example:
A flashlight battery has 1000J of energy in it. What does this mean? There are two possibilities:
1) It could be a powerful battery: Maybe every Coulomb of charge it sends out has 100J of energy.
It could be used to do heavy work like run a motor. This would be a 100V battery.
2) It could be a weak battery: Maybe every Coulomb of charge it sends out has only 10J of energy.
It couldn’t lift anything, but maybe could light a small bulb. It would be a 10V battery.
The first battery is very powerful, but would not last long. The second battery is weak, but would last
longer. Both batteries would spend 1000J by the time they stopped working.
As this example shows, the energy of a battery does not tell us much.
The voltage of a battery is a measure of how much energy its electrons are given. Since it’s the
electrons that do the work, voltage is a more useful measure of what a battery can be used for.
Review Questions
1. What is the unit for:
a) Electric Force
b) Electric field strength
c) Electric Potential Energy
d) Electric Potential (difference)
2.
a)
b)
c)
d)
Electric Field Strength is closely related to:
Force
Energy
Mass
Voltage
3.
a)
b)
c)
d)
Electric Field strength is to electricity what _________________ is to gravity:
Force
g
mass
G
4.
a)
b)
c)
d)
Voltage is closely related to:
Force
Energy
Mass
Field Strength
5.
a)
b)
c)
d)
Voltage is loosely analogous to _______________________ in a gravitational system:
Weight
Mass
Height
Speed
6. There is an electric field of 300N/C at a point 8.0cm away from my computer screen. How
much of a force would that impose on a blood cell in my finger that is charged with a 3.5μC
charge?
7. My WiFi gives off an electric field with strength 220N/C at a point 15cm from the modem. If I sit
30cm from the modem, the field is:
a)
b)
c)
d)
about the same
about half as strong
about twice as strong
about one quarter as strong
8. The electric field at a point P is non-zero. What would happen if a positively charged object
were placed at point P?
9. An electric field equal in strength and direction to the gravitational field exists at a point P. If I
place a 1kg mass with charge -1C at point P, what will happen?
10. My car battery is 12V. This means that every _______________ of charge that leaves the
battery contains ________________ of energy.
11. A 12V battery sends 10C of charge into my cell phone. How much energy is used by my phone?
12. Consider the two charges below then answer the questions:
-4μC
a)
b)
c)
d)
+2μC
What direction does the force on the +2μC charge point?
What direction does the force on the -4μC charge point?
Which force is bigger?
Is the potential energy of this system positive, negative or zero?
13. Consider the picture below then answer the questions:
-3μC
a)
b)
c)
d)
+3μC
Is the voltage at point “X” positive, negative or zero?
What direction is the electric field at point “X”?
If an electron was placed at the “X”, what direction would it move at first?
Is the energy of the two-charge system positive, negative or zero?
14. The square below has a side length of 10cm. Find the:
a) E field strength at the centre of the square.
b) Electric potential at the centre of the square.
+7μC
+7μC
-7μC
-7μC
15. Four charges are placed in the corners of a square 40cm wide as shown. In roughly what
direction is the -4μC charge being pulled? Do not calculate.
-2μC
-4μC
+2μC
+2μC
16. Consider the two charges at rest shown below, then answer the following True/False questions:
-3μC
+10μC
a) Positive work was done to bring the two charges together.
b) If I release the charges they will accelerate toward each other.
c) If I increase the distance between the charges, I increase the potential energy of the
system.
d) If I increase the distance between the charges, I increase the force between them.
e) The force on the left charge is repulsive.
f) The force on the right charge is bigger than the force on the left charge.
g) The total energy of the system is negative
h) The system is bound.
17. Susie wishes to map the gravitational field strength throughout the world. She uses the internet
to contact people from all over the world and she asks them to weigh themselves to get the
earth’s force of gravity in different locations. From this she finds that gravity is stronger in
America than it is in Asia.
a) Why is her conclusion incorrect?
b) What should she do to map the earth’s gravitational field properly?
18. A region has a constant electric field of 140N/C [East] going through it.
a) What force would a -3.0C charge feel in this electric field? Include direction.
b) What force would a +12.0C charge feel in this field?
19. A region has a constant electric field of 2500N/C [North]. A speck of dust with a -5.0C charge on
it is placed in the field and let go.
a) What direction does the dust speck go?
b) Does the dust speck go at constant speed?
20. Two billiard balls of mass 150g are each given a +45μC charge and then fired at each other from
a distance of 1.0m. They are both given an initial speed of 12.0m/s. How close will they get to
each other?