Marc and Annie
Chapters 17­19; The Electric Field, Electric Potential, and Current
Final Review Chapter 17 and Chapter 18
Vocabulary:
● Charging by conduction: involves the contact of a charged object to a neutral object.
Suppose that a positively charged aluminum plate is touched to a neutral metal sphere. The
neutral metal sphere becomes charged as the result of being contacted by the charged aluminum
plate
● Charging by induction: a method of charging a neutral object, using a charged object, without
establishing physical contact between them.
● Conductor: A material through which an electric charge can flow
○ Good conductors of heat are usually good conductors of charge
● Coulomb: The SI unit of charge
○ One coulomb (C) is equal to the total charge of 6.25*1018 electrons
● Coulomb’s law: The relationship among electrical force, charges, and distance: The electrical
force between two charges varies directly as the product of the charges and inversely as the
square of the distance between them
● Electroscope: a device that uses the repulsion of like charges to show the presence of charge.
It consists of a metallic rod with two small pieces of gold leaf at the bottom. When charged, the
leaves repel each other
● Electrostatics: The study of electric charges at rest
● Insulator: a material that is a poor conductor of heat and that delays the transfer of heat and is
also a poor conductor of electricity
● Neutral: no charge
● Capacitance: a measure of how much charge a capacitor can store for a given potential
difference V.
● Capacitor: a simple device for storing charge. It consists of two conductors separated by a
small space.
○ Each conductor carries a net charge. The charges are equal in magnitude but opposite in
sign. T
○ The net charge on the entire capacitor is zero.
● Electric field: A force field that fills the space around every electric charge or group of charges
● Electric field line: potential energy of the charge in an electric field
○ Flow from positive to negative
● Electric potential difference: the difference in electric potential energy of a unit test charge
transported between two points.
● Equipotent: Equally powerful
● Grounding: Allowing charges to move freely along a connection between a conductor and the
ground
● Volt: The SI unit of electric potential
○ One volt is the electric potential at which on coulomb of charge would have one joule of
potential energy
Equations and Variables:
● Fe=K qiq2/r2
○ Unit:N (Neutron)
○ Steps to solve:
■ Decide it’s an Fe problem
■ Write down the givens
■ Identify the unknowns
■ Find the appropriate equation(s)
■ Plug in values WITH the correct units
■ Solve the equation with sig figs and units
■ Check: Does it make sense
●
●
●
●
●
○ + force value=repealed
○ – force value=together
E=Fe/q
○ E=electric field strength
○ qt=test particle
○ q=charge
○ UnitN/C
C=q/v
○ UnitJ(joules)/C(coulombs) F (Farad)
e­= 1.6*10­19C=fundamental charge
k=8.99*109 nm2/c2
Fe=electrostatus force (N)
● K=electrostatus constant=8.99*109 Nm2/c2
Chapter Review 19
● Ampere: The SI unit for electric current is the coulomb per second, which is called an “ampere”
(amp for short) and denoted by A
● Battery: A battery is quantitatively described by its “emf” value.* The emf, denoted by E, is
defined to be the energy per unit charge provided by a source as charge crosses the source’s
terminals.
● Conventional current: The concept of current as the transfer of positive charge, so that its
direction of flow is opposite to that of electrons which are negatively charged.
● Electric circuit: An electronic circuit is composed of individual electronic components, such as
resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces
through which electric current can flow
● Electric current: The rate of flow of charge through a cross section of the wire is called electric
current. This term applies to the motion of any charge through any cross­sectional area.
● Kilowatt­hour: A convenient unit of work or energy is the kilowatt­hour, abbreviated kWh. It
is defined as the work or energy delivered at the rate of 1 kilowatt for a period of 1 hour. Since
W P– t,
● Photovoltaic cell: a cell that converts solar energy into electrical energy
● Potentiometer: instrument for determining a potential difference or electromotive force by
measuring the fraction of it that balances a standard electromotive force
● Resistance: Units of resistance are volts per amp, which we call ohms and abbreviate by the
Greek letter: For a given applied voltage, the greater the resistance of a piece of material, the
less current will
● Resistor: small devices designed to provide electrical resistance. When inserted in an electric
circuit, resistors are used to control the amount of current in various parts of the circuit. Some
resistors consist of a very long coil of fine wire. Others are made of a composition material
containing carbon.
Overview and Equations:
● Ohm’s law states that current is proportional to potential difference.
○ On a graph with current as the x­axis and voltage as the y­axis the slope is the
resistance
● V=IR
○ Voltage=current*resistance
○ Use to find voltage drops
● R=V/I
○ Resistance (ohms)= voltage (volts)/current (amps)
● R=pl/a
○ Resistively=resistive of the material*length of the piece/sectional area
Problems:
1. Draw the electric field lines for these two electric fields, labeling any axises and asymptotes.
+
2. A proton and an electron are 1.8 nm apart. What is the force of attraction between them?
A. 2.77 x 10^27 N
B. ­1.28 x 10^­19 N
C. ­7.1 x 10^­11 N
D. ­7.1 x 10­29 N
3. A particle with a charge of 15 uC is in an electric field with a test particle of ­1.6 x 10^­19 C,
and the distance between them is 4.6 x 10^­11 m. What is the force of attraction between
these particles? What is the electric field strength? What is the qualitative difference between
the force of attraction and the electric field strength?
4. The three particles A of +9e, B of ­8e, and C of ­e are arranged in a triangle. The distance
between A and B is 4.00 um, between B and C is 3.00 um, and between A and C is 4.50 um.
The angle of point C is 60.0 degrees. What are Feac, Febc, and Feab? What is the resultant
force on C?
5. In a row are three charged particles along the X axis. The first has a charge of ­2.0 uC. The
second, of charge 3.0 uC, is 0.30 m from the first. The third, of charge ­1.0 uC, is 0.20 m from
the second one. What is the magnitude and direction of the net force on the third charge?
6. A negatively charged ball is hanging in between a positive and negative plate. Where will the
ball’s next two swings go?
A. To the negative side and attach there.
B. To the positive side and attach there.
C. To the positive side and then back to the middle.
D. To the positive side and then to the negative side.
How will the ball’s movement continue? Why?
A. It will sway back and forth because the energy cannot be destroyed.
B. It will then attach to the positive side to get rid of the negative charge.
C. It will sway back and forth until it transfers all of its charge and then hang in the
center.
D. It will sway back and forth until the friction due to the air causes it to stop.
7. What is the charge of an atom that loses 1.5 x 10^6 electrons?
A. ­1.5 x 10^6 C
B. 2.4 x 10^­13 C
C. 9.375 x 10^24 C
D. 1.07 x 10­25 C
8. Particle A, of 5.00 x 10^­3 C, is 5 mm away from Particle B, of ­7.00 x 10^­2 C. What is the
magnitude and direction of the force exerted by Particle B on Particle A?
9. If the electric field strength between two plates 0.500 m apart is 2.50 x 10^3 N/C, what is the
electric potential difference between them?
What does this value represent?
A. The total charges of the two plates.
B. The amount of energy required to move one columb across the plate.
C. The amount of current established between the plates.
D. The amount of electric energy stored in one columb anywhere in the field.
10. What charge must be added to a 2.2 uF capacitor so that the electric potential difference in the
capacitor equals the one found above?
What additional charge must be added to that capacitor to achieve a voltage difference of 5.00
x 10^3V
11. How much work has to be done on a 10uC particle to move it across an electric field with a
voltage difference of 4.0 x 10^3 V?
What is the qualitative difference between the voltage difference of 4.0 x 10^3 V and the work
value found above?
12. The electric field strength 0.2 m from the center of a particle is .05 N/C. If the test particle’s
charge is doubled, what is the value of the electric field strength?
A. 0.1 N/C
B. 0.025 N/C
C. 0.4 N/C
D. It does not change
13. Draw the electric potential lines between these particles
Answers:
1. 2. C
3. Fe = ­1.0 x 10^7 N, E = 6.4 x 10^25 N/C. Fe is the force of attraction between those two
particles, whereas E is the strength of the electric field anywhere, based on the particles and
their location.
4. Feac = ­1.02 x 10^­16 N, Febc = 2.05 x 10^­10 N, and Feab = ­1.04 x 10^­15N. R = 1.78
x 10­10 N at 29.8 degrees.
5. ­0.60 N
6. D, C
7. B
8. ­1.26 x 10^11 N
9. 1.25 x 10^3 J/C or 1.25 x 10^3V, B
10. 2.75 x 10^­3 C, 8.25 x 10^­3 C
11. 0.04 J. The electric potential difference of 4.0 x 10^3 V is the amount of energy to move a 1 C
particle, while the 0.04 J is how much energy it takes to move a 10 C particle.
12. D .
13.