Download Chapter 5 - Fayetteville State University

-1FAYETTEVILLE STATE UNIVERSITY
COLLEGE OF BASIC AND APPLIED SCIENCES
DEPARTMENT OF NATURAL SCIENCES
CSCI 110-04
FALL 2005
Oct. 21-05
Chapter 5 Electricity and Magnetism
I) Basic concepts and Equations
1) The unit of charge is the Coulomb(C): The charge of an electron or a proton is:
1.6x10-19 C
2) Coulomb's Law: F = KQ1Q2/R2 (Newtons), where Q stands for charges
K = 9x109 N m/C2, proportionality constant, F electrostatic force
3) Intensity = charge transfer/over time interval
I = Q/t, where Q stands for charge and t for time. The unit of intensity is the Ampere
1 ampere (A) = coulomb/second = C/s
4) Potential difference: gives a measurement of the tendency of the electrons to flow in a
conductor. It is also called voltage (V)
1 volt = joule/coulomb = J/C
5) Ohm’s Law: I = V/R
I: electric current
amperes
V: potential difference or voltage volts
R the conductor resistance
ohms
Resistance (R) in a conductor depends inversely on the conductor length, directly in the
conductor cross-section, and on the type of material the conductor is made up (copper
wires have less resistance than iron wires)
6) Electric Power: Indicates the rate at which electric current produces work
P = VI (Watts)
7) Magnetic Field: Magnetic Fields are generate by magnets
8) Magnets: are more likely made up of metals such as Iron, Nickel Cobalt
Magnets exhibit two poles: North and South
Like magnets repel and alike attract each other
9) Magnetized material: stroking two iron bars polarize them given origin to two poles
north and south, this polarization creates magnets. A magnetized material can be
depolarized by increasing the temperature or by hammering it. Increasing the temperature
-2randomizes the orientation of a collection of small magnets that made up a big magnet
this will decrease the magnetization of a given material.
9) Oersterd’s Experiment: This experiment demonstrated by first time the relationship
between electricity and magnetism. A current along a wire generates a magnetic field that
circles around the wire (this can be determined using the right hand rule).
10) Electromagnetic Field: static charges have associated an electric field; charges in
movement have associated the magnetic filed due to the current and also the electric
field. The last two effects are called electromagnetic field and are associated to charges in
movement.
11) Electromagnets: These are magnetic devices constructed using a wire with many
turns, which makes the magnetic field generated by the current proportional to the
number of turns. The magnetic field can still be increased more if an iron bar is placed
through the wire turns this whole array is called an electromagnet.
12) Magnetic field and work: Magnetic fields can produce work n many different ways,
one of the simplest is through the interaction of a current that flow through a wire with a
magnetic field, which generate a force perpendicular to the magnetic field that make the
wire to oscillate. This interaction is important in the design of electric motors that
transform magnetic energy into mechanical energy.
II) Multiple Choice Test
Question 5.01 If an electrically neutral rubber rod is rubbed against an electrically
neutral piece of fur, we know that
A) the fur and the rod both pick up the same charge.
B) the fur and the rod both remain uncharged.
C) the fur and the rod pick up equal but opposite charges
D) the fur and the rod each pick up different amounts of opposite charge.
Question 5.02 A neutral piece of iron
A) does not have any charge.
B) has equal amounts of positive and negative electrons.
C) has equal numbers of protons and electrons.
D) has equal numbers of protons and neutrons.
Question 5.03 A proton and an electron are separated by 0.03 mm. If the
distance between them changes to 0.06 mm, the force between the charges is
A) one fourth of the original force.
B) half the original force.
C) equal to the original force.
D) twice the original force.
E) four times the original force.
Question 5.04 If a charged object is held near (but not touching) small bits of
aluminum foil, the foil pieces
A) are attracted to the charged object because they become charged as they are ripped up.
-3B) are attracted to the charged object because one side of each aluminum piece becomes
slightly more negative than the other side.
C) are not attracted to the charged object because the aluminum has no net charge.
D) are repelled by the charged object because one side of each aluminum piece becomes
slightly more negative than the other side.
E) are repelled by the charged object because they become charged as they are ripped up.
Question 5.05 In a conducting material, such as a piece of copper wire,
A) some of the electrons are free to move readily.
B) all of the electrons are free to move readily.
C) all of the protons are free to move readily.
D) some of the protons are free to move readily.
E) both electrons and protons move readily.
Question 5.06 If you pump water through a pipe that has an open end, water will
spill out and run all over the ground. If you connect a battery to one end of a wire
and set the other end on a table top,
A) no electrons will flow.
B) electrons will flow out of the wire and spill out over the table top.
C) electrons will flow but build up at the end of the wire instead of spilling out.
Question 5.07 A 6 V battery is connected in a circuit. If the 6 V battery is
replaced by a 12 V battery,
A) the electrons move twice as fast in the circuit.
B) the electrons move half as fast.
C) twice as many electrons are moved around the circuit in the same amount of time.
D) half as many electrons are moved around the circuit in the same amount of time.
Question 5.08 A thick copper wire is connected in a circuit to a battery. If the
thick wire is removed and replaced by a thinner copper wire of the same length
then we know that
A) the resistance is smaller and the current is smaller.
B) the resistance is bigger and the current is smaller.
C) the resistance is smaller and the current is bigger.
D) the resistance is bigger and the current is bigger.
Question 5.09 A copper wire is connected in a circuit to a battery. If the wire is
removed and replaced by a longer copper wire of the same thickness then we
know that
A) the resistance is smaller and the wire is hotter.
B) the resistance is bigger and the wire is hotter.
C) the resistance is smaller and the wire is not as hot.
D) the resistance is bigger and the wire is not as hot.
-4Question 5.10 A strong positive charge is placed at a point in space. We know
that the magnetic field that this charge produces at a nearby point in space is
A) strong.
B) weak.
C) zero.
Question 5.11 Two current carrying wires are placed parallel to each other. The
force between the two wires
A) is zero.
B) is attractive.
C) is repulsive.
D) Can be attractive or repulsive depending on the direction of the current in the wires
Question 5.12 To produce a current in a single closed loop of wire (with no
batteries) you would have to
A) place it in a magnetic field.
B) move it towards a magnet.
C) move it away from a magnet.
D) A and B but not C.
E) B and C but not A.