Download Newton`s first law of mechanics: An object moves at the same

EGN 100 Essential Concepts
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Newton’s first law of mechanics: An object moves at the same velocity, including zero,
unless acted upon by external forces that add up to be something other than zero.
Newton’s second law of mechanics: F = ma
Mass attracts mass.
Opposite charges attract. Like charges repel.
What are the following: displacement, velocity, acceleration, mass, force, voltage,
current, resistance, charge, work, energy, power?
Ohm’s law: V = IR
Krichoff’s voltage law: In a circuit, the sum of the voltage rises equals the sum of the
voltage drops. A voltage rise occurs when there is a source of energy, such as the
chemical energy of a battery or the mechanical energy of a generator.
Work and energy have the same units: Joules
Power is the rate at which work is done or energy is used.
Mechanical work: Work = Fd Joules
Mechanical power: P = Fd/t = Fv, F is force in Newtons, v is velocity in m/s and t is time
in seconds.
Mechanical energy is Pt = F(d/t)t = Fd Joules
Electrical work: Work = QV, where Q is the charge in Coulombs and V is the voltage in
Volts. Not always a useful measure since we don’t actually move charge around except
as current.
Electrical Power: P = IV, were I is current in Amperes and V is voltage in Volts.
Electrical energy it Pt Joules
First law of thermodynamics: Energy is not created or destroyed but is converted from
one form to another.
Second law of thermodynamics: When constraints are removed, energy goes from a
higher level to a lower level.
A battery contains chemicals that react with the battery electrodes with the result that
electrons are added to one electrode and taken from the other, producing a voltage
difference between the electrodes. If we connect the terminals with an electrical circuit,
the charges will flow through the circuit, doing useful work and/or producing heat.
A radian is a measure of angle defined as the angle subtended by and arc of length equal
to the radius of the circle.
Angular velocity is measured in radians per second.
v = r where v is the velocity of a point on the circumference,  is the angular velocity,
and r is the radius.
When two gears or two pulleys interact, the larger gear (or pulley) has an angular velocity
smaller than the smaller gear and a torque greater than the smaller gear.
Without losses, the power is the same in two interacting gears.
P = T where T is torque and  is angular velocity
If P1 = P2, then T11 = T22 or T1/T2 = 2/1
Efficiency is Power out / Power in multiplied by 100 to express it in percent. Because of
the second law of thermodynamics, it is impossible for efficiency to be greater than
100%.
There is a magnetic field around a current.
We can create an electromagnetic by forming the wire through which a direct current
flows into a coil. According to the right hand rule, the direction of the magnetic field
inside the coil is in one direction, creating a north and a south pole of the magnet. We can
increase the strength of the electromagnet by increasing the current. We can change the
magnetic polarity of the electromagnet by changing the direction of the current.
The principles of an electromagnet can be put to good use in a dc motor. The armature is
the part of the motor that is connected to the shaft and has coils of wire around it and
placed between magnets. When a voltage is applied to the ends of the coil wires a current
flows and the armature is magnetized. The interaction of the armature magnetic field and
the other magnetic fields creates torque which causes the armature and the shaft to rotate.
As the motor speeds up, the counter electromotive force (voltage) increases in proportion
to the speed. Since the electrical polarity of the counter emf is opposite to that of the
applied voltage, the net voltage decreases and the current decreases and a steady state or
equilibrium voltage, current and angular velocity are maintained.
The relationship between angular velocity and the torque to maintain that angular
velocity is portrayed as the performance curve of the motor driven system. An example
of such a curve is shown on the next page.
Angular Velocity,
rad/s
Performance Curve 64:1
25.0
20.0
15.0
10.0
5.0
0.0
0.00
w= -244T + 21.7
0.02
0.04
0.06
0.08
Torque, n-m
Since power is the product T, we can plot mechanical power as a function of torque
(load) and find the maximum power at a torque equal to one-half the “stall” torque, i.e.
one-half the torque at which the angular velocity drops to zero. We want to design and
build our system to achieve maximum output power in order to maximize the
performance of the system.
Mechanical Power Curve
Power, W
0.6
0.4
0.2
0
-0.2
0
0.02
0.04
0.06
Torque, N.m
0.08
0.1