Download 17. APC Summary

APC Physics
Basic Physical Principles
Presentation 2004 R. McDermott
st
1
Semester Concepts:
Mechanics
What is a “System”?
A system is any object or set of objects that is of
interest (arbitrary choice).
The system can consist of a box.
Or the box and the table it is on.
Or the box, table it is on, and the Earth.
Each choice will affect the analysis of the
problem (making it easier or harder to solve),
as well as making the solution more or less
useful.
What is a “Field”?
The idea of a “field” is a way of dealing with
forces that act over a distance without contact
between two objects.
A field can be thought of as being like a spring
that exerts forces on objects at each end and
can store and release energy.
Alternatively, a field can be thought of as being
an “object”.
Field (cont.)
The strength of a field is determined by
measuring the force on an object in the field,
and dividing that force by the characteristic of
the object that produces the force:
(gravitational field strength) = weight/(object mass)
g = F/m
(electric field strength) = electrostatic force/(object
charge) E = F/q
B (magnetic field strength) = magnetic force/(object
charge times object velocity) B = F/qv
What are “Free-body”
Diagrams?
Free-body (force) diagrams show forces acting
on the system, and are used to produce
equations (for Newton’s laws).
Identify forces due to fields (gravitational, electric,
magnetic).
Identify forces due to contact (tension, friction,
normal, etc).
Set up axes based on motion (or potential motion).
Resolve forces into components on axes.
Write equations for each axis.
How Are They Used?
If no unbalanced net force acts on the
system from outside, then the system’s
motion will not change (F = 0).
Stationary system remains stationary.
Moving system continues moving at constant
velocity (same speed in a straight line).
How Are They Used (cont.)?
If an unbalanced net force acts on the system
from outside, then the system will accelerate
(undergo a change in velocity). F=ma
Change in direction without a change in speed
(circular motion – force perpendicular to motion).
Speed up (force component acts in same direction as
motion).
Slow down (force component acts in direction
opposite to motion).
The size of the acceleration (observed change)
depends on mass (small objects display a greater
observed change for the same force).
What is “Momentum”?
Momentum is the product of mass and
velocity for a system (p = mv).
The change in momentum indicates the effect
of external net forces on the object (or
system).
Changing velocity also changes momentum,
so an external, unbalanced force causes a
change in momentum (Ft=mv).
What are “Force Pairs”?
Forces develop between pairs of objects such
that each object feels an identical force (but
acting in opposite directions)
Since both the object and agent feel the same
force, for the same time interval, they
experience the same change in momentum (but
they may not experience the same acceleration
if the masses are different).
What is “Working”?
Working is the transfer of energy from one
system to another (also done by radiating and
heating).
This changes the energy of both systems.
Positive working (work on a system) increases the
energy of the system.
Negative working (work done by the system)
decreases the energy of the system.
W=EK+EG+EE+EI
2nd Semester Concepts:
Electricity and Magnetism (E&M)
What is “Charge”?
Charge is the result of the relative
number of electrons and protons
(ultimately of quarks).
Charge generates an electric field, which
produces a force on other charges.
E=F/Q
What is “Current”?
Current is movement of charge through a
conductor.
Charge “flows” when there is a difference in
electric potential (electrical height). I=V/R
(I=Q/T)
There is resistance to the movement of charge in
a conductor. R=L/A
Work is done when a charge moves through a
potential difference. W=VQ
What is a “Circuit”?
A circuit is a closed path, within which charge
may flow.
A series circuit is one in which current has only
one path to follow.
RT=R1+R2+…
IT=I1=I2=…
VT=V1+V2+…
A parallel circuit is one in which current has
multiple paths to follow.
1/RT=1/R1+1/R2+…
IT=I1+I2+…
VT=V1=V2=…
Kirchoff’s Laws
The current entering a junction must
equal the current exiting a junction:
 Iin=Iout
The sum of the electric potentials in any
circuit loop must equal zero:
Each loop is a series circuit
Electrical “height” (V) provided by the
battery must be “used up” by the resistors.
What is “Magnetism”?
A magnetic field results from moving charge.
Magnetic fields exert forces on moving charges.
F=Bqv
Magnetic fields exert forces on current-carrying
wires. F=BIL
A potential difference is produced when a
conductor is moved through magnetic field
lines. =BLv