Physics 8: Midterm Study Guide
Spring Term, 2006
The midterm will cover lecture material up through Thursday,
04/27 on Electrical Devices. The quantitative level will be
no worse than what you've seen in the "mock quizzes" during
discussion section, and also similar to homework and transmitter
You may also want to study the transmitter questions from class:
See entry on Lectures Page just after
Below are the topics that are likely to appear in some form on the
midterm. The midterm will consist of 20 multiple choice, 10 true/false,
and 5 short answer questions. Note that 35 questions and 35 bullets suggest
a nearly one-to-one correspondence. I'll give you a bucket of equations on
the exam. See Front Page of Midterm
to see exactly what you'll get.
- Understand what inertia is, and know Newton's three laws of motion.
Expect to apply F = ma (Newton's 2nd law).
- Understand that an acceleration is any change in velocity.
Know how to quantify acceleration (if velocity changes by 3 m/s over
the course of one second, the acceleration is 3 m/s2). Any
change in velocity (direction or speed) implies a net force.
- Understand how forces can add or cancel, etc.
- Be able to compute work: either from lifting or pushing (with some force
through some distance)
- Work is a force times distance: applied force times distance through
which object moves (in direction of applied force). For example, the
work required to lift a mass m a vertical height h
against gravity, g = 9.8 m/s2 is simply mgh
(mg is force of gravity, or weight, by F = ma, and
h is the distance through which the force of lifting acts).
- Understand that work measures energy, and that energy comes in both
kinetic and potential forms. Energy is the capacity to do work, measured
in J (Joules) = N m (Newton-meters) = (kg m2)/s2
- Kinetic energy of a mass in motion is ½mv2.
- Energy is exchanged between potential and kinetic forms, always adding to
the same amount, given no frictional losses.
- Heat is really just kinetic energy (motion) on a microscopic scale:
individual atoms/particles rattling around.
- The energy content associated with heat is characterized by heat
capacity. Water has a heat capacity of 4184 J/kg/°C, meaning that
4184 Joules (1 Calorie) will raise the temperature of 1 kg of water (1
liter) by 1°C. Most materials have a heat capacity around 1000
J/kg/°C, so for instance raising the temperature of a 0.2 kg coffee mug
by 40°C would take about 8000 Joules (about 2 Cal).
- Understand the concept of power as work per unit time,
with units of J/s = W (Watts). This is by far my favorite exam topic, since it
ties a lot of physics concepts together.
- Understand energy conservation among various forms of potential
and kinetic energy. Appreciate that most energetic processes involve friction
and end up converting their energy content to heat. Know what ultimately
happens to all this heat.
- All hot objects radiate power in the form of infrared light. The amount
of power is given by: P = σAT4,
where σ = 5.67×10-8 in MKS units, A
is the surface area of the radiator, and T
is in degrees Kelvin.
- Humans on average consume about 75100 W of power just sitting
still. This energy expenditure ultimately ends up as heat.
- The force of air drag at sea level for most objects will be about:
Fdrag=0.65Av2. This comes out in Newtons if
A is in square meters and v is in meters per second.
- Terminal velocity is reached when the force of drag exactly equals (and
therefore cancels) the force of gravity.
- Springs supply a restoring force proportional to the imposed
displacement. This gives rise to a quadratic (parabolic) potential energy
function, which leads to natural oscillation. You'll want to know that the
frequency of oscillation is proportional to the square root of the spring
constant (stiffness) divided by the mass. Cut the mass by a factor of four
and the oscillation will have twice the frequency.
- Applying a cyclic force to a system with a natural oscillation
frequency could lead to resonance if the pushing frequency is close to the
natural frequency. If the system is "clean", with little damping
and a sharply defined resonant frequency, it may be ripped apart by even a
small force at the resonant frequency.
- A reasonable mental model for molecules and solid crystal lattices is a
bunch of atoms connected by springs. This is because nearby atoms (even
neutral ones) are attracted, but become repulsive when they're too close.
- An electric force exists between charged particles so that like charges
repel, and unlike (different sign) charges attract. Electrons and protons
have exactly the same charge magnitude (measured in Coulombs) but opposite
sign. Neutral atoms have equal numbers of protons and electrons. The
force between two charges is proportional to the product of their charges
and inversely proportional to the square of their separation.
- The electric field points away from positive charges and
toward negative charges. The electric field serves as a roadmap telling
positive charges which way to move (negative charges move
opposite). The magnitude of the field indicates how much force would be
experienced by a charge placed in that location.
- Electric current is simply the flow of charge, measured in amperes
(amps), which is Coulombs per second. Current is defined as
positive charge flow, so electrons actually flow in a direction
opposite that indicated by the direction of the current arrow.
- The way to get a bulb to light up is to force current through the
filament. This generally involves a loop for current to flow that includes
both the bulb and the battery as integral parts of the loop.
- Because current is simply the flow of charge, currents always have to
add up at junctions. You don't lose any of the flowing charge. In all the
circuits we've seen (with one voltage source), all the current
flows through the battery, but may split up later on.
- Bulbs shine because they're hot. They glow in the infrared via
blackbody radiation. They get hot because the electrons racing through the
filament (basically a resistor) bump into atoms and make them vibrate
(which is heat). More current means a brighter bulb.
- Blackbody radiation works so that hotter means bluer, cooler means
redder (into infrared). The relation governing power output
is the P = σAT4 equation from above.
- Incandescent bulb filaments are much cooler than the surface of the
sun, which means their light is redder/duller. Why don't we crank them
up to be as hot as the sun and therefore whiter (and more efficient,
consequently)? Because we have no metals that would remain solid at such
high temperatures. Tungsten barely makes it to 3,000 K. Halogen bulbs
help a bit by re-depositing tungsten back onto the filament, so we can run
halogen bulbs hotter/whiter.
- The power dissipated in an electrical component depends on the voltage
drop across that component and the current running through it.
Specifically, P = VI: power is voltage times current.
- Know Ohm's Law inside out: V = IR. V represents the
voltage drop across the component in question. I is current in
amps (A), and R is resistance in Ohms (Ω).
- Know how resistors in series and in parallel combine into effective
resistances. You can use this to figure out how much total resistance a
battery sees, and thus how much current it will deliver (in accordance with
- Be able to combine P = VI with V = IR to get that
power is equal to both I2R and
V2/R, depending on which is more convenient to
use. For a fixed resistor, this means that power is a quadratic function
of either current or voltage. Understand why this is so.
- Be able to rank bulb brightness in a network of bulbs. It is generally
sufficient to rank brightness by current considerations alone. Also be on
top of scenarios in which a bulb is added or removed, and its impact on the
brightness of other bulbs in the system.
- Know why we want to deliver electricity to homes at high voltage
rather than low voltage: what does this gain us?
- Know why this choice for high voltage demands AC rather than DC. It's
a multi-part story. Most simply put, we need a way to convert high voltage
to low. We can do this with transformers, but the transformer's secondary
coil needs to see a changing magnetic field to generate any
voltage/current action. The way to get this is to have an oscillating
voltage/current on the primary coil. So we need AC to perform this trick.
- Diodes are one-way current "gates," and are useful for
controlling flow in circuits. They are also used to "rectify" an
AC voltage and turn it into DC (with the help of capacitors to smooth it