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 questions.

You may also want to study the transmitter questions from class: See entry on Lectures Page just after Lecture 9.

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 75–100 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: F_drag=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 Ohm's Law).
• 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 out).