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Electrostatics
- Charge, conductors, charge conservation
- Charges are either positive or negative. Zero charge is neutral.
- Like charges repel, unlike charges attract.
- Charge is quantized, and the unit of charge is the Coulomb.
- Conductors are materials in which charges can move freely. Metals are good conductors.
- Charge is always conserved. You can't create or destroy charge, you can only transfer charge from one source to another.
- Insulators
- Insulators are materials in which charges can not move freely. Nonmetals are good insulators.
- Coulomb's law (F = kq1q2/r2, sign conventions)
- F = kq1q2/r2
- k = 9E9 Nm2/C2
- If the charges have the same sign, the force is repulsive.
- If the charges have opposite signs, the force is attractive.
- Electric field
- field lines
- Electric field is denoted by the vector E.
- Lines that are closer together denote stronger fields than lines that are farther apart.
  
- Electric fields come out of positive charges, and goes into negative charges.
- The unit for electric field is N/C, or Newtons per Coulomb.
- field due to charge distribution
- Field lines come out of the positive end and goes into the negative end of a dipole.
- Field lines for two negative charges are the same as those for two positive charges except that the direction of the field lines would be reversed.
- The direction and magitude of the field at any point in space can be calculated as the vector sum of all the field components there.
- Electric field in between a capacitor is uniform until it reaches the ends of the capacitor.
- Electric field for wires runs radially perpendicular to the wire.
- Electric field for a cylinder runs radially perpendicular to the cylinder, and is zero inside the cylinder.
- Potential difference, absolute potential at point in space
 
- Absolute potential (V) is the amount of energy per charge that something possesses.
- V = U/q0 = kq/r
- V is the electric potential (absolute potential) caused by q, which is experienced by q0.
- q is the charge that is causing the potential, not the charge that's experiencing the potential.
- Traditionally, q0 is the charge experiencing the potential. The magnitude of q0 is very small.
- U is the electrical potential energy possessed by q0.
- r is the distance between the potential-causing charge and the charge that's experiencing the potential (r is always positive).
- if there are multiple charges contributing to the potential, then calculate the potentials each of them causes (positive charges cause positive potentials, and negative charges cause negative potentials), and sum them together.
- The unit for potential is Volts (V) or Joules per Coulomb (J/C).
- Potential difference (ΔV) is the difference between two potentials.
- ΔV = VB - VA
- Potential difference is used in scenarios such as the difference in potential between the two plates of a capacitor, or the positive and negative terminals of a battery.
- Equipotential lines
- Equipotential lines are places where the potential is the same.
- Equipotential lines are always perpendicular to electric field lines.
- Electric dipole
- definition of dipole
- dipole = a positive charge and a negative charge separated by some distance.
- behavior in electric field
- A dipole in an electric field will want to align itself with the electric field, such that the positive end of the dipole is in the direction of the electric field.
- potential due to dipole
- To calculate the exact potential at a given point, just calculate the individual potential due to the positive charge and the negative charge, then add them together.
- Electrostatic induction
- Induction does not involve any type of conduction.
- Electrostatic induction is where a charged object induces the movement / redistribution of charges in another object.
- The classical example of electrostatic induction is picking up pieces of paper using a comb rubbed against fur.
- It's called electrostatic induction because it's static - the charged species polarizes non-charged species by simply being there. This is not the same as electromagnetic induction, which is how electric generators work. Luckily electromagnetic induction is not listed as an official AAMC topic.
- Gauss' law
- ΦE = EA cosθ
- ΦE is electric flux.
- E is electric field, A is area that the field goes through, and θ is the angle between the field and the normal of the area.
- ΦE = q/ε0
- For an enclosed surface, the electric flux is equal to q, the charge inside the enclosure, over the permitivity of free space.
- The net electric flux through any enclosed surface is totally dependent on the charge inside. If there's no charge inside, then the net electric flux through the enclosure is zero.
- An important application of Gauss's law is the Faraday cage. Basically, the electric field inside a closed conducting cage is zero. This is because the charges on the conducting cage will rearrange to cancel out any external field.
Magnetism
- Definition of the magnetic field B
- Magnetic field B exists in a region of space if a moving charge experiences a force due to its motion in that region.
- The unit for magnetic field is the Tesla (T) or N·s/m·C
- Existence and direction of force on charge moving in magnetic field
- F = qvB sinθ
- θ is the angle between the charge velocity and the magnetic field. Sometimes the sinθ is omitted as θ is assumed to be 90°.
- The force is always perpendicular to both the magnetic field and to the velocity of the charge.
- You can use the right hand rule to predict the direction of the force. The thumb is the direction of a positive charge, the middle finger is the direction of the magnetic field, and the palm faces the direction of the force.
- Special scenarios / cases
- Charge moving in a circle
- F = qvB = mv2/r
- You are setting the electromagnetic force equal to the centripetal force, which maintains the orbit. Using this equation, you can solve for whatever the question asks you.
- Current carrying wires
- F = qvB sinθ = (it)vB sinθ = (it)(L/t)B sinθ = iLB sinθ
- i is current, L is length of wire.
- Consider the current in the wire as moving positive charges (by tradition, the direction of the current is defined as the direction of moving positive charges).
- You can calculate the direction of the force on the wire in the same way using the right hand rule. Just treat the direction of the current the same as the direction of velocity of a positive charge.
- Two wires will attract each other if the current is in the same direction.
- Two wires will repel each other if the current is in opposite directions.
Electromagnetic Radiation (Light)
- Properties of electromagnetic radiation (general properties only)
- radiation velocity equals constant c, in vacuo
- Electromagnetic radiation travels fastest in a vacuum, at a velocity equals c, or 3x108m/s
- Light slows down when it travels in a medium other than in vacuo.
- n = c/v, where n is the index of refraction for the medium, and v is the speed of light travelling in that medium.
- radiation consists of oscillating electric and magnetic fields that are mutually perpendicular to each other and to the propagation direction
- Classification of electromagnetic spectrum (radio, infrared, UV, X-rays, etc.)
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| Lower frequency, longer wavelength, less energy |
| Radio | Causes electronic oscillations in the antenna |
| Microwave | Causes molecular rotation |
| Infrared | Causes molecular vibration |
| Visible | Can excite electrons to orbits of higher energy. Visible light ranges from 400-700 nm. 400ish being violet, 700ish being red. |
| Ultraviolet | Can break bonds and excite electrons so much as to eject them, which is why UV is considered ionizing radiation. |
| X-rays | Ionizing radiation, photoelectric effect |
| Gamma rays | Even more energetic than X-rays |
| Higher frequency, shorter wavelength, more energy |
Old AAMC Topics: the topics below have either been removed or modified from the official AAMC outline.
Magnetism
- Orbits of charged particles moving in magnetic field
- Perfect orbit occurs when qvB = mv2/r
- When qvB < mv2/r, there isn't enough centripetal force, and the charged particle flies out of orbit.
- When qvB > mv2/r, there's too much centripetal force, and the charged particle spirals inward.
- General concepts of sources of the magnetic field
- Anything that involves a moving charge creates a magnetic field
- Moving charges.
- Current carrying wire.
- Solenoids and toroids.
- The Earth (electric current in the liquid core).
- Atoms with unpaired electrons is the other source of magnetic fields. This is basically the same deal as moving charges, since the unpaired electrons orbiting the nuclei is the same thing as moving charges.
- Magnets.
- Individual atoms of Ferromagnetic and Paramagnetic create magnetic fields because they have unpaired electrons. Ferromagnetic materials have domains of aligned atoms that make them even more susceptible to be magnetized. Both Ferro and paramagnetic material are attracted to magnetic fields.
- Diamagnetic atoms don't create magnetic fields because the electrons are paired, so their individual fields cancel out. Diamagnetic fields actually is repeled by an external magnetic field.
- Nature of solenoid, toroid
- Solenoid
- The solenoid is just a coil of current-carrying wire.
- B = μ0nI.
- n is the number of loops per meter. I is current.
- The magnetic field produced by a solenoid is directly proportional to the number of coils, and to the current.
- Toroid
- Toroid is just a solenoid in a circle.
- B = μ0NI/circumference
- N is the total number of loops, I is the current.
- More loops, smaller circle → greater magnetic field.
- Ampere's law for magnetic field induced by current in straight wire and other simple configurations
- Ampere's law lets you calculate the magnetic field at a radius r from a current-carrying wire: B = μ0I/2πr
- Comparison of E and B relations
- force of B on a current
- F = qvB sinθ = (it)vB sinθ = (it)(L/t)B sinθ = iLB sinθ
- i is current, L is length of wire.
- Consider the current in the wire as moving positive charges (by tradition, the direction of the current is defined as the direction of moving positive charges).
- You can calculate the direction of the force on the wire in the same way using the right hand rule. Just treat the direction of the current the same as the direction of velocity of a positive charge.
- Two wires will attract each other if the current is in the same direction.
- Two wires will repel each other if the current is in opposite directions.
- energy
- Oscilations of electric and magnetic fields (electromagnetic radiation) has energy.
- E = hν
- E is energy per photon, h is Planck's constant, and ν is the frequency of the electromagnetic wave.
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