Electrostatics + Electric currents + Electromagnetism

\*Law of Conservation of Charge

The total electric charge of an isolated system remains constant.

Conductor

material through with electric charge flows freely

Insulator

material through which electric charge does **not** flow freely

\*Coulomb’s Law

The electric force between two point charges is directly proportional to the product of the two charges and inversely proportional to square of the distance between them, and directed along the line joining the two charges.  (F = k q1 q2 / r2)

Insulator

material through which electric charge does **not** flow freely

\*Electric Field Strength (E)

Electric force per positive unit test charge  (E = F/q)

Radial Field

field that extends radially (like the electric field around a point charge or the gravitational field around a planet)

\*Electric Potential (V)

work done per unit charge moving a small positive test charge in from infinity to a point in an electric field. (V = W/q)  (V = kq/r) (NOTE: the work done is path independent)

\*Electric Potential Energy (Ee)

energy that a charge has due to its position in an electric field

\*Electric Potential Difference (ΔV)

electric potential energy difference per unit charge between two points in an electric field (ΔV = ΔEe / q   OR   ΔV = W / q)

\*Electronvolt (eV)

energy gained by an electron moving through an electric potential difference of one volt.  (OR: Work done moving an electron through an electric potential difference of one volt.)  (1 eV = 1.60 x 10-19 J)

\*Electric Current (I)

current is defined in terms of the force per unit length between parallel current-carrying conductors (NOTE: one ampere of current is the amount of current in each of two infinitely long straight wires one meter apart experiencing a magnetic force per unit length of 2 x 10-7 newtons)

\*Resistance (R)

ratio of potential difference applied to a device to the current through the device (R = V/I)

Resistor

device with a constant resistance (Ohmic device) over a wide range of potential differences

\*Ohm’s Law

The current flowing through a device is proportional to the potential difference applied across it providing the temperature is constant.  (NOTE: R = V/I is **not** a statement of Ohm’s Law)

Ohmic Device

one whose resistance remains constant over a wide range of potential differences (eg – resistor)

Non-Ohmic Device

one whose resistance does **not** remain constant over a wide range of potential differences (eg – filament lamp)

\*Electromotive Force (emf) (ε)

Total energy difference per unit charge around a circuit  (total energy per unit charge made available by the chemical reaction in the battery) (ε = ΔEe/q    OR   ε = W/q)

Total energy difference per unit charge around a circuit  (total energy per unit charge made available by the chemical reaction in the battery) (ε = ΔEe/q    OR   ε = W/q)

resistance inside a battery that causes the battery’s terminal potential difference to be less than its emf (NOTE: internal resistance in a meter causes it not to act as an ideal meter)

Ideal Ammeter

one with zero internal resistance – must be placed in series

Ideal Voltmeter

one with infinite internal resistance – must be placed in parallel

Potential Divider

two resistors placed in series that divide up the battery’s potential difference (R1 / R2 = V1 / V2)Light-Dependent Resistor (LDR)

Light-Dependent Resistor (LDR)

sensor whose resistance depends on amount of light shining on its surface – increase in light causes a decrease in resistance

\*Magnitude of a Magnetic Field (magnetic field strength, magnetic field intensity, magnetic flux density) (B)

ratio of magnetic force on a current carrying conductor to the product of the current and length of wire and sine of the angle between the current and the magnetic field  (B = FB / Ilsinθ) (OR: ratio of magnetic force on a charged particle to the product of the charge and its velocity and the sine of the angle between the velocity and the magnetic field)  (B = FB / qvsinθ)

\*Direction of a Magnetic Field

the direction that the North pole of a small test compass would point if placed in the field  (N to S)

\*Magnetic Flux (Φ)

product of the magnetic field strength and a cross-sectional area and the cosine of the angle between the magnetic field and the **normal** to the area (Φ = B A cosθ)

\*Magnetic Flux Linkage

product of the magnetic flux through a single coil and the total number of coils (flux linkage = N Φ)

\*Faraday’s Law

The emf induced by a time changing magnetic field is proportional to the rate of change of the flux linkage. (ε α  N ΔΦ/Δt)

\*Lenz’s Law

The direction of an induced emf is such that it produces a magnetic field whose direction opposes the change in magnetic field that produced it. (NOTE: This is the negative sign added to Faraday’s law.  ε= - N ΔΦ/Δt)

Root Mean Square (rms) Value of an Alternating Current (or Voltage)

 the value of the direct current (or voltage) that dissipates power in a resistor at the same rate (NOTE: The rms value is also known as the “rating.”)