Electricity and Electric Ciruits Flashcards
1
Q
Electric Current
A
- An electric current, I, measures the flow of charge from one point to another point during a certain amount of time
- I = ΔQ/Δt
- Current flows in the same direction as positive charges
- Current flows in the opposite direction as negative charges
- Current flows from a region of higher electrical potential to lower electrical potential
2
Q
Electromotive Force
What Generates emf?
A
- The electromotive force, emf, is not really a force, but rather an energy-per-unit-charge quantify (much like a potential)
- Consider emf as a voltage source dervied from a chemical reaction
- Unit volts (J/C)
- Batteries are sources of emf
- Long line represents cathode and short line represents anode
3
Q
Resistivity
A
- Resistivity, ρ, of a material is a measure of how difficult it is for charges to conduct through the material; higher resistivities are associated with better electrically insulating materials
- ρ = E/J
- E = electiric field
- J = current density
4
Q
Conductivity
A
- Conductivity, σ, is the reciprocal of the resistivity
- A material with low resistivity has high conductivity
- σ = 1/ρ
5
Q
Electrical Resistance
A
- Electrical resistance, R, of a conductor differs from resistivity in that it considers both the material’s condutivity and the dimensions of the resistive device
- R = ρL/A
- Units ohm, Ω
6
Q
Resistor
A
- A resistor is any device in a circuit that hinders current and drains power from the circuit
- Almost everything is a resistor to some degree
7
Q
Ohm’s Law
A
- Ohm’s law describes the relationship between voltage, current, and resistance
- V = IR
8
Q
Electrical Power
A
- Power is the rate of conversion of electrical potential energy into some other type of energy
- P = IV
9
Q
Ohm’s Law and Power
A
- P = V2/R
- P = I2R
10
Q
Capacitors and Capacitance
A
- A capacitor is formed from two conductors separated by an insulator
- The capacitance, C, of a capacitor is the amount of charge q that can be stored per volt of potential differnce V across the two parallel palates
- C = q/V
11
Q
Dielectrics
A
- An added dielectric effectively decreases the electric field of the capacitor, effectively increasing the capacitor’s polarity, and also increasing the capacitor’s capacitance
- A capacitor’s capacitance will increase by a factor κ, the dielectric’s specific constant
- q = CV = κCvacuumV
12
Q
Charging a Capacitor
A
- An emf source can charge a capacitor
13
Q
Kirchhoff’s Loop Rule
A
- States that the algebraic sum of the potential differnces (voltage changes) in any closed circuit is equal to zero
14
Q
Kirchhoff’s Junction Rule
A
- The total current flowing through the pathways leaving the junction must equal the current that entered the junction
- i.e., all of the elctrons entering the junction must be accounted for - no electrons fall off of the wire or jump onto another wire
15
Q
Series and Parallel
A
- Circuit units can be grouped in both series and parallel
- Circuit elements in series share the same current
- Circuit elements in parallel share the same voltage drop
16
Q
Resistors in Series and Parallel
A
- We can treat resistors in series like one large resistor; electrical resistance of series resistors is additive
- Requivalent = R1 + R2 + R3 + …
- We can treat resistors in parallel like a fatter resistor; electrical resistance decreases with increasing cross sectional area
- 1/Reqivalent = 1/R1 + 1/R2 +1/R3 + …
- Requivalent = (R1x R2)/(R1 + R2)
17
Q
Capacitors in Series and Parallel
A
- Capacitors in series is calculated in the same fashion as resitors in parallel
- Capacitors in series will store the same amount of charge; larger capacitors will have a smaller voltage gain and also stores less energy
- 1/Cequivalent = 1/C1 + 1/C2 + 1/C3 + …
- Capacitors in parallel is like making one fat capictor, and capcitance increases with cross sectional area; capacitance is additve in parallel
- Cequivalent = C1 + C2 + C3 + …
18
Q
Average Power
A
- Pavg = Irms Vrms
- Irms = I/sqrt(2)
- Vrms = V/sqrt(2)
- Pavg = 1/2(IV)
