Electrical Circuits
85 flashcards by MentisLibrary
From Ohm's law to phasors, Thevenin equivalents, resonance, and power factor correction. The circuit analysis fundamentals that carry you through the whole electrical curriculum.
- What is electric charge, and what unit measures it?
- Charge is the fundamental electrical property of matter, carried by electrons (negative) and protons (positive). The SI unit is the coulomb (C). One electron carries 1.602e-19 C.
- Define electric current.
- Current is the rate of flow of charge past a point in a circuit. One ampere is one coulomb per second.
- What is voltage (potential difference)?
- Voltage is the energy per unit charge needed to move charge between two points. One volt is one joule per coulomb.
- State Ohm's law.
- The current through a resistor is proportional to the voltage across it and inversely proportional to its resistance.
- Give the three forms of the electrical power formula.
- Power is the rate of energy transfer, measured in watts. Substituting Ohm's law into P = VI gives the other two forms.
- How do you compute electrical energy consumed, and what is a kilowatt hour?
- Energy is power multiplied by time. A kilowatt hour is 1000 W drawn for one hour, the unit on an electricity bill.
- How does the resistance of a wire depend on its dimensions and material?
- Resistance rises with resistivity and length, and falls with cross sectional area.
- What are conductance and conductivity?
- Conductance is the reciprocal of resistance, measured in siemens (S). Conductivity is the reciprocal of resistivity.
- How does temperature change the resistance of a metal conductor?
- For metals, resistance increases roughly linearly with temperature over normal ranges.
- How do resistors combine in series?
- Series resistances add directly, because the same current flows through each and the voltages add.
- How do resistors combine in parallel?
- Reciprocals add. For exactly two resistors the product over sum shortcut applies.
- Write the voltage divider rule.
- In a series string, each resistor takes a share of the source voltage proportional to its own resistance.
- Write the current divider rule for two parallel resistors.
- Current splits inversely with resistance, so each branch gets the share set by the OTHER resistor over the sum.
- State Kirchhoff's current law (KCL).
- The algebraic sum of currents entering a node is zero, since charge cannot accumulate at a junction.
- State Kirchhoff's voltage law (KVL).
- The algebraic sum of voltages around any closed loop is zero, because energy per charge returns to its starting value.
- Outline node voltage analysis.
- Choose a reference node (ground), label unknown node voltages, write KCL at each unknown node with branch currents expressed as (Vnode - Vother)/R, then solve the linear system.
- Outline mesh current analysis.
- Assign a circulating current to each independent mesh (usually clockwise), write KVL around each mesh, and solve. Shared branches carry the difference of two mesh currents.
- State the superposition theorem.
- In a linear circuit, the response to several independent sources equals the sum of the responses to each source acting alone, with the others deactivated.
- What is source transformation?
- A real voltage source (Vs in series with R) and a real current source (Is in parallel with the same R) are interchangeable at their terminals.
- State Thevenin's theorem.
- Any linear two terminal network can be replaced by a single voltage source Vth in series with a single resistance Rth.
- How do you find Rth when the circuit has only independent sources?
- Deactivate every independent source (short voltage sources, open current sources) and compute the resistance looking back into the terminals.
- State Norton's theorem and its relation to Thevenin.
- Any linear two terminal network reduces to a current source IN in parallel with RN, where RN = Rth and IN is the short circuit current.
- State the maximum power transfer theorem for DC.
- A load draws maximum power when its resistance equals the Thevenin resistance of the source network.
- Define capacitance.
- Capacitance is the charge stored per volt applied, measured in farads. One farad is one coulomb per volt.
- What sets the capacitance of a parallel plate capacitor?
- Plate area and dielectric permittivity increase it, plate separation decreases it.
- How much energy is stored in a capacitor?
- Energy is one half the capacitance times the voltage squared.
- How do capacitors combine in series and parallel?
- The rules are the reverse of resistors: parallel capacitances add, series capacitances add as reciprocals.
- Write the current voltage relationship for a capacitor.
- Capacitor current is proportional to the RATE OF CHANGE of its voltage, so a constant DC voltage gives zero current.
- What is the time constant of an RC circuit?
- Tau is the product of resistance and capacitance, the time for the exponential to move about 63.2 percent of the way to its final value.
- Give the charging equation for an RC circuit driven by a step Vs.
- The capacitor voltage rises exponentially from zero toward the source voltage.