Concept Summary

Current

• Current is the movement of charge that occurs between two points that have different electrical potentials.

  • By convention, current is defined as the movement of positive charge from the high-potential end of a voltage source to the low-potential end.
  • In reality, it is negatively charged particles (electrons) that move in a circuit, from low potential to high potential.

• Current flows only in conductive materials.

  • Metallic conduction relies on uniform movement of free electrons in metallic bonds.
  • Electrolytic conduction relies on the ion concentration of a solution.
  • Insulators are materials that do not conduct a current.

• Kirchhoff’s laws express conservation of charge and energy.

  • Kirchhoff’s junction rule states that the sum of currents directed into a point within a circuit equals the sum of the currents directed away from that point.
  • Kirchhoff’s loop rule states that in a closed loop, the sum of voltage sources is always equal to the sum of voltage drops.

Resistance

• Resistance is opposition to the movement of electrons through a material.
• Resistors are conductive materials with a moderate amount of resistance that slow down electrons without stopping them.
• Resistance is calculated using the resistivity, length, and cross-sectional area of the material in question.
• Ohm’s law states that for a given resistance, the magnitude of the current through a resistor is proportional to the voltage drop across the resistor.

• Resistors in circuits can be combined to calculate the equivalent resistance of a full or partial circuit.

  • Resistors in series are additive and sum together to create the total resistance of a circuit.
  • Resistors in parallel cause a decrease in equivalent resistance of a circuit.
• Across each resistor in a circuit, a certain amount of power is dissipated, which is dependent on the current through the resistor and the voltage drop across the resistor.

Capacitance and Capacitors

• Capacitors have the ability to store and discharge electrical potential energy.
• Capacitance in parallel plate capacitors is determined by the area of the plates and the distance between the plates.
• Capacitors in series cause a decrease in the equivalent capacitance of a circuit.
• Capacitors in parallel sum together to create a larger equivalent capacitance.
• Dielectric materials are insulators placed between the plates of a capacitor that increase capacitance by a factor equal to the material’s dielectric constant, κ.

Meters

• Ammeters are inserted in series in a circuit to measure current; they have negligible resistance.
• Voltmeters are inserted in parallel in a circuit to measure a voltage drop; they have very large resistances.
• Ohmmeters are inserted around a resistive element to measure resistance; they are self-powered and have negligible resistance.