current

Examples of current in the following topics:

• Current and Voltage Measurements in Circuits

  • An electrical circuit is a type of network that has a closed loop, which provides a return path for the current.
  • Resistance is inversely proportional to current.
  • More specifically, Ohm's law states that R in this relation is constant, independent of the current.
  • Using this equation, we can calculate the current, voltage, or resistance in a given circuit.
  • Describe the relationship between the electrical current, voltage, and resistance in a circuit

• Energy Stored in a Magnetic Field

  • This changing magnetic flux produces an EMF which then drives a current.
  • When a conductor carries a current, a magnetic field surrounding the conductor is produced.
  • The resulting magnetic flux is proportional to the current.
  • If the current changes, the change in magnetic flux is proportional to the time-rate of change in current by a factor called inductance (L).
  • Thus, inductors oppose change in current by producing a voltage that,in turn, creates a current to oppose the change in magnetic flux; the voltage is proportional to the change in current.

• RL Circuits

  • We know from Lenz's law that inductors oppose changes in current.
  • is the current in an RL circuit when switched on.
  • The current will be 0.632 of the remainder in the next time.
  • In each successive time $\tau$, the current falls to 0.368 of the preceding value, and in a few multiples of $\tau$, the current becomes very close to zero.
  • (a) An RL circuit with a switch to turn current on and off.

• Inductors in AC Circuits: Inductive Reactive and Phasor Diagrams

  • The graph shows voltage and current as functions of time.
  • The current then becomes negative, again following the voltage.
  • Current lags behind voltage, since inductors oppose change in current.
  • Changing current induces an emf .
  • (b) Graph of current and voltage across the inductor as functions of time.

• Electric Currents and Magnetic Fields

  • Electric current produces a magnetic field.
  • Each segment of current produces a magnetic field like that of a long straight wire, and the total field of any shape current is the vector sum of the fields due to each segment.
  • Integral calculus is needed to sum the field for an arbitrary shape current.
  • You can think of the "surface" as the cross-sectional area of a wire carrying current.
  • A current-carrying wire feels a force in the presence of a magnetic field.

• Root Mean Square Values

  • The root mean square (RMS) voltage or current is the time-averaged voltage or current in an AC system.
  • Unlike direct current (DC), where the currents and voltages are constant, AC currents and voltages vary over time.
  • Here, I is the current at time t, and I0=V0/R is the peak current.
  • (a) DC voltage and current are constant in time, once the current is established.
  • Relate the root mean square voltage and current in an alternating circut with the peak voltage and current and the average power

• Back EMF, Eddy Currents, and Magnetic Damping

  • If motional EMF can cause a current loop in the conductor, we refer to that current as an eddy current.
  • Moreover, adjacent loops have currents in opposite directions, and their effects cancel.
  • A common physics demonstration device for exploring eddy currents and magnetic damping.
  • As it enters and leaves the field, the change in flux produces an eddy current.
  • Magnetic force on the current loop opposes the motion.

• Different Types of Currents

  • Direct current (DC) is the unidirectional flow of electric charge.
  • A term formerly used for direct current was galvanic current.
  • A direct current circuit is an electrical circuit that consists of any combination of constant voltage sources, constant current sources, and resistors.
  • A particular circuit voltage or current does not depend on the past value of any circuit voltage or current.
  • The current i flowing through the circuit is given by Ohm's law.

• The Junction Rule

  • Kirchhoff's junction rule states that at any junction (node) in an electrical circuit, the sum of the currents flowing into that junction is equal to the sum of the currents flowing out of that junction.
  • Thus, Kirchoff's junction rule can be stated mathematically as a sum of currents (I):
  • where n is the total number of branches carrying current towards or away from the node.
  • This flow would be a current, thus violating Kirchhoff's junction law.
  • Kirchhoff's Junction Law illustrated as currents flowing into and out of a junction.

• Ohm's Law

  • What drives current?
  • The electric field, in turn, exerts force on charges, causing current.
  • Recall that while voltage drives current, resistance impedes it.
  • Ohmic materials have a resistance R that is independent of voltage V and current I.
  • Or if the current is forced to some value I, then the measured voltage V divided by that current I is also R.