direct current

Examples of direct current in the following topics:

• Different Types of Currents

  • Direct current (DC) is the unidirectional flow of electric charge.
  • The electric charge flows in a constant direction, distinguishing it from alternating current (AC).
  • 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.
  • Describe structure of an electrical circuit and identify elements of a direct current circuit

• Induced EMF and Magnetic Flux

  • It was found that each time the switch is closed, the galvanometer detects a current in one direction in the coil on the bottom.
  • Each time the switch is opened, the galvanometer detects a current in the opposite direction.
  • The current is a result of an EMF induced by a changing magnetic field, whether or not there is a path for current to flow.
  • Each point on a surface is associated with a direction, called the surface normal; the magnetic flux through a point is then the component of the magnetic field along this normal direction.
  • When the switch is opened and closed, the galvanometer registers currents in opposite directions.

• Resistors in AC Circuits

  • Direct current (DC) is the flow of electric charge in only one direction.
  • Alternating current (AC) is the flow of electric charge that periodically reverses direction.
  • (a) DC voltage and current are constant in time, once the current is established.
  • (b) A graph of voltage and current versus time for 60-Hz AC power.
  • Apply Ohm's law to determine current and voltage in an AC circuit

• Ampere's Law: Magnetic Field Due to a Long Straight Wire

  • Current running through a wire will produce a magnetic field that can be calculated using the Biot-Savart Law.
  • Current running through a wire will produce both an electric field and a magnetic field.
  • This relationship holds for constant current in a straight wire, in which magnetic field at a point due to all current elements comprising the straight wire is the same.
  • As illustrated in the direction of the magnetic field can be determined using the right hand rule—pointing one's thumb in the direction of current, the curl of one's fingers indicates the direction of the magnetic field around the straight wire.
  • The direction of the magnetic field can be determined by the right hand rule.

• Electric Currents and Magnetic Fields

  • One way to explore the direction of a magnetic field is with a compass, as shown by a long straight current-carrying wire in .
  • Another version of the right hand rule emerges from this exploration and is valid for any current segment—point the thumb in the direction of the current, and the fingers curl in the direction of the magnetic field loops created by it.
  • where the integral sums over the wire length where vector dℓ is the direction of the current; r is the distance between the location of dℓ, and the location at which the magnetic field is being calculated; and r̂ is a unit vector in the direction of r.
  • The right hand rule can be used to determine the direction of the force on a current-carrying wire placed in an external magnetic field.
  • (b) Right hand rule 2 states that, if the right hand thumb points in the direction of the current, the fingers curl in the direction of the field.

• Magnetic Force on a Current-Carrying Conductor

  • The expression for magnetic force on current can be found by summing the magnetic force on each of the many individual charges that comprise the current.
  • Since they all run in the same direction, the forces can be added.
  • The direction of the magnetic force can be determined using the right hand rule, demonstrated in .
  • The thumb is pointing in the direction of the current, with the four other fingers parallel to the magnetic field.
  • Curling the fingers reveals the direction of magnetic force.

• Magnetic Force Between Two Parallel Conductors

  • Parallel wires carrying current produce significant magnetic fields, which in turn produce significant forces on currents.
  • Parallel wires carrying current produce significant magnetic fields, which in turn produce significant forces on currents.
  • If the currents are in the same direction, the force attracts the wires.
  • If the currents are in opposite directions, the force repels the wires.
  • Currents I1 and I2 flow in the same direction, separated by a distance of r.

• Electric Motors

  • Current in a conductor consists of moving charges.
  • The direction of the Lorentz force is perpendicular to both the direction of the flow of current and the magnetic field and can be found using the right-hand rule, shown in .
  • Using your right hand, point your thumb in the direction of the current, and point your first finger in the direction of the magnetic field.
  • The force on opposite sides of the coil will be in opposite directions because the charges are moving in opposite directions.
  • The force on opposite sides of the coil will be in opposite directions because the charges are moving in opposite directions.

• 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.
  • As it enters from the left, flux increases, and so an eddy current is set up (Faraday's law) in the counterclockwise direction (Lenz' law), as shown.
  • But when the plate leaves the field on the right, flux decreases, causing an eddy current in the clockwise direction that, again, experiences a force to the left, further slowing the motion.
  • Moreover, adjacent loops have currents in opposite directions, and their effects cancel.
  • Magnetic force on the current loop opposes the motion.

• 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 .
  • Again, the phasors are vectors rotating in counter-clockwise direction at a frequency $\nu$ (you can see that the voltage leads the current).