Self Inductance Solenoid Flux Coils of wire carrying

Self Inductance

Solenoid Flux ] Coils of wire carrying current generate a magnetic field. NI, l • Strong field inside solenoid B ] If the current increases then F the magnetic field increases. • Increased magnetic flux ] The magnetic flux through all coils depends on the coil area and the number of turns.

Inductance ] The ratio of magnetic flux to current is the inductance. ] Inductance is measured in henrys. • 1 H = 1 T m 2 / A • More common, 1 H = 1 V / A / s ] The inductance can be derived for an ideal solenoid.

Coil Length ] An inductor is made by wrapping a single layer of wire around a 4. 0 -mm diameter cylinder. The wire is 0. 30 mm in diameter. ] What coil length is needed to have an inductance of 10 m. H? ] The formula is based on the radius and length of the coil. • Radius r = 2. 0 x 10 -3 m • Turns N = l / d ] Substitute for N in the formula.

Electric Inertia ] A changing current will Increasing I, F create a changing flux. B ] Faraday’s law states that the changing flux will create an emf. • Direction from Lenz’s law ] The emf acts to oppose the change in flux. • Inertial response Decreasing I, F B

Back-Emf ] Motors have internal coils. ] The self-inductance will oppose a change in current by creating an emf. ] This back-emf is responsible for excess power draw when a motor starts.

Induced EMF ] Faraday’s law gives the magnitude of the induced emf. • Depends on rate of change ] The definition of inductance gives a relationship between voltage and current. • More useful in circuits ] Inductive elements in a circuit act like batteries. • Stabilizes current

Mutual Inductance ] The definition of inductance applies to transformers. • Mutual inductance vs selfinductance ] Mutual inductance applies to both windings.

Stored Energy ] Electrical power is voltage times current. • • • True for emf from inductance Average current is approximately one half maximum Use one half to get average power ] Magnetic energy is stored in a magnetic field. • Energy is the power times the time

Energy Density ] The energy density in a solenoid is based on its volume. ] The energy density in a capacitor is based on its volume. ] The energy density can be expressed in terms of the magnetic field. ] The energy density can be expressed in terms of the electric field. next