Electricity & Magnetism Flashcards
Magnetic Inductance
The process by which a magnetic substance acquires magnetic properties temporarily due to the presence of a magnet close to it.
Faraday’s Law
The magnitude of the emf induced in a circuit is proportional to the rate of change with time, t, of the magnetic flux, “phi”, that cuts across the circuit.
Vector Torque acting on a magnetic dipole placed in a magnetic field
The cross product between vector dipole moment ,µ, and vector Magnetic field, B. Where the direction of Torque is perpendicular to that of Magnetic field, B
Displacement Current
When potential difference between the plates of a capacitor changes with time then variable electric field is set up between the plates and this variable electric field gives rise to displacement current.
Voltage change when a bar magnet is dropped through a solenoid coil
When the bar falls through the solenoid coil, due to Faraday’s Law, the change in magnetic flux induces an emf across the coil which in turn produces a voltage across its end.
Initially a huge spike is formed on the voltage as it enters, decreasing to zero until when the bar is symmetrically centred in the coil. Then the voltage increases in the opposite direction to a peak.
Conductor (properties)
In metals, about one electron per atom is delocalised
Insulator (Property)
All electrons are bound to nearby atoms
Dipole
… is a system of two equal but opposite charges separated by a small distance
Polar Molecules
… have permanent dipole moments because their centres of positive and negative charge do not coincide. They behave like simple dipoles in an electric field.
Nonpolar Molecules
… do not have permanent dipole moments, but they acquire induce dipole moments in the presence of an electric field.
Gauss’ Law
The net outward electric flux through a closed surface equals the net charge within the surface divided by ε_0
Electrostatic Potential Energy
… of a system of point charges is the work needed to bring the charges from an infinite separation to their final positions.
Capacitor
A capacitor is a device for storing charge and energy. It consists of two conductors that are insulated from each other and carry equal and opposite charges.
Dielectrics (Macroscopic Behaviour)
A nonconducting material is called a dielectric. When a dielectric is inserted between the plates of a charged, electrically isolated capacitor, the electric field between the plates is weakened and the capacitance is thereby increased by a factor κ.
Dielectric (Microscopic View)
The electric field in the dielectric of a capacitor is weakened because the molecular dipole moments tend to align with the applied field and thereby produce a second electric field inside the dielectric that opposes the applied field.
Piezoelectric Effect
In certain crystals a mechanical stress changes the polarisation of the material, which results in a voltage across the crystal.
Pyroelectric Effect
In certain crystals an increase in temperature changes the polarisation of the material, which results in a voltage across the crystal.
Electric Current
The rate of flow of electric charge through a cross-sectional area.
Drift Velocity
The average velocity attained by charge particles in a material due to an electric field.
Ideal Battery
An ideal battery is a source of emf that maintains a constant potential differences between its two terminals, independent of the current through the battery.
Kirchhoff’s 1st Law
When any closed loop is traversed, the algebraic sum of the changes in potential around the loop must equal zero.
Kirchhoff’s 2nd Law
At any junction (branch point) in a circuit where the current can divide, the sum of the currents into the junction must equal the sum of the currents out of the junction.
The Hall Effect
When a conducting strip carrying a current is placed in a magnetic field, the magnetic force on the charge carriers causes a separation of charge called the Hall effect.
Lenz’s Law
The induced emf and induced current are in such a direction as to oppose, or tend to oppose, the change that produces them.
Transformers
A transformer is a device used to increase or decrease the voltage in a circuit without an appreciable loss in power.
