Electricity & Magnetism

Question 1

Magnetic Inductance

Answer 1

The process by which a magnetic substance acquires magnetic properties temporarily due to the presence of a magnet close to it.

Question 2

Faraday’s Law

Answer 2

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.

Question 3

Vector Torque acting on a magnetic dipole placed in a magnetic field

Answer 3

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

Question 4

Displacement Current

Answer 4

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.

Question 5

Voltage change when a bar magnet is dropped through a solenoid coil

Answer 5

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.

Question 6

Conductor (properties)

Answer 6

In metals, about one electron per atom is delocalised

Question 7

Insulator (Property)

Answer 7

All electrons are bound to nearby atoms

Question 8

Dipole

Answer 8

… is a system of two equal but opposite charges separated by a small distance

Question 9

Polar Molecules

Answer 9

… have permanent dipole moments because their centres of positive and negative charge do not coincide. They behave like simple dipoles in an electric field.

Question 10

Nonpolar Molecules

Answer 10

… do not have permanent dipole moments, but they acquire induce dipole moments in the presence of an electric field.

Question 11

Gauss’ Law

Answer 11

The net outward electric flux through a closed surface equals the net charge within the surface divided by ε_0

Question 12

Electrostatic Potential Energy

Answer 12

… of a system of point charges is the work needed to bring the charges from an infinite separation to their final positions.

Question 13

Capacitor

Answer 13

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.

Question 14

Dielectrics (Macroscopic Behaviour)

Answer 14

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.

Question 15

Dielectric (Microscopic View)

Answer 15

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.

Question 16

Piezoelectric Effect

Answer 16

In certain crystals a mechanical stress changes the polarisation of the material, which results in a voltage across the crystal.

Question 17

Pyroelectric Effect

Answer 17

In certain crystals an increase in temperature changes the polarisation of the material, which results in a voltage across the crystal.

Question 18

Electric Current

Answer 18

The rate of flow of electric charge through a cross-sectional area.

Question 19

Drift Velocity

Answer 19

The average velocity attained by charge particles in a material due to an electric field.

Question 20

Ideal Battery

Answer 20

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.

Question 21

Kirchhoff’s 1st Law

Answer 21

When any closed loop is traversed, the algebraic sum of the changes in potential around the loop must equal zero.

Question 22

Kirchhoff’s 2nd Law

Answer 22

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.

Question 23

The Hall Effect

Answer 23

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.

Question 24

Lenz’s Law

Answer 24

The induced emf and induced current are in such a direction as to oppose, or tend to oppose, the change that produces them.

Question 25

Transformers

Answer 25

A transformer is a device used to increase or decrease the voltage in a circuit without an appreciable loss in power.