Laws of physics

Question 1

Faraday’s Law

Answer 1

When an E.M.F is induced, the magnitude of the induced E.M.F is equal to the rate of change of flux linkage.

Question 2

Lenz’s Law

Answer 2

An induced E.M.F always opposes the change that causes it

Question 3

Newton’s 3rd Law

Answer 3

If a body A exerts a force on body B, then B exerts an equal and opposite force on A

Question 4

Newton’s 2nd Law

Answer 4

The rate of change of momentum of an object is proportional to the resultant force acting on it, and takes place in the direction of that force

Question 5

First law of thermodynamics

Answer 5

The increase, U, in internal energy of a system is U = Q – W in which Q is the heat entering the system and W is the work done by the system. Any of the terms in the equation can be positive or negative, e.g. if 100 J of heat is lost from a system Q = –100 J.

Question 6

Ohm’s law

Answer 6

The current in a metal wire at constant temperature is proportional to the pd across it.

Question 7

Hooke’s law

Answer 7

The tension in a spring or wire is proportional to its extension from its natural length, provided the extension is not too great.

Question 8

Newton’s law of gravitation

Answer 8

The gravitational force between two particles is proportional to the product of their masses, m1 and m2, and inversely proportional to their separation squared, r2.
F  Gm m in which G is the gravitational constant. r2
G = 6.67  10-11N m2 kg-2.
12

Question 9

Coulomb’s law

Answer 9

The electrostatic force, F, between two small bodies is proportional to the product of their charges, Q1 and Q2, and inversely proportional to their separation squared, r2.
F  Q Q in which  is the permittivity of free space = 8.85 40r2 0
 10-12 F m-1

Question 10

Wien’s displacement law

Answer 10

The wavelength of peak emission from a black body is inversely proportional to the absolute (kelvin) temperature of the body.
  W [W  the Wien constant = 2.90  10-3 m K] max T [W=theWienconstant=2.9010-3 mK]

Question 11

Stefan’s law

Answer 11

The total electromagnetic radiation energy emitted per unit time by a black body is given by power = A σT4 in which A is the body’s surface area and σ is a constant called the Stefan constant. [σ = 5.67  10-8 W m-2 K-4]

Question 12

Kepler’s laws of planetary motion: 1

Answer 12

Each planet moves in an ellipse with the Sun at one focus.

Question 13

Kepler’s laws of planetary motion: 2

Answer 13

The line joining a planet to the centre of the Sun sweeps out equal areas in equal times.

Question 14

Kepler’s laws of planetary motion: 3

Answer 14

T2, the square of the period of the planet’s motion, is proportional to r3, in which r is the semi-major axis of its ellipse. [For orbits which are nearly circular, r may be taken as the mean distance of the planet from the Sun.]

Question 15

Snell’s law

Answer 15

At the boundary between any two given materials, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant.

Question 16

Faraday’s law

Answer 16

When the flux linking an electrical circuit is changing, an emf is induced in the circuit of magnitude equal to the rate of change of flux linkage.
E   (N) [Note: the  sign is from Lenz’s law, see below]

Question 17

Lenz’s law

Answer 17

The direction of any current resulting from an induced emf is such as to oppose the change in flux linkage that is causing the current.