# Unit 4 Definitions

## Unit 4 Definitions

Force and Momentum

Newton's First Law
An object remains at rest or in uniform motion unless acted on by a resultant force.

Newton's Second Law

The rate of change of momentum of an object is proportional to resultant force acting on it. (Resultant force is proportional to the change of momentum per second)

Force Time graphs
The area under the line of a force time graph represents the change in momentum or the impulse of the force.

Newton's Third Law
When two objects interact, they exert equal and opposite forces on each other.

The Principle of Conservation of Momentum
For a system of interacting objects, the total momentum remains constant, providing no external resultant force acts on the system.

An Eleastic Collision
Is a collision in which there is no loss of kinetic energy

A Totally Inelastic Collision

Is one where the colliding objects stick together.

A Partially inelastic collision

Is where the colliding objects move apart after the collision and have less kinetic energy after the collision than before.

Circular Motion

A Object rotating at a steady rate in said to be in uniform circular motion.

Angular Displacement
The angle an object moving in circular motion turns through.

Angular Speed

the rate of change of angular displacement of an object in circular motion.

Simple Harmonic Motion

An object moves in simple harmonic motion if its acceleration is proportional to the displacement from the equilibrium position and is always directed towards the equilibrium position.

Amplitude
Is the maximum displacement o an oscillating object from its equilibrium position.

Time Period
Is the time for one complete cycle of oscillations.

Frequency
Is the number of cycles per second

Phase Difference
Is the fraction of a cycle between 2 oscillating objects measured in radians (or degrees)

Free Oscillations
Oscillations where there is no damping and no periodic force acting on the system so that the amplitude of oscillations is constant.

Damped Oscillations
Oscillations that reduce in amplitude as a result of resistive forces such as friction or drag.

Light Damping
Amplitude of oscillations gradually decreases, reducing by the same fraction each cycle

Critical Damping
is just enough to stop the system oscillating after it has been displaced and released from the equilibrium position. The oscillating object returns to the equilibrium positin in the shortest possible time without overshooting.

Heavy Damping
The damping is so strong that the displaced object returns to the equilibrium positin slowly. No oscillating motion occurs

Forced Oscillations
Oscillations of a system that is subjected to an external periodic force.

Natural Frequency
When a system oscillates without a periodic force being applied to it, its frequency is refered to as the natural frequency.

Resonance

If the applied frequency of a periodic force is equal to the natural frequency of the system, the system will oscillate with very large amplitude vibrations.

Gravitational Fields

Gravitational field strength
The strength of a gravitational field, g, is the force per unit mass on a small test mass placed in the field

Gravitational Potential
The gravitational potential, V, at any point is the work done per unit mass to move a small object from infinity to that point.

Gravitational Potential Energy
At a point in a gravitational field is the work done to move a small object from infinity to that point

At a point in a gravitational field is the change in potential per metre at that point. (gravitational field strength is the negative of potential gradient)

Newton's Law of Gravitiation
The gravitational force between two point objects is proportional to the product of their masses and inversley proportional to the square of their distance apart. It is always attractive.

Kepler's Third Law
Fr any planet, the cube of its mean radius of orbit, r, is directly proportional to the square of its time period, T.

Satellite
A smaller object in orbit around a larger object

Geostationary satellite
A satellite which stays above the same point on the Earth's equator. The time period is 24 hours

Electric Field

An electric field is the region in which a charged particle experiences a force.

Electric Field Strength
The electric field strength at a point in the field is defined as the force per unit charge on a positive test charge placed at that point.

Electric Potential
The Electric potential at a point in an electric field is defined as the work done per unit positive charge on a positive test charge when it is moved from infinity to that position. (It is a scalar)

At any position in an electric field is the change of potential per unit change of distance in a given direction.

Coulomb’s Law
For two point charges states that the force between the charges is proportional to the product of the charges and inversely proportional to the square of the distance apart. The force can be attractive or repulsive.

Capacitors
Capacitance
The capacitance of a capacitor is defined as the charge stored per unit pd.

Time Constant
Time constant is the time taken for a quantity that decreases exponentially to decrease to 0.37 (1/e) of its initial value.

Magnetic Fields
A magnetic field is a region in which a: magnetic material, magnet, current carrying wire or moving charged particle experience a force.

Magnetic field line
A magnetic field line or line of force of a magnetic field is a line along which a free north pole would move in the field.

Magnetic Flux Density
The magnetic flux density or magnetic field strength is defined as the force per unit length per unit current on a current carrying conductor at right angles to the magnetic field lines.

Fleming’s left hand rule
Relates to the directions of force, magnetic field and current on a current carrying conductor in a magnetic field

Motor Effect
The force on a current carrying conductor due to a magnetic field

Electromagnetic Induction
When a magnet is moved near a wire, a small current passes through the wire. This happens because an emf is induced in the wire. This effect is called electromagnetic induction. (The generation of an emf when the magnetic flux linkage through the coil changes or a conductor cuts across magnetic field lines).

Flemings Right hand Rule
The rule that relates the directions of the induced current, magnetic field and velocity of the conductor when the conductor cuts across magnetic field lines and an emf is induced in it.

Lenz’s Law
When a current is induced by electromagnetic induction, the direction of the induced current is always such as to oppose the change that causes the current.

The induced emf in a circuit is equal to the rate of change of magnetic flux linkage through the circuit.

Magnetic Flux
The product of the magnetic flux density and the area swept out is the magnetic flux (BA).

BAN

Back EMF
Emf induced in the spinning coil of an electric motor or in any coil in which the current is changing. A back emf acts against the applied pd.

Transformer
Converts the amplitude of an alternating pd to a different value. It consists of 2 insulated coils, the primary coil and the secondary coil, wound around a soft laminated core.

Transformer Rule
The ratio of the secondary voltage to the primary voltage is equal to the ratio of the number of secondary turns to the number of primary turns.

Step Down Transformer
A transformer in which the rms pd across the secondary coil is less than the rms pd applied to the primary coil.

A Step Up Transformer
A transformer in which the rms pd across the secondary coil is greater than the rms pd applied to the primary coil.

Transformer Efficiency
For an ideal transformer: output power= input power. Transformer inefficiencies are due to: resistance heating of the current in each coil, the heating effect of the eddy currents in the core, repeated magnetisation and demagnetisation of the core.

Eddy Currents
Eddy currents are unwanted induced currents in metal parts of alternating current machines.

Grid System
The network of transformers and cables that is used to distribute electrical power from power stations to users.