Chapter 4
What are the 3 main types of magnets
Ferromagnetic, Paramagnetic, Nonmagnetic
What is the SI unit for frequency and how is it defined?
Hertz; 1 Cycle per second.
What is LENZ' LAW
INDUCED CURRENT FLOWS in a direction such that it OPPOSES the Action that induces it. (LEFT HAND RULE)
What are 3 ways to increase the strength of an electromagnet?
Increasing the number of loops (turns) in the coil. Increasing the current strength Increasing the permeability of the core.
Why is alternating current required for mutual induction to occur?
It creates a VARYING magnetic field which, when it PASSES THROUGH A COIL, INDUCES a current flow in that coil.
What is mechanical means?
It is a method of magnetization, Common mechanical method is heating or hammering a piece of steel while its near a magnet. This causes magnetization.
What is magnetic induction?
It is a method of magnetization. When a non magnetized iron bar is brought within the lines of force of a strong magnet, the dipoles will temporarily align themselves with the lines of force passing through the iron bar. Removed=unmagnetized
What does pulsating Direct Current mean?
It means the current flows in one direction only but still fluctuates in value.
Describe the motor principle (motor effect) which explains why the armature in the motor moves.
It results from the interaction of the magnetic field around the current in the conductor (armature) and the magnetic field of the field magnet.
What is an advantage of an electromagnet as compared to artificial permanent magnets (ALNICO)?
Its magnetic field can be adjusted or turned ON or OFF by varying the current flow through its coil of wire.
what rule determines the direction of the induced current?
Left hand generator/Dynamo rule
What always surrounds a conductor carrying a Current?
Magnetic Field
What are two methods of Magnetization?
Magnetic Induction, Mechanical Means
What are the 3 laws of magnetism?
Magnetic poles, Repulsion-Attraction, Inverse Square Law.
When using Ohm's law in AC circuits, why must Impedance be substituted for resistance in the DC version of the formula?
Necessary since resistance in the conductor is not the only opposition to the flow of alternating current.
The direction of lines of (magnetic) force outside a magnet are from _______ to _______.
North; South
What 4 factors determine the magnitude (strength) of an induced current?
Speed Strength Number of turns Angle- lines of force
armature
rotates through the magnetic field
In the US, power is supplied at 60hz frequency. This means that the generators armature makes _____ rotations in one second resulting in ______impulses or alternations in that one second. This means the alternating current reverses its direction once every ____ second.
60; 120; 1/120th
What is permeability?
A MEASURE of the ABILITY of a material to BECOME MAGNETIZED or to FORM LINES OF INDUCTION.
What is retentivity?
A MEASURE of the ABILITY of a material to RETAIN ITS MAGNETISM after the magnetizing force is REMOVED
What is a motor?
A device that converts ELECTRICAL ENERGY into MECHANICAL ENERGY.
What is mutual induction?
A process of INDUCING a CURRENT FLOW through a SECONDARY coil by passing a VARYING current through the primary coil.
A synchronous AC motor has the same components as the ____ __________
AC generator
What are 4 examples of ferromagnetic (magnetic) substances?
ALNICO, Steel, Iron, Permalloy
How do the lines of force react when they are in opposing direction?
ATTRACT
What is FARADAYS LAW.
An ELECTRIC CURRENT will be INDUCED to FLOW in a CIRCUIT if some part of that current is in a CHANGING MAGNETIC FIELD.
What do we mean by Alternating Current (AC)?
An electric current which reverses its direction at regularly occurring intervals.
What is meant by the term phase?
Any selected point on the sine curve at the corresponding angle through which the armature has rotated.
What is Electromagnetism?
BRANCH OF PHYSICS that deal with the RELATIONSHIP between ELECTRICITY AND MAGNETISM
When using Ohm's law in AC circuits, why must average or rms values be used for current and voltage?
Because the values of current and voltage are CONSTANTLY CHANGING
Why is the average or Root-Mean Square (RMS) value necessary when measuring Alternating Current?
Because value current is continually changing.
Explain the relationship between permeability and retentivity.
Both are inversely proportional.
the amount of opposition offered by a capacitor in an AC circuit is called ______ reactance
Capacitive
A simple DC motor has the same components as the ___ ________
DC generator
The sine curve represents _____ complete rotation of a generators armature in the magnetic field.
ONE
Impedance due to coils and capacitors and the conductors resistance in AC circuits must be considered when the figuring the formulas for ________ Law
Ohm's
How do the lines of force react when they are in the same direction?
REPEL
To induce current in a conductor what needs to occur between the magnetic field and the conductor?
Relative motion.
A basic DC generator is different from a basic AC generator that the DC generator has a(n) ______ __________ supplying the current to the external circuit while the AC generator has two ______ _______ supplying the current to the external circuit. The AC generator delivers ________ current while the DC generator delivers ________ _______ current.
Single commutator, slip rings, alternating current, pulsating direct
The difference between a synchronous AC motor and an induction (AC) motor is that the synchronous motor's armature is ________with a current while the current in the induction motors rotor (its armature) is ________ by the action of the stator. The current in the induction motor's rotor creates a _________ field surrounding the rotor.
Supplied, induced, magnetic
Field magnet
Supplies the magnetic field
What is the SI unit for Magnetic Flux density?
TESLA (T)
What are 3 ways to induce a current and electromagnetic field by means of electromagnetic induction?
The CONDUCTOR may move or cut across a stationary MAGNETIC FIELD. A MAGNETIC FIELD may move or cut across a stationary CONDUCTOR. The MAGNETIC FIELD may vary in STRENGTH while a stationary CONDUCTOR lies in the field.
What is meant by the term frequency?
The frequency of an Alternating Current. The number of cycles per second.
What is SELF INDUCTION?
The induction of an OPPOSING EMF in a single coil by its own changing magnetic field. When alternating current flows through the coil.
Flux Density (Intensity)
The number of LINES OF FORCE per unit area in the magnetic field. The stronger the magnetic field, more lines of flux
What is the formula for Ohm's law as it is applied to AC circuits.
Vrms=(Irms) (Z)
What is an electromagnet?
When a ferromagnetic core is added to a solenoid, increasing a flux density.
What is a solenoid?
When a flux density is greatly increased and when a current is flowing through a coil.
What are 4 examples of nonmagnetic substances?
Wood, Glass, rubber, plastic
What will happen if a conductor moves across a magnetic field but the conductor does not form a closed circuit?
a POTENTIAL DIFFERENCE (Electromagnetic field) will be induced but no current will flow.
What is an electric generator or dynamo?
a generator that converts MECHANICAL ENERGY into ELECTRICAL ENERGY by electromagnetic induction.
slip rings
connects to the ends of the armature, allowing continual contact between the armature and the metal brushes.
2 stationary metal brushes
contact slip rings but allows each ring to rotate; conduct induced current from armature to external circuit.
What does it mean when we say the current is induced in the generators armature?
current is caused to flow
The difference between Alternating Current (AC) and Direct Current (DC) generators is the method of delivering the induced current in the armature to the ________ circuit.
external
An instrument which measures either electric current or voltage is called a _______ When it is connected in series it measures _______ in ________ and it is called a __________ When it is connected in parallel it measures ______ in ______ and it is called a _________.
galvanometer, current in amperes, ammeter , voltage in volts, voltmeter.
The opposition to any change in the amount of alternating current flowing in a coil is called ___________ The actual quality of opposition due to the property of induction in a coil is called _________ reactance.
inductance, inductive
Impedance is the total opposition offered in an Alternating Current (AC) circuit by _________ reactance, ________ reactance, and ___________ to the flow of alternating current.
inductive; capacitive, resistance
The armature in a motor is actually in the form of a ______ of wire.
loop/winding
The rotor in an induction motor rotates because its magnetic field is trying to align itself with the __________ _______ of the stator's electromagnets.
magnetic field
A charged particle in motion creates a _________ ________ which is _____________ to the motion
magnetic field; perpendicular
A coil in the galvanometer is located within a ________ field. When current flows through the coil in the galvanometer the coil becomes a __________. The coil is able to move or rotate as it is attracted by the magnetic fields pole. A pointer is attached to the coil. The pointer moves over a calibrated scale. The greater the current through the coil, the ________the rotation.
magnetic, conductor, greater.
A galvanometer works according to the ______ principle.
motor
In a generator the armature is ________ through a magnetic field. This ________ an electric current. In a motor the electric current is _______ to the armature. this causes the armature to ________ __________
rotated, induces , supplied, rotate mechanically
The component in the induction motor which revolves is the ______
rotor
In an induction motor, the armature is replaced with a ______ and the field magnet is replaced with a _______ . There are no slip rings or brushes.
rotor, stator
What is a helix?
series of loops (coil)
The component in the induction motor which establishes a rotating magnetic field is the ______
stator
Magnetism
the ABILITY of certain metals to ATTRACT CERTAIN other metals such as iron, and alloys of iron.
magnetic field
the SPACE SURROUNDING a magnet in which it FORCES of attraction and repulsion can be DETECTED OR MEASURED
What does direct current mean?
the current flows in one direction
