Union Test Prep Electronics Information Part 2

What is direct current? (DC)

The current that flows in only one direction

What is alternating current? AC)

The current that alternates in the direction it flows.

What do batteries produce?

Direct current (DC)

What do alternators produce?

Alternative current (AC)

What is a rectifier?

An electronic device that changes AC into DC by allowing current to flow in only one direction.

What is a commutator?

A mechanical device found in DC generators and DC motors that reverses the connections each half-turn, producing DC output.

What does an alternator have?

• Magnetic field

• Wire inside the magnetic filed

Which circuits have impedance?

Alternating current

Which circuits have resistance?

Direct current

How does an alternator produce AC?

A wire (coil) rotates inside a magnetic field. As it rotates, the direction electrons are pushed changes every half-turn, producing alternating current (AC).

What is the frequency of a waveform when it goes through one complete cycle of change in a period of one second?

It has a frequency of 1 hertz (Hz).

When two complete cycles of the wave form occur in one second, what is the frequency sand the time period?

Frequency = 2 Hz

Time = 0.5 s

What is the equation for a waveform that completes two cycles in one second?

What is the equation of a waveform that has a time period of 2 seconds, and its frequency is one cycle per 2 seconds.

How is the frequency of the waveform found?

It is found by inverting the time period.

What is the equation for frequency?

where f is in hertz and T is in seconds.

What is the usual (North American) frequency for domestic and industrial AC supplies?

60 Hz

In electronics, what is the frequency range of AC wave forms?

The waveform can range from zero (i.e., DC) through kilohertz (kHz), megahertz (MHz), and gigahertz (GHz).

What is grounding?

A method for neutralizing a charged object by connecting an object to the Earth so excess electrical charge can safely flow away, neutralizing the object.

How does connecting a changed object to a larger source of charge neutralize it?

The immense size of earth allows excess electrons to flow to or from the Earth until the object has a balanced (neutral) charge.

What does "neutralizing" mean?

Removing excess electrical charge so the object is no longer positively or negatively charged.

What type of resistance path do ground wires provide?

A low resistance path to the earth

What do ground wires prevent in the case of faulty circuits?

They help prevent electric shock

What are resistors useful for?

They are useful for modifying circuits to produce a desired amount of current.

What are resistors?

They are passive circuit components used to reduce current flow and voltage levels in a circuit.

How many color bands do resistors have?

They come marked with four or five colored bands.

What do the colored bands in a resistor signify?

They signify the amount of resistance (and tolerance) they provide.

What does the last band indicate in a resistor?

The band indicates the tolerance in the resistor.

What does the second to last band in a resistor represent?

The band represents the multiplier, or what the first two or three bands are multiplied by to find the resistance.

If the resistivity of the resistor’s material is known, the resistance can be found using what following equation?

where ⍴ represents the resistivity of the conducting material, L is the length, and A is the cross-sectional area.

What type of circuits can resistors be used for?

They can be used in a circuit in series, in parallel, or in series-parallel fashion.

What is a fixed resistor?

A resistor that has only one resistance value.

Can the resistance value in a fixed resistor be changed or adjusted?

No

Fixed resistors include….

carbon composition, wire-wound, and metal film resistors.

What is a carbon composition resistor?

A resistor made of carbon graphite and clay.

What is a wire-wound resistor?

A resistor constructed using various types of wire wounds on an insulating form.

What is a metal film resistor?

A type of resistor that is made up of a metal film from a specific metal alloy or carbon that is deposited in a vacuum located on a small ceramic cylinder that has leads connected to it

What is a variable resistor?

A resistor that’s resistance can be changed to an value within its range.

When are variable resistors used?

When making frequent adjustments, such as adjusting the volume on a television set.

What are the resistance elements of variable resistors?

Carbon composition, wire-wound, or deposited film.

What are the types of variable resistors?

There are low-power resistors, high power-resistors, and are also manufactured with a wide range of resistance values.

Variable resistors can be changed from a resistance of….

0 Ω to the stated resistance value of the device.

What is a potentiometer?

A single resistors with terminals at each end and a moveable contact that can be set to any point on the resistor.

What are potentiometers most commonly used for?

They are most commonly used as voltage dividers.

In the diagram, what are the functions for Terminals A, B, and C?

Terminals A and B in the following figures are the end terminals, and terminal Cis the adjustable contact.

What is a rheostat?

A variable resistor with two terminals.

What type of device is a rheostat?

A current -limiting device

Wah i the typical power rating of a rheostat?

2 W

What are rheostats used for?

They are used for controlling the light level of incandescent lamps.

How can a potentiometer be wired as a rheostat?

They can be wired if only one end of the resistance element and the movable arm are connected to the circuit.

What is the difference between a rheostat and a potentiometer?

Rheostats are often wire-wound and have higher power rating than potentiometers.

When using a potentiometer as a rheostat….

the power rating must be sufficient for the application.

What are fuses and circuit breakers?

They are safety features that preserve the integrity of a circuit in the case of a power overload.

What are the functions of a fuse and circuit breaker?

The features of a fuse and circuit breaks are included as safety switches in order to disrupt or break the circuit before any large scale damage is done.

why is a short circuit undesirable?

This is because the surge in current running through the circuit and the possible dangers associated with this surge.

Why would a circuit be shorted?

A circuit would be shorted when the intended pathway of the current in a circuit is “shorter” than it is designed to be.

What are capacitors?

They are passive circuit components that serve to temporarily store electrical energy in a circuit.

What happens when you connect a voltage source?

A voltage source (like a battery) pushes electric charges into the capacitor.

What does the battery do when connected to a voltage source?

The battery forces positive charges to build up on one side of the capacitor and negative charges to build up on the other side. This results in the capacitor being charged and holding electrical energy.

What does "voltage source can then be disconnected" mean for capacitors?

Once the capacitor is full of stored energy, you can remove the battery because the capacitor functions similarly to a better in that it can discharge and provide energy to a circuit.

How does the capacitor act like a battery?

Capcitors function similarly to a better in that it can discharge adn provide enrgy to a circuit.

What is the equation for calculating capacitance?

where C is capacitance, Q is charge, and V is voltage.

What is capacitance measured in?

Farads

What are capacitors made up of?

They are made of two conducting plates separated by a distance containing a dielectric.

What does "polarized" mean?

Polarized means the charges inside the material slightly rearrange. Therefore, The material is still neutral overall, but its positive and negative charges are separated slightly.

What does the dielectric do in a capacitor?

1. Prevents the plates from touching (which would cause a short circuit).

2. Allows the capacitor to store more electrical energy.

3. Becomes polarized when the capacitor is charged.

What are examples of dielectrics?

• Glass

• Plastic

• Rubber

• Ceramic

• Air

What is a dielectric?

A material that acts as an insulator, blocks the flow of electricity, but can become polarized and helps store energy in a capacitor.

What is the formula for capacitive reactance?

Do capacitor reactance allow AC current to pass freely?

No, it pushes back against the flow of electricity.

What is capacitive reactance?

Capacitive reactance is the opposition a capacitor creates against AC current. It depends on the frequency of AC as well as the size (capacitance) of the capacitor

What are semiconductors?

They are components that can function as either an insulator or a conductor.

What are the most commonly used semiconducting materials?

Silicon and germanium

When semiconductors increase in conductivity what happens to their temperature?

The temperature increases

What are examples of semiconductors?

Diodes and transistors

Through doping, what happens to a semiconductor?

They can become more or less conductive

What is doping?

The process of adding impurities to a semiconducting material in order to alter its electrical properties

What are the impurities used in doping?

Extra or absent electrons

How is a diode formed?

It is formed by joining N- and P-type semiconductor materials together

How is a junction developed?

When N- and P -type semiconductor materials come in contact with each other.

What is this device known as?

Junction diode

What are the type of diode leads?

Anode lead and cathode lead

What material is an anode lead connected to?

P -type material

What material is a cathode lead connected to?

N -type material

What are diodes?

They are electrical components that restrict the movement of current to only one direction and block the motion of current in the opposite direction.

when are diodes used?

They are used in bridge rectifiers to convert AC to DC.

When is a diode considered to be forward-biased?

When the polarity of the battery allows electrons to flow freely through it.

When is a diode described as reverse-biased?

When the polarity of the battery prevents electrons from flowing through it.

What would you do in order to make the circuit forward-biased?

Connect the positive terminal of the battery to the P-type material (anode) and the negative terminal of the battery to the N-type material (cathode)

What is the most important application of diodes?

Rectification

What is rectification?

The process of converting AC into DC.

How does rectification occur?

A diode blocks the part of AC that goes the wrong direction.

What does the diode do to AC flow?

The diode "flips" the AC into a one-direction flow.

Why do we need rectification?

Most homes have AC electricity from wall outlets (like 120 V AC), yet many devices need DC electricity. Therefore power supply uses diodes to rectify AC into DC so electronics can use it.

Why are diodes used in rectification?

They are used because they allow electricity to flow in only one direction, blocking the reverse current and creating a one-direction output.