Post-test: Electrical SystemsQuiz
What is an electromagnet?
A coil of wire, usually wound around an iron core, that temporarily becomes magnetic when an electric current flows in the wire.
Select all of the following that correctly describe an electric circuit.
A generator, a lamp and two connecting wires. Must have a return path. If the path is "open" at any point, the current will not flow. A path through which electricity may flow from a source and back to the source.
What happens when current exists in a conductor?
A magnetic field is formed around the conductor.
What is a blackout?
A malfunction in the grid resulting in large areas being without electrical service.
What happens to a speaker when 6 VDC is briefly connected?
A monotone hum is produced from the steady flow of DC current.
What is the purpose of a commutator in an electric motor?
A shaft-mounted, rotating switch that controls the current in the armature coils.
What advantage and disadvantage does a fluorescent lamp have compared to an incandescent?
A simple incandescent has a lower initial cost than a fluorescent. A fluorescent generates more light and less heat than an incandescent, for a given amount of power.
What is a power grid?
A system of interconnecting power generators from separate locations so that they can contribute to or consume from the system. Enables separate utility companies to contribute to or consume from the power distribution system as the need arises.
Select all answers that describe the circuit operation.
AC or DC makes no difference to the switch, which simply connects conductors The right switch position creates an open circuit The lamp lights for the left switch position.
How should a circuit breaker or fuse be chosen for an application?
According to the amperage rating of the wiring. According to the manufacturer recommendation for an appliance.
What are advantages of a digital meter over an analog?
Accurate, easy to read, and optional advanced features.
By what process does a battery produce electricity?
Chemical reaction.
What is the difference between a conductor and an insulator?
Conductors have a low resistance to the flow of electricity, while insulators have a very high resistance.
What is meant by the "coil terminals" on a relay?
Connections to the relay coil. Where voltage is applied to activate the relay.
What is a transducer?
Converts energy from one form to another. For example, a speaker.
Which of the following correctly describe and explain the circuit behavior?
Current in the circuit causes the bulb filament to get hot enough to emit light. Electrical energy is being converted to light and heat. The circuit is complete, enabling current from the source, through the lamp and back to the source.
How do AC and DC voltages differ?
DC voltage has a current that flows in only one direction. AC voltage has a current that flows in both directions. Transformers pass AC, but not DC.
What control functions does the potentiometer serve?
Drops voltage or current. Can be used to control the volume of a radio, the balance of a stereo amplifier.
How is electric current related to magnetism?
Electric current produces a magnetic field and magnetic fields can produce a current in a conductor.
What best describes the flow of electricity?
Electron flow.
A compass needle is not affected by an iron-core electric magnet.
False
All fuses blow instantly when the current is above the rated value.
False
The LED is turned on and off by the potentiometer switch.
False
The lamp converts electricity into easily audible sound.
False
By what process is AC current generated?
Generators containing coils that are rotated within a magnetic field.
What is the difference between a half-wave rectifier and a full-wave rectifier?
Half-wave rectifiers allow only one half of the AC current to pass. A full-wave allows all the AC current to pass, but routes it so that all flows in the same direction.
In what units is frequency expressed?
Hertz refers to the number cycles of direction change per second of an AC current or voltage.
What must the source voltage be with respect to the voltage rating of a load?
If a single value is specified as the device voltage rating, the supplied voltage should be equal to or less than the specified value. Not lower or higher than the specified range.
What simple procedure must be done before reading Ohms on an analog multimeter?
If not possible to adjust the needle to a zero reading, the internal battery should be replaced. Zero the Ohmmeter. Short the probe tips and turn the zero adjust until the needle reads exactly zero.
Select all of the following that explains how to measure the resistance of a motor.
Make sure the power is off. Touch probe tips to the motor terminals and read the ohms scale.
How does a battery work and what type of voltage does it produce?
Produces voltage and current output based on the number of cells in the battery. Produces only DC. Produces electric potential and current flow by a chemical process.
Which of the following functions is NOT provided by a relay?
Remote or automated switching.
Select all responses that correctly describe the behavior of the circuit shown.
Right switch position turns on the bottom light. Left switch position turns on the top light.
Why does an acrylic rod, after being rubbed with a cloth, attract the pieces of paper?
Rubbing the rod imparted electrons to it, resulting in a net negative static charge on the rod. The paper was neutral, or less negative, and thus the effect was if the paper had an opposite charge.
The multimeter is set to the 50 VDC range. What is the value indicated?
Slightly less than 6 Volts. About 5.6 to 5.8.
Which of the following are parts of a motor?
Stator comprised of a fixed magnet or field coils. Shaft mounted rotating commutator. Rotor or armature comprised of an armature coil on the motor shaft.
Which of the following are true of voltage regulators?
They are available as small solid state devices, having three terminals. They are used to control the output of battery chargers, so the charge rate and maximum voltage is safe for the battery. Some have more than three terminals enabling extra functions, such as adjustment of the output voltage.
When you set the multimeter to read current, why should you always start on the higher range?
To avoid damage to the meter by a current that is too high for the range.
What is the purpose of fuses and breakers?
To break the circuit in which the fuse or breaker is installed, when excessive current flow occurs. To provide protection against fire or other damage that excessive current can cause.
What is the purpose of a voltage source or power source?
To maintain the potential difference so that the flow of electrons is continuous. To supply the potential difference required to produce a current in a circuit.
What is the function of a voltage regulator?
To prevent damage to equipment that would occur from application of excessive voltage. To control the charging of batteries on cars and trucks. To maintain a voltage output to a specified value.
A LED behaves as a diode, allowing current to pass in only one direction.
True
A motor is a device that converts electric power into motion.
True
A piezoelectric buzzer will operate on DC if the polarity is correct.
True
An incandescent lamp lights with either DC or AC.
True
Electric current produces magnetism.
True
Electron flow produces visible light when a common table light is switched on.
True
Ground-fault interrupters are typically used in bathrooms, kitchens or around swimming pools.
True
In order for a table light to function, the current flows not only from the wall plug to the lamp, but also from the lamp back to the wall plug.
True
In this circuit, the LED is lit when the relay is OFF.
True
In this circuit, the LED varies in brightness as you rotate the knob.
True
LEDs are available that produce light in colors other than red.
True
Obviously, this LED lights with a current of less than 0.01 amp.
True
Rubbing one substance against another can actually impart electrons to the surface.
True
The function of a rectifier is to convert AC to DC.
True
The lamp decreases in brightness as the rheostat shaft is rotated CW.
True
When a compass is near an energized electric magnet, the needle points to the magnet, not Earth north.
True
What happens if DC is applied to a speaker?
Unless it is a very low DC voltage, it can overheat and damage the coil. No normal sound is produced.
What is voltage drop?
Voltage drop is the difference in voltage between two points of a conductor through which electrons are flowing. It is due to the resistance of the circuit.
What does a multimeter measure?
Voltage, Resistance, Current
Which type of functions in your home may use relays, switches, potentiometers and rheostats?
Volume controls, stereo balance control. Service adjustments inside of electronic devices. Light dimmer controls.
What happens when 6 V AC is applied to a 12 VDC pieozoelectric buzzer?
. It produces a buzz, higher pitched than the power line frequency.
What is the formula expressing Impedance as a function of voltage and current?
.Z = V/I
For what current is the fuse rated?
1 amp.
A SPST switch has how many terminals?
2
With the multimeter set to measure 250 DC mA, what is the value displayed in the figure?
220 mA.
A DPDT switch has how many terminals?
4
What is an electrical output device?
An electric oven. Any device that is powered by electricity. An electric drill motor.
What is impedance?
An expression of the combined effects of inductance, capacitance and resistance to the flow of current. The ratio of voltage to current.
What are the differences between analog and digital multimeters?
Analog meters are less expensive and useful when high accuracy is not required or when displaying slow moving trends. Digital meters are more expensive, but more accurate, easier to read and feature rich.
What are characteristics of an SSR?
Available with small or large activation voltages, in AC or DC No moving parts. No contacts to wear out. Some types allow a small leakage current.
How is a LED operation affected by the connection polarity?
Being a diode, it allows current in only one direction.
Select all the answers that describe how resistance and impedance are related.
Both are measured in Ohms. Impedance can be expressed as the ratio of the voltage across a pair of terminals to the current flow between the terminals. Resistance expresses the opposition to the flow of current in a conductor, while impedance includes the effects of capacitance and inductance.
Indicate the terminal that connects to the potentiometer wiper.
C
What did Joseph Henry's experiment prove about electromagnets?
Increasing the number of turns of a coil produces a stronger magnet for a given current. Increasing the current through a coil proportionally increases the magnetic field.
Why is current electricity used more by man than static?
It can be consistently generated, controlled and directed.
How does the brightness of a lamp compare when connected to a 7812 regulator instead of a 7805?
It is brighter.
When a LED is connected to a battery instead of an incandescent lamp:
It lights continuously, just as the lamp. Nothing if the polarity is reversed.
If the meter shown is set on the 10 DC mA range, what current does it indicate?
Just under 7 mA
What is the advantage of fluorescent lamps over incandescent?
Less heat generated. Require less current to generate the same light, and are thus cheaper to operate.
How does a rheostat change the brightness of a lamp?
The rheostat regulates the current by varying the amount of resistance in the circuit.
What are semiconductors and why are they useful?
Materials that can be made to function as conductors or as insulators. They are extremely useful in the electronics and computer industries.
To connect a relay so that a device is turned OFF when the relay is turned ON, you must use which contacts?
Not possible to do this.
What happens to a speaker if a low voltage AC is applied?
Nothing.
What is AC?
The common type of current supplied commercially for use in homes and industry. Alternating Current, meaning that it periodically changes direction of flow.
Under what conditions does a magnetic field produce an electric current?
The conductor must be in a circuit. The conductor must be plugged into a wall outlet or equivalent source.
How do you connect a multimeter in order to read current?
The current must actually flow through the meter. Select DC mA, starting with the highest range. The meter must be in series with the circuit (unless it is a clamp-on). The polarity must be correct (or the needle moves off scale).
How is an iron-core electromagnet different from a wood-core version?
The iron-core version is stronger.
If the current stops flowing in an electromagnet, what happens to the magnetic field?
The magnetic field stops.
What happens when like poles of two magnets are brought close?
The magnets repel one another. The closer the magnets, the stronger the repelling force.
What happens to a DC motor if you connect it to an AC supply?
The motor hums and vibrates, but the shaft does not spin normally.
What happens if a DC motor is powered by a rectifier?
The motor runs nicely.
What determines the amount of voltage produced during electromagnetic induction?
The number of turns in the conductor coil. The speed at which the magnetic flux lines are crossed. The closeness of the magnet to the conductor.
What is electromagnetic induction?
The process by which magnetism produces electric current in a nearby conductor.
What is meant by the frequency of an AC current?
The rate at which the current changes direction.
Select all answers that describe the circuit operation.
The relay clicks on when the switch is closed. The LED lights when the relay is energized. The relay contacts close, completing the LED to battery circuit.
Select all answers that describe what happens to the LED in this circuit when you move the switch.
The relay still operates, but the LED is not energized. These two relay terminals are never connected, the LED circuit can not be completed.
When does electromagnetic induction occur?
When the magnetic field of a magnet moves past a stationary conductor. When a conductor, in a closed circuit, moves through a magnetic field. When an unchanging current is flowing, and a conductor is near.
What is the advantage of a clamp-on ammeter?
You need only to clamp around a wire, making this meter quick and easy to use. You do not have to cut a wire, or directly connect into a circuit.
What are the two types of electricity?
static and current