DC Fundamentals Midterm
What are the three principal parts of an atom?
Protons, Electrons, and Neutrons
What type of material is used to coat the aluminum drum of a copy machine?
Selenium
A complete circuit is often referred to as a(n) ________ circuit.
closed
What part of an atom has a negative charge?
electron
An electric circuit has a resistance of 20. The current is measured to be 6A. How many volts are applied to the circuit?
120V
An electric motor has an apparent resistance of 15. If 8A of current are flowing through the motor, what is the connected voltage?
120V
How many times more does a proton weigh than an electron?
1838 times more in weight
An ammeter is connected in _____ with the load to permit the load to limit current flow.
Series
How many valence electrons are generally contained in materials used for insulators?
Seven or eight valence electrons
Is the north magnetic pole of the earth a north polarity or a south polarity?
South polarity
State three rules concerning parallel circuits.
-The total current flow in the circuit is equal to the sum of the currents through all the branches -The voltage drop is the same as the applied voltage through all branches -The total resistance is always less than the resistance of the lowest value resistor, or branch, in the circuit
A 0.5 W, 2000-fl resistor has a current flow of 0.01A through it. Is this resistor operating within its power rating?
0.2 W; yes it is
A parallel circuit has three branches. An ammeter is connected in series with the output of the power supply and indicated a total current flow of 2.8A. If Branch 1 has a current of 0.9A and Branch 2 has a current of 1.05A, what is the current flow through Branch 3?
0.85A
A 1 W, 350-fl resistor is connected to 24 V. Is this resistor operating within its power rating>
1.65 W; no it is not
How many times smaller is an electron than a proton?
1000 times smaller
A 240 V circuit has a current flow of 20 A. How much resistance is connected in the circuit?
12 ohms
Define reluctance.
Resistance to magnetism
A parallel circuit contains four branches. One branch has a current flow of 0.8A, another has a current flow of 1.2A, the third has a current flow of 0.25A, and the fourth has a current flow of 1.5A. What is the total current flow in the circuit?
3.75A
A 240 V air-conditioning compressor has an apparent resistance of 8. How much current will flow in the circuit?
30 A
What type of current reverses its direction of flow at regular intervals?
Alternating current (AC)
A thundercloud has a negative charge, and an object on the ground has a positive charge. Will the lighting discharge from the cloud to the ground or from the ground to the cloud?
From the cloud to the ground.
A voltmeter has a very ________ resistance connected in series with the meter movement.
High
What is the advantage of a wire-wound resistor?
Higher power rating to withstand more heat
Define residual magnetism.
The magnetic flux remaining in the magnetized element
Define ohm.
The unit of measure for electrical resistance
What is the advantage of a metal film resistor over a carbon resistor?
They do not change their value with age and their tolerance is generally better.
What is measured on the X axis of an oscilloscope?
Time
A grounding conductor helps prevent a shock hazard in the event that an ungrounded, or hot, conductor comes in contact with the case or frame of an appliance. (True or False)
True
One of the rules for Ohm's law in a series circuit is that the applied voltage is equal to the sum of the voltage drops across all the resistors. (True or False)
True
One of the rules for Ohm's law in a series circuit is that the current is the same at any point in the circuit. (True or False)
True
Resistors have a power rating in watts that should not be exceeded, or the resistor will be damaged. (True or False)
True
The value of a resistor can often be determined by the "color code." (True or False)
True
How should tubular wire-wound resistors be mounted and why?
Vertically to permit air to rise through the center.
In total resistance, each time another element is connected in parallel, there is ___________ to the flow of current through the entire circuit.
less opposition
In a parallel circuit, the total resistance is _________ value resistor.
less than the lowest
What charge does an electron have?
negative charge
What charge does a neutron have?
no charge
What type of meter contains its own separate power source?
ohmmeter
State the law of charges.
opposite charges attract and like charges repel
Most devices such as a light receptacle at home and office buildings are connected in ________ circuits
parallel
A circuit that has only one path for current flow is a __________ circuit.
series
In a parallel circuit, the total current flow is equal to the ______ of the currents through all the branches.
sum
The voltage drop across any branch of a parallel circuit is the same as ________ voltage.
the applied
In a parallel circuit, the voltage across each branch must be ____________.
the same/equal to one another
Which of the following is one function of a resistor in a circuit?
to limit the flow of current
What is indicated by the fourth color band on a resistor?
tolerance
If a circuit is open, current ________ flow
will not
A waveform shown on the display of an oscilloscope completes one cycle in 50us. What is the frequency of the waveform?
20,000Hz
A 5 kW electric heating unit is connected to a 240 V line. What is the current flow in the circuit?
20.83A
Four resistors having a value of 100 n each are connected in parallel. What is the total resistance of the circuit?
25 ohms
A force of 1 ounce is equal to how many dynes?
27,800 dynes
A parallel circuit contains four resistors. The total resistance of the circuit in 120. Three of the resistors have values of 820, 750, and 470. What is the value of the fourth resistor?
273.76 ohms
A resistor has color bands of orange, blue, yellow, and gold. What are the resistance and tolerance of this resistor?
360,000U at +/-5%
An electric heating element has a resistance of 16 and is connected to a voltage of 120 V. How much current will flow in this circuit?
7.5A
A 10,000-fl resistor has a tolerance of 5%. What are the minimum and maximum values of this resistor?
9,500U-10,500U
Four resistors having values of 270, 330, 510, and 430 are connected in parallel. What is the total resistance in the circuit?
90.741 ohms
What is a potentiometer?
A 3 terminal variable resistor that is used as a voltage divider.
Define permeability.
A measure of a materials ability to become magnetized
Define coercive force.
A measure of a materials ability to retain magnetism
Define flux density.
A measure of the density of the field lines around a magnet
Define watt.
A measure of true power
What meter must NEVER be connected to a circuit when the circuit power is turned on?
An Ohmmeter
What meter measures an amount of voltage during a period of time and produces a two-dimensional image?
An Oscilloscope
A test meter that uses a scale and pointer to indicate values is a(n) ________ meter.
Analog
Why are circuits in homes connected in parallel?
Because each device receives the same voltage.
Why do static charges accumulate on insulator materials only?
Because insulators are the only materials that can hold the electrons stationary and keep them from flowing to a different location.
Why is static electricity considered to be a charge and not a current?
Because it refers to electrons that are sitting still and not moving. Since it doesn't move it can't be a current; so it is a charge.
The meter is only connected to one of the lines supplying power when using a(n) __________ ammeter.
Clamp-on
What type of voltage must be connected to a d'Arsonval meter movement?
DC voltage
What type of current flow in only one direction?
Direct current (DC)
A lesser amount of current will flow through a low-value resistor, and a greater amount of current will flow through a high-value resistor. (True or False)
False
You are given the total voltage (50V) and the resistance for R1-R3 (40, 10, & 50); find the rest of the information. (This is a series chart)
Fill out chart
You are given the total current (0.6A) and the resistance for R1-R4 (470, 360, 510, & 430); find the rest of the information. (This is a combination chart)
Fill out the chart.
You are given the total voltage (160V) and the resistance for R1-R3 (1200, 300, & 120); find the rest of the information. (This is a parallel chart)
Fill out the chart.
A circuit contains a 1200, 2200, and a 3300 resistor connected in parallel. The circuit has a total current flow of 0.25A. How much current flows through each of the resistors?
I1=0.131A I2=0.0714A I3=0.0476A
An electroscope has been charged with a negative charge. An object with an unknown charge is brought close to the electroscope. The leaves of the electroscope come closer together. Does the object have a positive or a negative charge?
It has a positive charge
What characterizes a parallel circuit?
It has more than one path for current flow.
How is an ammeter connected into a circuit?
It is a low-impedance device used to measure current and must be connected in series with the load to permit the load to limit the current flow.
What is an ampere?
It is a measurement of the amount of electricity that is flowing through a circuit
What is an electroscope?
It is a metal ball attached to the end of a metal rod. The other end is attached to two thin metal leaves. These leaves are inside a transparent container to permit the action of the leaves to be seen and to prevent air currents from affecting their movement.
What special property does this material have that makes it useful in a copy machine?
It is a semiconductor material that charges its conductivity with a charge of light intensity. When it is in the presence of light, it has very high conductivity. When it is in darkness it has a very low conductivity.
What is a gluon?
It is a subatomic particle that acts as a mediator to hold the nucleus together (also known as a quark)
Define voltage.
It is an electrical measurement of potential difference, electrical pressure, or electromotive force
How is a voltmeter connected into a circuit?
It is connected directly across the power source because it has a very high resistance connected in series with the meter movement.
What is electricity?
It is the flow of electrical power or charge. It is a driving force that provides most of the power
What is a coulomb?
It is the measure of charge
What electrical quantity does the oscilloscope measure?
It measures an amount of voltage during a period of time and produces a two-dimensional image.
How can the polarity of an electromagnet be determined if the direction of current flow is known?
Left hand rule
Name two devices used for lighting protection.
Lighting rods and lightning arrestor
What were early natural magnets known as?
Lodestone
Name three types of fixed resistors.
Metal film Carbon film Metal glaze
Most voltmeters are _____ voltmeters, meaning they are designed to use one meter movement to measure several ranges.
Multirange
What meter measures resistance?
Ohmmeter
The voltage section and the time base are the two main sections of what measuring instrument?
Oscilloscope
On the Power formula chart, what do you use to find power when current and voltage are known?
P = I x E
What is measured on the Y axis of an oscilloscope?
Voltage
What meter is designed to be connected directly across the power source?
Voltmeter
Define saturation.
When the magnet reaches a point where its strength, increases only slightly with an increase in current saturation occurs when all molecules of the magnet material are lined up
Can one thundercloud contain both positive and negative charges?
Yes
The sum of the currents in a parallel circuit must ___________ the total current.
be equal to
How many valence electrons are generally contained in materials used for conductors?
between one and three valence electrons
Electrical current is the flow of _________.
electrons
In a series circuit, the sum of the voltage drops is _________ the applied voltage.
equal to
If electrons are removed from an object, is the object positively or negatively charged?
positive charge
What charge does a proton have?
positive charge
What is another term for a variable resistor?
potentiometer
Since the invention of calculators, the formula has become the most popular?
reciprocal formula
The south pole of on magnet is brought close to the south pile of another magnet. Will the magnets repel or attract?
repel
The basic law of physics states that like charges _________ and opposite charges _________.
repel; attract
