Chapter 23 part 1 Flashcards
What is ohm’s law?
Ohm’s law states that current in a circuit is proportional to the voltage and inversely proportional to the resistance. Any value in this relationship can be found when the other two are not known.
(E)
Voltage
(R)
Resistance
(I)
Current
What formula should be used to calculate ohm’s law?
E = I x R
E = voltage ( in _ )
V
I = current (in _)
A
R = resistance (in _)
Q
Voltage =
Current x Resistance
Current =
Voltage / Resistance
Resistance =
Voltage / Current
Calculating resistance example: What is the voltage in a circuit that includes a 3 Q heating element that draws 4 A?
E = I x R E = 4 x 3 E = 12 V
I =
E / R
Calculating Current example: What is the current in a circuit with 3 Q heating element connected to a 12 V supply?
I = E/R I = 12/3 I = 4 A
Calculating Resistance Example: What is the resistance of a circuit in which a load that draws 4 A is connected to a 12 V supply?
R = E / I R = 12 / 4 R = 3 Q
Ohm’s law states that if the ______ in a circuit remains constant, a change in current is directly proportional to a change in in voltage.
resistance
A fixed load resistance of 4 Q is connected to a variable power supply which may be varied from 0 V to 24 V. The current in the circuit may be found for any given voltage by applying Ohm’s law. For example if the voltage in the circuit is set at 8 V, the current equals _____.
2A (8 / 4 = 2A)
Ohm’s law states that if the ______ in a circuit remains constant, a change in resistance produces an inversely proportional change in current.
voltage
The _____ in a circuit decreases with an increase in resistance, and the _____ in the circuit increases with a decrease in resistance.
Current
In this example, a variable of resistance of 2 Q and 24 Q is connected to a fixed power supply. The current in the circuit may be found for any resistance using Ohm’s law. For example, if the resistance in the circuit is set at 24 Q, the current equals ____.
5A because (120 (V) / 24 (I) = 5 (A)
What is used for design applications and troubleshooting applications.
Ohm’s Law
Ohm’s law is used in design applications to solve for the proper values of voltage, current, or resistance during circuit design and to predict circuit characteristics when ____ of the three electrical values are known.
two
When using Ohm’s law in design applications, calculating values helps determine the correct size of ______ and components and the _____ level that must be used for proper circuit operation.
Conductors
voltage
In trouble shooting applications, Ohm’s law is used to determine how a circuit should operate and how it is operating under power. When trouble shooting a circuit, the circuit is already designed. To determine the problem, _____ and current measurements are taken. These measured values are used to calculate the circuit’s _____ and help locate the problem.
- voltage
2. resistance
Changing a circuit’s resistance changes the amount of _____ flowing though the circuit.
current
Changing a circuit’s resistance changes it’s current. This principle is used in any circuit that uses a variable resistor _________ to control a load.
(potentiometer or rheostat)
Name three items that resistors control.
- The sound of speakers
- The brightness level of lamps (dimmer switches)
- Amount of heat output of heating elements (temperature controls)
When designing a circuit like the automobile lighting circuit, the designer engineer uses ______ to determine the resistance range required to control the lamps through their full range of brightness.
Ohm’s law
If a car designer is trying to figure out the required resistance range for designing headlights on a car, what are the known electrical values?
The battery voltage and the current drawn from the lamps selected.
If a cars headlight system requires four light bulbs and the bulbs are rated at 0.25 per bulb what is the total lamp current?
1A because (0.25 x 4 = 1A)
What is the total Q required to reduce lamps by 50%, if the total resistance is 12Q, and the current is 1 A?
(1 x 0.5 = 0.5A) then (12 / 0.5 = 24) 24Q
So 50% lamp brightness resistance equals 24Q.
Thus 24Q total resistance is required to reduce the brightness of the lamps by 50%. If the lamps produce 12 Q then a ____ that produces 12 Q or resistance at mid-range is required.
variable resistor
Using ohm’s law we know that the variable resistor will have a full range of ____, and a midrange of ____.
(0 Q - 24Q)
12Q
When Ohm’s law is used for troubleshooting to determine circuit conditions, _____ and _____ measurements are taken because _____ cannot be measured on a circuit that is powered.
voltage and current measurements
resistance
If a heater has 240 V and 4 A, how would Ohm’s law be used to trouble shot the heater?
(240 / 4 = 60) 60 Q is the circuit resistance if everything is working as it should.
A trouble shooter can use this 60 Q resistance value to figure out the condition of the circuit. For example if an ammeter measured 3A instead of 4A what does this mean?
(240 V / 3 A = 80Q) If the resistance value is 80 Q we know that it is 20 Q too high because it is supposed to be 60Q.
When troubleshooting what are the reasons a resistance value has increased?
A loose or dirty connection, or open coil sections.
If the ammeter measured 6A when it is supposed to measure 4A and the volts is 240. What does this mean?
This means that the resistance has decreased. (240V / 6A = 40Q) The resistance is supposed to be 60Q.
What does it mean if you have a decreased resistance?
Partially shorted coil or insulation breakdown
The power formula is a relationship between what three variable in a circuit?
Power (P)
Voltage (V)
Current (I)
The power formula maybe visualized by presenting the power formula in a _____.
Pie Chart
When using the power formula:
E = voltage (in __)
P = power (in __)
I = current (in __)
Voltage = V power = W Current = A
When using the power formula voltage equals:
E = P / I
When using the power formula power equal:
P = E x I
When calculating current using the power formula:
I = P / E
What is the power of a load that draws 0.5 A when connected to a 120V supply?
P = E x I P = 120 x 0.5 P = 60 W
What is the voltage in a circuit in which a 60 W load draws 0.50 A?
E = P / I E = 60/0.5 E = 120 V
What s the current in a circuit in which 60W load is connected to a 120V supply?
I = P/E I = 60/120 I = 0.5 A
The power formula states that if the voltage in a circuit remains constant and the power required from the circuit changes, _______.
the current in the circuit also changes.
The power required from a circuit changes anytime loads are added or loads are removed. What does adding load do? What does removing loads do?
Adding loads is a power increase and removing loads is a power decrease.
There is a circuit with a 240W fan, a 120 W sign, and a 1200 W motor, they are all connected to a 120V power supply. What is the current of all of these appliances together?
240 / 120 = 2
120 / 120 = 1
1200 / 120 = 10
2 + 1 + 10 = 13A
The power formula is useful when determining expected ____ values because most electrical equipment lists voltage and power rating.
current
Ohm’s law and the power formula are limited to circuits in which electrical _____ is the only significant opposition to the flow of current.
resistance
DC
direct current circuit
AC
alternating current circuit
What are AC circuits tat do not include inductance and/or capacitance are devices such as_____________.
heating elements and incandescent lamps
AC circuits that include inductance are any circuits that include what?
a coil as the load
Motors, transformers, and solenoids all include what?
coils
AC units that include capacitance are any circuits that include a _______.
capacitor (s)
In DC and Ac circuits that do not contain a significant amount of inductance and / or capacitance, the opposition to the flow of current is ______.
Resistance (R)
In circuits that contain inductance or capacitance, the opposition to the flow of current is _____.
Reactance (X)
In circuits tat contain resistance (R) and reactance (X), the combined opposition to the flow of current is ____.
Impedance (Z)
Ohm’s law is used in circuits that contain impedance, however, Z is substituted for __ in the formula.
R
Switches, loads, meters, fuses, circuit breakers, and other electrical components can be connected in _____.
series
Has two or more components connected so there is only one path for the current to flow.
Series connection
With a series circuit what happens when a fuse blows, a circuit breaker trips, or a switch or load opens?
Current stops flowing
A fuse blows or a circuit breaker trips when the circuit is _____.
Overloaded
These are normally used when illustrating a series connected circuit.
A schematic or line diagram
All switches connected in series must be _____ before current flows in the circuit.
closed
series connected switches include what?
mechanical, manual, or automatic switch
Switches are usually connected in series to build ____ into a circuit.
safety
How are switches used in a series on a microwave oven?
A microwave oven includes includes a limit switch in the door. The limit switch is connected in series with the oven ON switch. The oven cannot be turned on unless the limit switch is closed and the oven On switch is closed.
What is an example of a manually-operated switch that is connected in series with automatic switches?
A manually operated system ON/OF switch is included with most thermostats. The switch an be used to turn OFF the automatically operated heating and air conditioning system.
The total resistance in a circuit containing series-connected loads equals the sum of the resistances of _____.
all loads
What formula is used to calculate resistance in series circuits?
RT = R1 + R2 + R3 …….
What is the total resistance of a circuit containing resistors of 25Q, 50 Q, and 75Q connected in series?
RT = R1 + R2 + R3 RT = 25 + 50 + 75 RT = 150 Q
When dealing with voltage in Series circuits the voltage across loads connected in series is _____ across all loads.
divided
When dealing with series circuits the exact voltage drop across each load, depends on _____.
the resistance of each load
The voltage drop across each load in a series circuit is the same if_______.
the resistance values are the same
The load with the lowest resistance has the _____ voltage drop and the load with the highest resistance has the ____.
lowest
highest
In an ideal circuit, the ___ should be the only devices that have resistance.
loads
What parts of a circuit can have voltage drop across it that are not supposed to?
Mechanical switches that are burnt, corroded, or not making good connection have voltage drop across them.
If a switch has a high voltage drop what does this mean?
It is really damaged
In theory the voltage drop across conductors should be ____. In fact, all conductors have some voltage drop across them.
zero
Small conductors have _____ resistance than large conductors made of the same material.
higher
Undersized conductors have a _____.
higher voltage drop
Conductors should be sized large enough to prevent no more than __ voltage drop from the power source to the load.
3%
The voltage drop across conductors and switches can be measured using a _____.
voltmeter
All loads have some ____.
resistance value
The lower the power rating of a load, the ___ the resistance value.
higher
The higher the power rating of a load, the ___ the resistance value.
lower
Loads are normally represented in a schematic diagram by the _____.
Resistor symbol
The ___ of all the ____ in a series circuit is equal to the applied voltage because the voltage in a series circuit is divided across all the loads in the circuit.
sum
voltage drops
What formula is used to calculate total voltage in a series circuit?
ET = E1 + E2 + E3 + …..
What is the total applied voltage of a circuit containing 2 V, 4 V, and 6 V drops across three loads?
ET = E1 + E2 + E3 ET = 2+4+6 ET = 12V
The current in the circuit _____ if the circuit’s resistance _____ and the current _____ if the cirucuit’s resistance _______.
decreases, increases
increases, decreases
To calculate total current of a series circuit what formula should be used?
IT = I1 =I2 =I3
What is the total current through a series circuit if the current measured at each of six different loads is 0.25A (250 mA)
IT = I1 = I2 = I3 = I4 = I5 = I6 IT = 0.25 = 0.25=0.25=0.25=0.25=0.25 It = 0.25A
