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T5D

ELECTRICAL PRINCIPLES

Ohm's Law; Series and parallel circuits

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T5D011 of 14

What formula is used to calculate current in a circuit?

Why Ohm's Law says current equals voltage divided by resistance, I = E / R, where E is in volts, R in ohms and I in amperes. Raising the voltage across a fixed resistance raises the current proportionally, while raising the resistance for a fixed voltage cuts the current down. The same relationship rearranges to E = I x R and R = E / I, so all three forms come from one equation.
Watch out Multiplying voltage by resistance is the tempting trap, but that gives no meaningful quantity here; multiplication belongs in E = I x R, where current and resistance are multiplied. Choices with E squared come from power formulas like P = E squared / R, not current.
Draw the triangle: E on top, I and R below. Cover what you want. Cover I and you see E over R.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T5D022 of 14

What formula is used to calculate voltage in a circuit?

Why Ohm's Law says voltage equals current multiplied by resistance, E = I x R, where E is in volts, I in amperes and R in ohms. Push more current through the same resistance and the voltage drop rises proportionally; raise the resistance at the same current and the voltage rises too. The same relationship rearranges to I = E / R and R = E / I, so knowing one form gives you all three.
Watch out Dividing current by resistance is just Ohm's Law scrambled, and the forms with I squared come from the power formula P = I squared x R, which gives watts, not volts.
Cover the letter you want in the E over I R triangle: E is on top, so E = I x R.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T5D033 of 14

What formula is used to calculate resistance in a circuit?

Why Ohm's Law relates voltage (E), current (I) and resistance (R) as E = I x R. Solving that for resistance means dividing both sides by current, giving R = E / I. So if 12 volts pushes 3 amps through a component, its resistance is 12 / 3 = 4 ohms.
Watch out Multiplying voltage by current, E x I, is the power formula (watts), not resistance. Adding or subtracting volts and amps is meaningless since they are different units.
Ohm's triangle: E on top, I and R below. Cover R and you see E over I.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T5D044 of 14

What is the resistance of a circuit in which a current of 3 amperes flows when connected to 90 volts?

Why Ohm's Law rearranged for resistance is R = E / I, voltage divided by current. Here 90 volts divided by 3 amperes gives 30 ohms. Notice the units work out: volts per ampere is the definition of the ohm.
Watch out The 270 ohm choice comes from multiplying 90 by 3 instead of dividing, and 1/30 ohm is the division done upside down (current divided by voltage).
Ohm's triangle: E on top, I and R below. Cover R and you get E divided by I.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T5D055 of 14

What is the resistance of a circuit for which the applied voltage is 12 volts and the current flow is 1.5 amperes?

Why Ohm's Law rearranged for resistance is R = E / I, so divide the voltage by the current: 12 volts / 1.5 amperes = 8 ohms. Whenever the question gives volts and amps and asks for ohms, division is the operation, and the answer must be smaller than the voltage number whenever the current is greater than 1 amp.
Watch out The choice of 18 ohms comes from multiplying 12 by 1.5, which is the power formula P = E x I and gives watts, not ohms; 0.125 is the division done upside down (1.5 / 12).
E over I gives R. Cover R in the E/(I x R) triangle and what's left is E divided by I.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T5D066 of 14

What is the resistance of a circuit that draws 4 amperes from a 12-volt source?

Why Ohm's Law rearranged for resistance is R = E / I, voltage divided by current. Here 12 volts divided by 4 amperes gives 3 ohms. A quick sanity check: with only 3 ohms of opposition, a fairly large current flows from a modest voltage, which matches the 4 amperes given.
Watch out The 48 ohm choice comes from multiplying 12 by 4, which actually gives power in watts (P = E x I), not resistance. The 16 and 8 ohm answers come from adding or subtracting the two numbers, which no Ohm's Law form does.
Resistance question means divide: volts over amps. If you multiplied, you found watts instead.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T5D077 of 14

What is the current in a circuit with an applied voltage of 120 volts and a resistance of 80 ohms?

Why Ohm's Law solved for current is I = E / R, so divide the applied voltage by the resistance: 120 volts / 80 ohms = 1.5 amperes. The voltage pushes, the resistance opposes, so current is always the voltage divided by the resistance, never the product.
Watch out The 9600 ampere choice comes from multiplying 120 by 80, and 0.667 amperes is the division done upside down (80/120); keep voltage on top.
I = E/R: bigger voltage, more current; bigger resistance, less current. 120/80 = 1.5.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T5D088 of 14

What is the current through a 100-ohm resistor connected across 200 volts?

Why Ohm's Law says current equals voltage divided by resistance, I = E/R. Here 200 volts divided by 100 ohms gives 2 amperes. The resistor is connected directly across the source, so the full 200 volts appears across it.
Watch out The 0.5 ampere answer comes from dividing the resistance by the voltage instead of the other way around, and 20,000 amperes comes from multiplying voltage by resistance.
Volts on top: I = E/R. If your answer is bigger than the voltage number, you probably multiplied.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T5D099 of 14

What is the current through a 24-ohm resistor connected across 240 volts?

Why Ohm's Law rearranged for current gives I = E / R. Here 240 volts divided by 24 ohms equals 10 amperes. Whenever you are given volts and ohms and asked for amps, divide the voltage by the resistance.
Watch out The 0.1 ampere choice comes from dividing the resistance by the voltage (24/240), which flips the formula upside down, and 5760 amperes is what you get by multiplying volts times ohms instead of dividing.
Amps = Volts over Ohms. 240/24 = 10. If your answer looks absurdly huge, you multiplied when you should have divided.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T5D1010 of 14

What is the voltage across a 2-ohm resistor if a current of 0.5 amperes flows through it?

Why Ohm's Law in the form E = I x R gives voltage when you know current and resistance. Here 0.5 amperes times 2 ohms equals 1 volt. Whenever the question gives you current and resistance and asks for voltage, you multiply.
Watch out The 0.25 volt choice comes from dividing the current by the resistance instead of multiplying, which is what you would do if you were solving for something else entirely.
Voltage question = multiply: E = I x R. Current and ohms go together as a product, never a quotient.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T5D1111 of 14

What is the voltage across a 10-ohm resistor if a current of 1 ampere flows through it?

Why Ohm's Law says voltage equals current times resistance, E = I x R. Here 1 ampere flowing through 10 ohms gives E = 1 x 10 = 10 volts. Whenever you know current and resistance, multiply them to get the voltage drop across the component.
Watch out The choices showing 11 volts and 9 volts come from adding or subtracting the two numbers instead of multiplying them, which is not what Ohm's Law says.
E = I x R: volts = amps times ohms. One amp through ten ohms is ten volts.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T5D1212 of 14

What is the voltage across a 10-ohm resistor if a current of 2 amperes flows through it?

Why Ohm's Law in the form E = I x R gives voltage when you know current and resistance. Here 2 amperes through 10 ohms yields 2 x 10 = 20 volts. Whenever the question gives current and resistance and asks for voltage, multiply.
Watch out The 0.2 volt choice comes from dividing 2 by 10 instead of multiplying, and the 8 and 12 volt choices come from subtracting or adding the two numbers, none of which is Ohm's Law.
Voltage asked, multiply: E = I x R. 2 A x 10 ohms = 20 V.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T5D1313 of 14

In which type of circuit is the current always the same through all components?

Why A series circuit provides only one path for charge to flow, so every electron that leaves one component must pass through the next. That means the same current flows through every component in the chain, while the source voltage divides among them in proportion to their resistances.
Watch out Parallel is the opposite case: the voltage is identical across each branch while the current splits among the branches according to each branch's resistance.
Series = Same current, one path. Parallel = same voltage, current splits.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T5D1414 of 14

In which type of circuit is the voltage always the same across all components?

Why In a parallel circuit every component connects between the same two nodes, so each one sees the identical potential difference. Current divides among the branches according to each branch's resistance (Ohm's Law, I = E/R), but the voltage across each branch is the same. That is why household outlets, all wired in parallel, each supply the same voltage regardless of what is plugged in.
Watch out A series circuit is the opposite case: the same current flows through every component, while the source voltage divides among them in proportion to their resistances.
Parallel = same Voltage, Series = same Current. Think "PV" and "SC".
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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