Laplace Transforms
Mathematics · FE Reference Handbook section
Handbook notes for this section
Definitions and conditions exactly as the handbook states them.
- The unilateral Laplace transform pair
- represents a powerful tool for the transient and frequency response of linear time invariant systems. Some useful Laplace
- The last two transforms represent the Final Value Theorem (F.V.T.) and Initial Value Theorem (I.V.T.), respectively. It is
- assumed that the limits exist.
Core formulas for this FE topic
Definitions, applicability, units, assumptions and worked examples for each relation.
Worked exam-style examples
The four ways this section is written on the real exam — thoughts first, then equations, then substitution.
An engineer applies the Laplace transform to an exponential decay response. Given decay constant (a) = 7.7000 1/s; complex frequency (s) = 8.5000 1/s, determine the transform value (F) in s.
Given
Find
transform value (F), in s
Start with the thinking
- The governing relation printed in this handbook section is Laplace transform of an exponential.
- Everything except F is given, so isolate F symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- The Laplace transform of e^{-at} is 1/(s+a), a standard entry used in transform tables.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for F:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning F = 0.0617 s to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 0.1235 — kept a factor of two that cancels in the correct rearrangement.
- 0.0309 — dropped that same factor in the other direction.
- 0.0679 — rounded an intermediate value before the final step.
Reference: FE Handbook — Laplace Transforms
A student looks up a Laplace transform pair for a first-order system. Given decay constant (a) = 1.5000 1/s; transform value (F) = 1.7150 s, determine the complex frequency (s) in 1/s.
Given
Find
complex frequency (s), in 1/s
Start with the thinking
- The governing relation printed in this handbook section is Laplace transform of an exponential.
- Everything except s is given, so isolate s symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- The Laplace transform of e^{-at} is 1/(s+a), a standard entry used in transform tables.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for s:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning s = -0.9169 1/s to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- -1.8338 — kept a factor of two that cancels in the correct rearrangement.
- -0.4585 — dropped that same factor in the other direction.
- -1.0086 — rounded an intermediate value before the final step.
Reference: FE Handbook — Laplace Transforms
The Laplace transform of a circuit's impulse response is evaluated at s. Given complex frequency (s) = 1.7000 1/s; transform value (F) = 1.0940 s, determine the decay constant (a) in 1/s.
Given
Find
decay constant (a), in 1/s
Start with the thinking
- The governing relation printed in this handbook section is Laplace transform of an exponential.
- Everything except a is given, so isolate a symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- The Laplace transform of e^{-at} is 1/(s+a), a standard entry used in transform tables.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for a:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning a = -0.7859 1/s to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- -1.5718 — kept a factor of two that cancels in the correct rearrangement.
- -0.3930 — dropped that same factor in the other direction.
- -0.8645 — rounded an intermediate value before the final step.
Reference: FE Handbook — Laplace Transforms
An engineer applies the Laplace transform to an exponential decay response. Given decay constant (a) = 1.3000 1/s; complex frequency (s) = 3.8000 1/s, determine the transform value (F) in s.
Given
Find
transform value (F), in s
Start with the thinking
- The governing relation printed in this handbook section is Laplace transform of an exponential.
- Everything except F is given, so isolate F symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- The Laplace transform of e^{-at} is 1/(s+a), a standard entry used in transform tables.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for F:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning F = 0.1961 s to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 0.3922 — kept a factor of two that cancels in the correct rearrangement.
- 0.0980 — dropped that same factor in the other direction.
- 0.2157 — rounded an intermediate value before the final step.
Reference: FE Handbook — Laplace Transforms
A student looks up a Laplace transform pair for a first-order system. Given decay constant (a) = 2.4000 1/s; transform value (F) = 1.8060 s, determine the complex frequency (s) in 1/s.
Given
Find
complex frequency (s), in 1/s
Start with the thinking
- The governing relation printed in this handbook section is Laplace transform of an exponential.
- Everything except s is given, so isolate s symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- The Laplace transform of e^{-at} is 1/(s+a), a standard entry used in transform tables.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for s:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning s = -1.8463 1/s to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- -3.6926 — kept a factor of two that cancels in the correct rearrangement.
- -0.9231 — dropped that same factor in the other direction.
- -2.0309 — rounded an intermediate value before the final step.
Reference: FE Handbook — Laplace Transforms
The Laplace transform of a circuit's impulse response is evaluated at s. Given complex frequency (s) = 5.2000 1/s; transform value (F) = 0.9280 s, determine the decay constant (a) in 1/s.
Given
Find
decay constant (a), in 1/s
Start with the thinking
- The governing relation printed in this handbook section is Laplace transform of an exponential.
- Everything except a is given, so isolate a symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- The Laplace transform of e^{-at} is 1/(s+a), a standard entry used in transform tables.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for a:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning a = -4.1224 1/s to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- -8.2448 — kept a factor of two that cancels in the correct rearrangement.
- -2.0612 — dropped that same factor in the other direction.
- -4.5347 — rounded an intermediate value before the final step.
Reference: FE Handbook — Laplace Transforms
An engineer applies the Laplace transform to an exponential decay response. Given decay constant (a) = 7.0000 1/s; complex frequency (s) = 0.6000 1/s, determine the transform value (F) in s.
Given
Find
transform value (F), in s
Start with the thinking
- The governing relation printed in this handbook section is Laplace transform of an exponential.
- Everything except F is given, so isolate F symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- The Laplace transform of e^{-at} is 1/(s+a), a standard entry used in transform tables.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for F:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning F = 0.1316 s to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 0.2632 — kept a factor of two that cancels in the correct rearrangement.
- 0.0658 — dropped that same factor in the other direction.
- 0.1447 — rounded an intermediate value before the final step.
Reference: FE Handbook — Laplace Transforms
A student looks up a Laplace transform pair for a first-order system. Given decay constant (a) = 1.7000 1/s; transform value (F) = 0.8070 s, determine the complex frequency (s) in 1/s.
Given
Find
complex frequency (s), in 1/s
Start with the thinking
- The governing relation printed in this handbook section is Laplace transform of an exponential.
- Everything except s is given, so isolate s symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- The Laplace transform of e^{-at} is 1/(s+a), a standard entry used in transform tables.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for s:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning s = -0.4608 1/s to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- -0.9217 — kept a factor of two that cancels in the correct rearrangement.
- -0.2304 — dropped that same factor in the other direction.
- -0.5069 — rounded an intermediate value before the final step.
Reference: FE Handbook — Laplace Transforms
The Laplace transform of a circuit's impulse response is evaluated at s. Given complex frequency (s) = 2.6000 1/s; transform value (F) = 1.6690 s, determine the decay constant (a) in 1/s.
Given
Find
decay constant (a), in 1/s
Start with the thinking
- The governing relation printed in this handbook section is Laplace transform of an exponential.
- Everything except a is given, so isolate a symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- The Laplace transform of e^{-at} is 1/(s+a), a standard entry used in transform tables.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for a:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning a = -2.0008 1/s to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- -4.0017 — kept a factor of two that cancels in the correct rearrangement.
- -1.0004 — dropped that same factor in the other direction.
- -2.2009 — rounded an intermediate value before the final step.
Reference: FE Handbook — Laplace Transforms
An engineer applies the Laplace transform to an exponential decay response. Given decay constant (a) = 8.9000 1/s; complex frequency (s) = 9.6000 1/s, determine the transform value (F) in s.
Given
Find
transform value (F), in s
Start with the thinking
- The governing relation printed in this handbook section is Laplace transform of an exponential.
- Everything except F is given, so isolate F symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- The Laplace transform of e^{-at} is 1/(s+a), a standard entry used in transform tables.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for F:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning F = 0.0541 s to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 0.1081 — kept a factor of two that cancels in the correct rearrangement.
- 0.0270 — dropped that same factor in the other direction.
- 0.0595 — rounded an intermediate value before the final step.
Reference: FE Handbook — Laplace Transforms