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Laplace Transforms

Mathematics · FE Reference Handbook section

Mathematics
14 formulas
10 exam-style examples
~60 min
All Mathematics lectures

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.

Example 1
Laplace transform of an exponential — solve for transform value — Laplace Transforms

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

  • decayconstant(a)=7.70001/sdecay constant (a) = 7.7000 1/s
  • complexfrequency(s)=8.50001/scomplex frequency (s) = 8.5000 1/s

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

  1. Step 1 — State the governing relation:

    F(s)=1s+aF(s) = \dfrac{1}{s + a}
  2. Step 2 — Rearrange symbolically for F:

    F=1s+aF = \dfrac{1}{s+a}
  3. Step 3

    Listthegivens:decayconstant(a)=7.70001/s,complexfrequency(s)=8.50001/sList the givens: decay constant (a) = 7.7000 1/s, complex frequency (s) = 8.5000 1/s
  4. Step 4 — Substitute the given values:

    F=18.5000+7.7000F = \dfrac{1}{8.5000+7.7000}
  5. Step 5 — Evaluate:

    F=0.0617 sF = 0.0617\ \text{s}
  6. Step 6 — Check: returning F = 0.0617 s to

    F(s)=1s+aF(s) = \dfrac{1}{s + a}

    reproduces the given quantities, and both sides carry the same units.

Answer:
F=0.0617 sF = 0.0617\ \text{s}

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

Example 2
Laplace transform of an exponential — solve for complex frequency — Laplace Transforms (2)

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

  • decayconstant(a)=1.50001/sdecay constant (a) = 1.5000 1/s
  • transformvalue(F)=1.7150stransform value (F) = 1.7150 s

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

  1. Step 1 — State the governing relation:

    F(s)=1s+aF(s) = \dfrac{1}{s + a}
  2. Step 2 — Rearrange symbolically for s:

    s=1F−as = \dfrac{1}{F} - a
  3. Step 3

    Listthegivens:decayconstant(a)=1.50001/s,transformvalue(F)=1.7150sList the givens: decay constant (a) = 1.5000 1/s, transform value (F) = 1.7150 s
  4. Step 4 — Substitute the given values:

    s=11.7150−1.5000s = \dfrac{1}{1.7150} - 1.5000
  5. Step 5 — Evaluate:

    s=−0.9169 1/ss = -0.9169\ \text{1/s}
  6. Step 6 — Check: returning s = -0.9169 1/s to

    F(s)=1s+aF(s) = \dfrac{1}{s + a}

    reproduces the given quantities, and both sides carry the same units.

Answer:
s=−0.9169 1/ss = -0.9169\ \text{1/s}

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

Example 3
Laplace transform of an exponential — solve for decay constant — Laplace Transforms (3)

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

  • complexfrequency(s)=1.70001/scomplex frequency (s) = 1.7000 1/s
  • transformvalue(F)=1.0940stransform value (F) = 1.0940 s

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

  1. Step 1 — State the governing relation:

    F(s)=1s+aF(s) = \dfrac{1}{s + a}
  2. Step 2 — Rearrange symbolically for a:

    a=1F−sa = \dfrac{1}{F} - s
  3. Step 3

    Listthegivens:complexfrequency(s)=1.70001/s,transformvalue(F)=1.0940sList the givens: complex frequency (s) = 1.7000 1/s, transform value (F) = 1.0940 s
  4. Step 4 — Substitute the given values:

    a=11.0940−1.7000a = \dfrac{1}{1.0940} - 1.7000
  5. Step 5 — Evaluate:

    a=−0.7859 1/sa = -0.7859\ \text{1/s}
  6. Step 6 — Check: returning a = -0.7859 1/s to

    F(s)=1s+aF(s) = \dfrac{1}{s + a}

    reproduces the given quantities, and both sides carry the same units.

Answer:
a=−0.7859 1/sa = -0.7859\ \text{1/s}

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

Example 4
Laplace transform of an exponential — solve for transform value (case 2) — Laplace Transforms (4)

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

  • decayconstant(a)=1.30001/sdecay constant (a) = 1.3000 1/s
  • complexfrequency(s)=3.80001/scomplex frequency (s) = 3.8000 1/s

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

  1. Step 1 — State the governing relation:

    F(s)=1s+aF(s) = \dfrac{1}{s + a}
  2. Step 2 — Rearrange symbolically for F:

    F=1s+aF = \dfrac{1}{s+a}
  3. Step 3

    Listthegivens:decayconstant(a)=1.30001/s,complexfrequency(s)=3.80001/sList the givens: decay constant (a) = 1.3000 1/s, complex frequency (s) = 3.8000 1/s
  4. Step 4 — Substitute the given values:

    F=13.8000+1.3000F = \dfrac{1}{3.8000+1.3000}
  5. Step 5 — Evaluate:

    F=0.1961 sF = 0.1961\ \text{s}
  6. Step 6 — Check: returning F = 0.1961 s to

    F(s)=1s+aF(s) = \dfrac{1}{s + a}

    reproduces the given quantities, and both sides carry the same units.

Answer:
F=0.1961 sF = 0.1961\ \text{s}

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

Example 5
Laplace transform of an exponential — solve for complex frequency (case 2) — Laplace Transforms (5)

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

  • decayconstant(a)=2.40001/sdecay constant (a) = 2.4000 1/s
  • transformvalue(F)=1.8060stransform value (F) = 1.8060 s

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

  1. Step 1 — State the governing relation:

    F(s)=1s+aF(s) = \dfrac{1}{s + a}
  2. Step 2 — Rearrange symbolically for s:

    s=1F−as = \dfrac{1}{F} - a
  3. Step 3

    Listthegivens:decayconstant(a)=2.40001/s,transformvalue(F)=1.8060sList the givens: decay constant (a) = 2.4000 1/s, transform value (F) = 1.8060 s
  4. Step 4 — Substitute the given values:

    s=11.8060−2.4000s = \dfrac{1}{1.8060} - 2.4000
  5. Step 5 — Evaluate:

    s=−1.8463 1/ss = -1.8463\ \text{1/s}
  6. Step 6 — Check: returning s = -1.8463 1/s to

    F(s)=1s+aF(s) = \dfrac{1}{s + a}

    reproduces the given quantities, and both sides carry the same units.

Answer:
s=−1.8463 1/ss = -1.8463\ \text{1/s}

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

Example 6
Laplace transform of an exponential — solve for decay constant (case 2) — Laplace Transforms (6)

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

  • complexfrequency(s)=5.20001/scomplex frequency (s) = 5.2000 1/s
  • transformvalue(F)=0.9280stransform value (F) = 0.9280 s

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

  1. Step 1 — State the governing relation:

    F(s)=1s+aF(s) = \dfrac{1}{s + a}
  2. Step 2 — Rearrange symbolically for a:

    a=1F−sa = \dfrac{1}{F} - s
  3. Step 3

    Listthegivens:complexfrequency(s)=5.20001/s,transformvalue(F)=0.9280sList the givens: complex frequency (s) = 5.2000 1/s, transform value (F) = 0.9280 s
  4. Step 4 — Substitute the given values:

    a=10.9280−5.2000a = \dfrac{1}{0.9280} - 5.2000
  5. Step 5 — Evaluate:

    a=−4.1224 1/sa = -4.1224\ \text{1/s}
  6. Step 6 — Check: returning a = -4.1224 1/s to

    F(s)=1s+aF(s) = \dfrac{1}{s + a}

    reproduces the given quantities, and both sides carry the same units.

Answer:
a=−4.1224 1/sa = -4.1224\ \text{1/s}

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

Example 7
Laplace transform of an exponential — solve for transform value (case 3) — Laplace Transforms (7)

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

  • decayconstant(a)=7.00001/sdecay constant (a) = 7.0000 1/s
  • complexfrequency(s)=0.60001/scomplex frequency (s) = 0.6000 1/s

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

  1. Step 1 — State the governing relation:

    F(s)=1s+aF(s) = \dfrac{1}{s + a}
  2. Step 2 — Rearrange symbolically for F:

    F=1s+aF = \dfrac{1}{s+a}
  3. Step 3

    Listthegivens:decayconstant(a)=7.00001/s,complexfrequency(s)=0.60001/sList the givens: decay constant (a) = 7.0000 1/s, complex frequency (s) = 0.6000 1/s
  4. Step 4 — Substitute the given values:

    F=10.6000+7.0000F = \dfrac{1}{0.6000+7.0000}
  5. Step 5 — Evaluate:

    F=0.1316 sF = 0.1316\ \text{s}
  6. Step 6 — Check: returning F = 0.1316 s to

    F(s)=1s+aF(s) = \dfrac{1}{s + a}

    reproduces the given quantities, and both sides carry the same units.

Answer:
F=0.1316 sF = 0.1316\ \text{s}

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

Example 8
Laplace transform of an exponential — solve for complex frequency (case 3) — Laplace Transforms (8)

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

  • decayconstant(a)=1.70001/sdecay constant (a) = 1.7000 1/s
  • transformvalue(F)=0.8070stransform value (F) = 0.8070 s

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

  1. Step 1 — State the governing relation:

    F(s)=1s+aF(s) = \dfrac{1}{s + a}
  2. Step 2 — Rearrange symbolically for s:

    s=1F−as = \dfrac{1}{F} - a
  3. Step 3

    Listthegivens:decayconstant(a)=1.70001/s,transformvalue(F)=0.8070sList the givens: decay constant (a) = 1.7000 1/s, transform value (F) = 0.8070 s
  4. Step 4 — Substitute the given values:

    s=10.8070−1.7000s = \dfrac{1}{0.8070} - 1.7000
  5. Step 5 — Evaluate:

    s=−0.4608 1/ss = -0.4608\ \text{1/s}
  6. Step 6 — Check: returning s = -0.4608 1/s to

    F(s)=1s+aF(s) = \dfrac{1}{s + a}

    reproduces the given quantities, and both sides carry the same units.

Answer:
s=−0.4608 1/ss = -0.4608\ \text{1/s}

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

Example 9
Laplace transform of an exponential — solve for decay constant (case 3) — Laplace Transforms (9)

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

  • complexfrequency(s)=2.60001/scomplex frequency (s) = 2.6000 1/s
  • transformvalue(F)=1.6690stransform value (F) = 1.6690 s

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

  1. Step 1 — State the governing relation:

    F(s)=1s+aF(s) = \dfrac{1}{s + a}
  2. Step 2 — Rearrange symbolically for a:

    a=1F−sa = \dfrac{1}{F} - s
  3. Step 3

    Listthegivens:complexfrequency(s)=2.60001/s,transformvalue(F)=1.6690sList the givens: complex frequency (s) = 2.6000 1/s, transform value (F) = 1.6690 s
  4. Step 4 — Substitute the given values:

    a=11.6690−2.6000a = \dfrac{1}{1.6690} - 2.6000
  5. Step 5 — Evaluate:

    a=−2.0008 1/sa = -2.0008\ \text{1/s}
  6. Step 6 — Check: returning a = -2.0008 1/s to

    F(s)=1s+aF(s) = \dfrac{1}{s + a}

    reproduces the given quantities, and both sides carry the same units.

Answer:
a=−2.0008 1/sa = -2.0008\ \text{1/s}

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

Example 10
Laplace transform of an exponential — solve for transform value (case 4) — Laplace Transforms (10)

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

  • decayconstant(a)=8.90001/sdecay constant (a) = 8.9000 1/s
  • complexfrequency(s)=9.60001/scomplex frequency (s) = 9.6000 1/s

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

  1. Step 1 — State the governing relation:

    F(s)=1s+aF(s) = \dfrac{1}{s + a}
  2. Step 2 — Rearrange symbolically for F:

    F=1s+aF = \dfrac{1}{s+a}
  3. Step 3

    Listthegivens:decayconstant(a)=8.90001/s,complexfrequency(s)=9.60001/sList the givens: decay constant (a) = 8.9000 1/s, complex frequency (s) = 9.6000 1/s
  4. Step 4 — Substitute the given values:

    F=19.6000+8.9000F = \dfrac{1}{9.6000+8.9000}
  5. Step 5 — Evaluate:

    F=0.0541 sF = 0.0541\ \text{s}
  6. Step 6 — Check: returning F = 0.0541 s to

    F(s)=1s+aF(s) = \dfrac{1}{s + a}

    reproduces the given quantities, and both sides carry the same units.

Answer:
F=0.0541 sF = 0.0541\ \text{s}

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

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