Chapter 4 - Graphs of the Circular Functions - Section 4.1 Graphs of the Sine and Cosine Functions - 4.1 Exercises - Page 146: 56a

Amplitude$=3.8$ Period$=\frac{1}{20}$

For the graph of $y=a\cos x$, the amplitude is $|a|$. Therefore, for the case of $E=3.8\cos 40\pi t$, the amplitude is $|3.8|=3.8$. In addition, the period of the graph $y=\cos bx$ is $\frac{2\pi}{b}$. Therefore, for the case of $E=3.8\cos 40\pi t$, the period is $\frac{2\pi}{40\pi}=\frac{2}{40}=\frac{1}{20}$.