Power in Electric Circuits — Question 1

Question 1

Terminal signs distinguish absorbed electrical power from delivered power

Define $v=V_A-V_B=12.0$ V and i positive when entering terminal A. A device carries i=+2.00 A for 5.00 s, then i=-.500 A for 8.00 s, at the same voltage.

1. Find absorbed power and energy in each interval.

2. Find the net energy absorbed and net charge entering A; verify their relationship.

Question 1 - Solution

Terminal signs distinguish absorbed electrical power from delivered power

(a) Method and calculation

$$p=vi,\quad p_1=24.0\,\mathrm W,\quad W_1=120\,\mathrm J.$$$$p_2=-6.00\,\mathrm W,\quad W_2=-48.0\,\mathrm J.$$

(b) Interpretation and further calculation

$$W_{\mathrm{net}}=120-48=72.0\,\mathrm J,\quad Q_A=2(5)-.5(8)=6.00\,\mathrm C.$$$$W_{\mathrm{net}}=vQ_A=(12)(6)=72.0\,\mathrm J.$$

Physical check

Negative absorbed power means the device delivers energy during that interval; the sign convention remains fixed.