3.2 Power in a Circuit 🔦

Electrical circuits are often used to convert electrical energy into other types of energy. Recall from your previous physics classes that

Using this, we can derive an equation for power in electrical terms.

Because of Ohm's Law we can also write this several different ways by subbing in

Choose the one that best fits the information that's given in the problem.

1. A hair dryer is rated at 1200W when connected to 120V. What is the resistance of the dryer?

Answer

2. A student assembles the circuit shown below

The switch is closed and the student records the current in the circuit as shown in the graph below. The equation for the graph is

where t is the time elapsed from the instant the switch is closed and I_0 and k are constants.

a) Using the information in the graph, determine the potential difference Vo across the resistor immediately after the switch is closed.

b) Determine the value of k from this best-fit curve. Show your work and be sure to include units in your answer.

c) Determine the following in terms of R, k, I_o, and t.

i. The power delivered to the resistor at time t = 0 ii. The power delivered to the resistor as a function of time t iii. The total energy delivered to the resistor from t = 0 until the current is reduced to zero

Answers

a) According to the graph, the current at t=0 is approximately 9.3A. Using V = IR that gives us a Voltage of 31V across the 3.3Ω Resistor

b) Using I_o=9.3A, I=7.5A, t=0.5hr

c)