DC Circuits

1.       A resistor with a 10 V potential difference across its ends develops thermal energy at a rate of 25 W.
(a) What is the current in the resistor?
(b) What is its resistance? (Chapter 19, problem 33)

2.       The wattage rating of a lightbulb is the power it consumes when it is connected across a 120 V potential difference. For example, a 60 W lightbulb consumes 60 W of electrical power only when it is connected across a 120 V potential difference.
(a) What is the resistance of a 90 W lightbulb?
(b) Without doing any calculation, would you expect a 100 W bulb to have more or less resistance than a 90 W bulb? Calculate and find it. (Chapter 19, problem 37)

3.    The battery for a certain cell phone is rated at 4 V. According to the manufacturer it can produce 3 X 10⁴ J of electrical energy, enough for 5 h of operation, before needing to be recharged.
Find the average current that this cell phone draws when turned on. (Chapter 19, problem 41)