Electric Circuits Legacy Problem #10 Guided Solution
Problem*
Determine the amount of electrical energy (in J) used by the following devices when operated for the indicated times.
- Hair dryer (1500 W) - operated for 5 minutes
- Electric space heater (950 W) - operated for 4 hours
- X-Box video game player (180 W) - operated for 2 hours
- 42-inch LCD television (210 W) - operated for 3 hours
Audio Guided Solution
Typically electrical appliances have a power rating. This power rating can usually be found on a metal plate that is on the back or underside of the device. It lists the power of that device in units of watts and if you wish to determine the amount of electrical energy used by the device you need to use the definition of electrical power that the electrical power is equal to the energy consumed divided by the time in seconds. That would be the power in watts is equal to the E in joules divided by the T in seconds. In this problem we have four different electrical devices for which we need to determine the amount of energy used. We use the equation P equal E over T and rearrange it to the form of the energy equal the power multiplied by the time. Now you'll note the time is given in minutes and hours for these four different devices so one thing you'll need to do is convert from minutes to seconds by multiplying by 60 or from hours to seconds by multiplying by 60 to get the minutes and by 60 again to get the seconds. Then all you need to do is go energy equal power times time and you have your answer.
Solution
- 4.5x105 J
- 1.4x107 J
- 1.3x106 J
- 2.3x106 J
Habbits of an Effective Problem Solver
- Read the problem carefully and develop a mental picture of the physical situation. If necessary, sketch a simple diagram of the physical situation to help you visualize it.
- Identify the known and unknown quantities and record them in an organized manner. Equate given values to the symbols used to represent the corresponding quantity - e.g., \(\descriptive{\text{δV}}{δV,change in voltage} = 9.0\unit{\volt}\); \(\descriptive{R}{R,resistance} = 0.025\unit{\ohm}\); \(\descriptive{I}{I,current} = \colorbox{gray}{Unknown}\).
- Use physics formulas and conceptual reasoning to plot a strategy for solving for the unknown quantity.
- Identify the appropriate formula(s) to use.
- Perform substitutions and algebraic manipulations in order to solve for the unknown quantity.
Read About It!
Get more information on the topic of Electric Circuits at The Physics Classroom Tutorial.