27. Resistors & DC Circuits

Power in Circuits

Electric Power (1 of 3) and Watts, An Explanation

Basic Electricity - Power and watts

An incandescent lightbulb produces 100 W of light. If this lightbulb operates at 25% efficiency (meaning that out of all the power it generates, only 25% is released as light), what resistance must the lightbulb have if it operates at 120 V?

(a) i=0.1 A; (b) R=8.3×10−5 Ω

A hair dryer operates at 120 V (the voltage produced by a household outlet), and outputs 1200 W of energy. For this problem, treat the hair dryer as a single resistor.&nbsp;(a) At what current does the hair dryer operate? (b) What is the resistance of the hair dryer?

A heart defibrillator is used to enable the heart to start beating if it has stopped. This is done by passing a large current of 12 A through the body at 25 V for a very short time, usually about 3.0 ms. (b) how much energy is transferred?

A typical small flashlight contains two batteries, each having an emf of 1.5 V, connected in series with a bulb having resistance 17 Ω. ((b) If the batteries last for 5.0 h, what is the total energy delivered to the bulb?

Consider the circuit of Fig. E25.30<img src="https://lightcat-files.s3.amazonaws.com/problem_images/44c53098aa439786-1672414959348.jpg" alt="" />. (b) What is the power output of the 16.0-V battery?

The heating element of an electric dryer is rated at 4.1 kW when connected to a 240-V line. (b) What is the resistance of the dryer’s heating element at its operating temperature?

A certain resistor is connected across a <math display=“inline”><mrow><mn>9</mn><mo>.</mo><mn>0</mn><mtext>&#x00A0;</mtext><mi mathvariant="normal">V</mi></mrow></math> battery and dissipates <math display=“inline”><mrow><mrow><mn>20</mn><mo rspace="thickmathspace">&#x2062;</mo><mi mathvariant="normal" class="MathML-Unit">&#x00A0;W</mi></mrow></mrow></math> of power. The battery is replaced with an <math display=“inline”><mrow><mn>18</mn><mtext>&#x00A0;</mtext><mi mathvariant="normal">V</mi></mrow></math> battery. How much power is dissipated in the resistor now?

Consider the circuit of Fig. E25.30<img src="https://lightcat-files.s3.amazonaws.com/problem_images/c517896af202613d-1674064363510.jpg" alt="" />. (a) What is the total rate at which electrical energy is dissipated in the 5.0-Ω and 9.0-Ω resistors?

A 1500-W electric heater is plugged into the outlet of a 120-V circuit that has a 20-A circuit breaker. You plug an electric hair dryer into the same outlet. The hair dryer has power settings of 600 W, 900 W, 1200 W, and 1500 W. You start with the hair dryer on the 600-W setting and increase the power setting until the circuit breaker trips. What power setting caused the breaker to trip?

Consider the circuit of Fig. E25.30<img src="https://lightcat-files.s3.amazonaws.com/problem_images/0248914014407053-1674064663729.jpg" alt="" />. (c) At what rate is electrical energy being converted to other forms in the 8.0-V battery?

A <math display=“inline”><mrow><mn>1</mn><mo>.</mo><mn>4</mn><mtext>&#x00A0;</mtext><mtext>kW</mtext></mrow></math> toaster is connected to a wall outlet, which provides a potential difference of <math display=“inline”><mrow><mn>120</mn><mtext>&#x00A0;</mtext><mi mathvariant="normal">V</mi></mrow></math>. What is the effective resistance of the toaster?

A typical small flashlight contains two batteries, each having an emf of 1.5 V, connected in series with a bulb having resistance 17 Ω. (a) If the internal resistance of the batteries is negligible, what power is delivered to the bulb?

The battery for a certain cell phone is rated at 3.70 V. According to the manufacturer it can produce 3.15 * 10^4 J of electrical energy, enough for 5.25 h of operation, before needing to be recharged. Find the average current that this cell phone draws when turned on.

Electric Power (2 of 3) Example Problems

Electric power | Circuits | Physics | Khan Academy

Electric eels generate electric pulses along their skin that can be used to stun an enemy when they come into contact with it. Tests have shown that these pulses can be up to 500 V and produce currents of 80 mA (or even larger). A typical pulse lasts for 10 ms. What power and how much energy are delivered to the unfortunate enemy with a single pulse, assuming a steady current?

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