AC/DC and Electric Power

AC/DC and Electric Power

By Rachel Wampler

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Power Formula

Power Formula

Multiple Choice

Let's say an electric heater has a resistance of 1430 Ω and operates at 120 V. What is the power rating of the heater? How much electrical energy does it use in 24 hours?

• 10.1 W

  242.4 KJ

• 0.099 W

  8.5 KJ

• 10.1 W

  873 kJ

• 0.099 W

  2.4 KJ

Kilowatt Hours

• The electric company charges you for the amount of electrical energy you use.

  • Electrical energy used is measured​ in kilowatt hours (kWh).

• Despite the name, it doesn't mean the number of kilowatts you're using per hour.​

  • 1 kilowatt hour is the amount of energy you’d use if you kept a 1,000 watt appliance running for an hour.​

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Multiple Choice

If electricity costs $0.15 per kWh, how much does it cost to operate the heater from before (P = 10.1 W) for 30 days?

• $0.19

• $0.001

• $1090.80

• $1.09

• Charge flow reverses direction periodically

• Due to the way that power plants generate power

• AC in a simple circuit​

Alternating Current

Periodicity

• Voltage, current, and power fluctuate with time

• So we usually talk about the averages ​

Average Power

Root Mean Square (rms)

USA conventions

• V₀ = 170 V

• V_rms = 120 V

• Most electronics in the U.S. specify 120 V, so they really mean V_rms

• We will always (unless otherwise noted) use average power, and root mean square current and voltage

• As as result, all of the previous equations work!​

Multiple Choice

A 60 W light bulb operates on a peak voltage of 156 V. Find the V_rms, I_rms, and the resistance of the light bulb. (HINT* Peak Voltage = Maximum Voltage)

• V_rms = 120 V

  I_rms = 0.55 A

  R = 1.83 Ω

• V_rms = 110 V

  I_rms = 0.55 A

  R = 202 Ω

• V_rms = 120 V

  I_rms = 0.5 A

  R = 202 Ω

• V_rms = 110 V

  I_rms = 0..5 A

  R = 1.83 Ω

Resistance