Power, energy and efficiency

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  1. Introduction to Power, Energy and Efficiency:
  2. What is electric power?
  3. What are the formulas relating to power?
  4. What is electrical energy?
  5. How can we measure the efficiency of an electric circuit?
  6. What are the differences between voltage, power and energy?
  1. Solving for Power and Resistance

    Power, Energy and Efficiency
    1. What is the power dissipated by resistor RA?
    2. What is the resistance of RA?
  2. Solving for Power, Current and Energy

    Power, Energy and Efficiency
    1. What is the current leaving the battery?
    2. How much energy is used by RB in 2 minutes?
  3. Solving for Power and Voltage

    Power, Energy and Efficiency
    1. What is the voltage of the battery supply?
    2. What is the current going through RC?
    3. What is the power dissipated by the circuit?
  4. Solving for Power and Efficiency
    An electric crane runs off of a 100V source at a current of 14A. The crane lifts a 180kg block 17.5m in the air (vertically) in 35 seconds. What is the efficiency of the crane?
    1. Solving for Power and Efficiency
      A lightbulb emits approximately 300 J of light energy in 5 seconds. If this bulb is 16% efficient, what is the power supplied to the lightbulb?
      Topic Notes

      In this lesson, we will learn:

      • How we can understand power as the rate of energy transformation.
      • The definition of power related to energy and the 3 version of the power formula (related to voltage, current, and resistance)
      • About energy as a property and the accumulation of power dissipation across a span of time that the circuit/device is operating
      • How power is related to the efficiency of an electric circuit.
      • How to solve for power, energy, and efficiency using:
        • The formula for power: P=IV=I2R=V2RP = IV = I^{2}R = \frac{V^{2}}{R}
        • The formula representing the relationship between energy and power: P=EtP = \frac{E}{t} and E=Pt=IVtE = Pt = IVt
        • The efficiency formula: efficiency=PoutputPinputx efficiency = \frac{P_{output}}{P_{input}} x\, 100%


      • Power is the rate at which energy is transformed (when the resistor/device transforms electric energy into another form of energy such as heat, light, etc.)
      • Thus, power is defined as:
        • P=EtP = \frac{E}{t}
      • Where:
        • PP is the power dissipated (in watts, W)
        • EE is the energy transformed (in joules, J)
        • tt is the time that the device/circuit is operating (in seconds, s)

      • The unit for power is in watts (W) which represents: 1 Watt = 1 Joule1 Second \frac{1 \, Joule}{1 \, Second}

      • Power can also be conceptualized as the product of current and voltage, giving the first power formula:
        • P=IVP = IV
      • The formula can be written in two other versions by substituting of Ohm’s Law into the power formula:
        • P=I2RP = I^{2}R and P=V2RP = \frac{V^{2}}{R}
      • Energy is the property of the ability to do work (where work refers to energy transferred to objects in order to move them, heat them up, etc.)
      • Energy is defined through rearranging the first power definition; energy is the accumulation of power dissipation for a duration of time:
        • P=EtP = \frac{E}{t} therefore, E=PtE = Pt
        • And by substituting P=IVP = I V (the power formula), it is given that: E=IVtE=I V t
      • The unit for energy is in joules (J) which represents a variety of physics concepts (gravitational potential energy; force and work; charge and voltage; power and time):
        • J=(kg)(m2)s2=Nm=CV=WsJ= \frac{(kg)(m^{2})}{s^{2}} = Nm = CV = Ws
      • For your monthly electricity bill, you pay for energy (and NOT power). You are paying for how much energy you’re using by keeping your electronics on for an amount of time (power is the rate at which your devices are transforming electrical energy). You are not billed for the number of joules, but rather in the units of kilowatt-hours (1kWh = 3.6x106J).

      • The efficiency of an electric circuit is a percentage that represents the proportion of power that is produced by a device (useful output of dissipated power) over how much power is actually supplied to that device (input power that is consumed):
        • efficiency=PoutputPinputxefficiency = \frac{P_{output} }{P_{input}} x 100%
      • The efficiency is not perfect (100%) because there is energy loss when electrical energy is transformed into other forms (i.e. a lightbulb transforms electrical energy into thermal energy to heat up its wire filament so that it will glow and produce light energy; the initial heating is lost partially to the environment).