Energy & Power (DP IB Physics)

Energy & Power

• The power of a mechanical process is the rate at which energy is transferred

• This energy transferred is the work done

• Therefore, power is:

  The rate of work done (energy transfer)

• Time is an important consideration when it comes to power

• Two cars transfer the same amount of energy, or do the same amount of work to accelerate over a distance

• If one car has more power, it will transfer that energy, or do that work, in a shorter amount of time

Two cars accelerate to the same final speed, but the one with the most power will reach that speed sooner

• Two electric motors:

  • lift the same weight

  • by the same height

  • but one motor lifts it faster than the other

• The motor that lifts the weight faster has more power

Two motors with different powers

• Power can be calculated using the equation:

• Where:

  • P = power (W)

  • ΔW = change in work done (J)

  • Δt = time interval (s)

  • F = force (N)

  • v = velocity (m s^–1)

• The equation with F and v is only relevant where a constant force moves a body at constant velocity

  • Power is required in order to produce an acceleration

• The force must be applied in the same direction as the velocity

• Power is also used in electricity

• Appliances are given a power rating, for example, 1000 W

  • The power ratings indicate the amount of energy transferred per second to the appliance

The Watt

• Power is measured in watts (W)

• The watt, W, is commonly used as the unit power (and radiant flux)

  • It is defined as 1 W = 1 J s^–1

• The SI unit for energy is kg m² s^–3

• One watt is defined as:

  A transfer of 1 joule of energy in 1 second