Energy & Power (HL IB Physics)

• 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

• 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, with labels on lightbulbs which indicate their power, such as 60 W or 100 W
  • These indicate the amount of energy transferred by an electrical current rather than by a force doing work

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 energy of 1 J in 1 s