Edexcel IGCSE Physics 复习笔记 4.2.5 Power

• Machines, such as car engines, transfer energy from one type to another every second
  • The rate of this energy transfer, or the rate of work done, is called power

   

Two identical cars accelerating to the same final speed will both gain the same amount of energy. But if one of them reaches that speed sooner, it will have a greater power

• Common power ratings are shown in the table below:

Power Ratings Table

• Power is defined as the energy transferred per unit time:

• Power can also be defined as the work done per unit time:

• Where:
  • E or W = The energy transferred, or work done, measured in joules (J)
  • t = time measured in seconds (s)
  • P = power measured in watts (W)

   

• This equation can be rearranged with the help of a formula triangle:

Work, power, time formula triangle

• An example that illustrates the definition of power is by comparing two electric motors if:
  • They lift the same weight
  • They are lifted by the same height
  • One lifts it faster than the other

   

• The motor that lifts the weight faster is said to have more power
• This is because the work done does not depend on time, only the force (weight) and the distance lifted

Two motors with different powers

Step 1: List the known values

• • Power, P = 2000 W
  • Time, t = 5 minutes = 5 × 60 = 300 s

   

Step 2: Write down the relevant equation 

Step 3: Rearrange for energy transferred, E

Energy transferred = power × time

E = Pt

Step 4: Substitute in the values

E = 2000 × 300 = 600 000 J