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CIE A Level Physics復習筆記9.1.4 Electrical Power

Category: A-level課程, 教材筆記, 福利干貨 Date: 2022年8月16日下午3:47

Calculating Electrical Power

In “Work, Energy and Power”, Power?P?was defined as the?rate of doing work
- Potential difference is the?work done per unit charge
- Current is the?rate of flow of charge
So, the power dissipated (produced) by an electrical device is defined as:

9.1.2.3-Power-equation

Power of a component in an electrical circuit

Using?V?=?IR?to rearrange for either?V?or?I?and substituting into the power equation means we also write power in terms of resistance?R

9.1.2.3-Power-in-terms-of-resistance

Power equation in terms of resistance

This means for a given resistance for example, if the current or voltage doubles the power will be four times as great

Worked Example

Two lamps are connected in series to a 150 V power supply. WE-power-question-image

Which statement most accurately describes what happens?

A.? ? ?Both lamps light normally

B.? ? ?The 15 V lamp blows

C.??? ?Only the 41 W lamp lights

D.? ? ?Both lamps light at less than their normal brightness

ANSWER:??A

9.1.2.3-Worked-example-power-2

Exam Tip

You can use the mnemonic “Twinkle Twinkle Little Star, Power equals?I?squared?R” to remember whether to multiply or divide by resistance in the power equationsWhich equation to use will depend on whether the value of current or voltage has been given in the question

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