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AQA A Level Physics复习笔记5.3.3 Electrical Energy & Power

Category: A-level课程, 教材笔记, 福利干货 Date: 2022年7月11日下午3:07

Energy & Power

In mechanics, power P is defined as the rate of doing work
- The potential difference is the work done per unit charge
- Current is the rate of flow of charge
Therefore, the electrical power is defined as the rate of change of work done:

5.1.5-Power-equation

The work done is the energy transferred so the power is the energy transferred per second in an electrical component

The power dissipated (produced) by an electrical device can also be written as

9.1.2.3-Power-equation

Using Ohm's Law V = IR to rearrange for either V or I and substituting into the power equation, means power can be written in terms of resistance R

9.1.2.3-Power-in-terms-of-resistance

This means for a given resistor if the current or voltage doubles the power will be four times as great.
- Which equation to use will depend on whether the value of current or voltage has been given in the question
Rearranging the energy and power equation, the energy can be written as:

E = VIt

Where:
- E = energy transferred (J)
- V = voltage (V)
- I = current (A)
- t = time (s)

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 lightsD. 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 mneumonic “Twinkle Twinkle Little Star, Power equals I squared R” to remember whether to multiply or divide by resistance in the power equations.When doing calculations involving electrical power, remember the unit is Watts W, therefore, you should always make sure that the time is in seconds

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