Power

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課程筆記

Definition of Power

Power is the rate at which energy is transferred.
The unit of power is the Watt (W).
One Watt equals one Joule per second (1 W = 1 J/s).

The motor that lifts the same weight through the same height in less time has more power

Power Equation

The equation linking power, energy and time is: Power = Energy ÷ Time.
In symbols: P = E ÷ t.
Energy is measured in Joules (J), time in seconds (s).
Rearranged: Energy = Power × Time (E = P × t).

Example Calculations

Example 1: A 5 W torch used for 30 s transfers 150 J of energy (E = 5 × 30).
Example 2: A toaster using 20,000 J in 100 s has a power of 200 W (P = 20,000 ÷ 100).

Kilowatts and Conversion

Power can be given in kilowatts (kW); 1 kW = 1000 W.
To convert kW to W: multiply by 1000.
To convert W to kW: divide by 1000.

Worked Examples from Workbook

Q3: Lamp: 2400 J in 40 s → Power = 2400 ÷ 40 = 60 W.
Q4: Motor: 4000 J in 200 s → Power = 4000 ÷ 200 = 20 W.
Q5: Microwave: 900 W for 15 s → Energy = 900 × 15 = 13,500 J.
Q6: Radiator: 2000 W for 90 s → Energy = 2000 × 90 = 180,000 J.
Q9: Washing machine: 2.2 kW for 20 min → Convert: 2.2 kW = 2200 W; 20 min = 1200 s; Energy = 2200 × 1200 = 2,640,000 J.

Key Points to Remember

Always use consistent units: Joules, seconds, Watts.
For time given in minutes, convert to seconds (×60).
For power in kW, convert to W (×1000) before calculation.

A simple circuit to measure power: P = V × I (though not directly covered, the circuit illustrates energy transfer).

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練習題

免費預覽——54 題中的 8 題。註冊即可查看全部。

1.What is the unit of power?
Easy
- AWatt (W)
- BJoule (J)
- CSecond (s)
- DKilowatt-hour (kWh)
2.Which equation correctly relates power (P), energy (E), and time (t)?
Easy
- AP = E × t
- BP = E ÷ t
- CP = t ÷ E
- DP = E + t
3.How many watts are in one kilowatt?
Easy
- A10 W
- B100 W
- C1000 W
- D1 000 000 W
4.A lamp uses 2400 J of energy in 40 s. What is its power?
Easy
- A60 W
- B96 000 W
- C0.0167 W
- D600 W
5.A microwave has a power of 900 W. How much energy does it use in 15 s?
Easy
- A60 J
- B13 500 J
- C900 J
- D135 J
6.An electric motor uses 4000 J of energy in 200 s. What is its power?
Easy
- A20 W
- B200 W
- C800 000 W
- D0.05 W
7.A radiator has a power of 2000 W. How much energy does it use in 90 s?
Easy
- A22.2 J
- B180 000 J
- C2000 J
- D22 222 J
8.A washing machine has a power of 2.2 kW. How much energy does it use in 20 minutes?
Medium
- A44 J
- B2640 J
- C2 640 000 J
- D44 000 J

複習筆記

## Power Power is the **rate at which energy is transferred**. The unit of power is the **watt (W)**. ### Key Equations - **Power = Energy ÷ Time** \( P = \frac{E}{t} \) - **Energy = Power × Time** \( E = P \times t \) Energy is measured in **joules (J)**, time in **seconds (s)**. ### Worked Examples 1. **Torch**: \( P = 5\,\text{W} \), \( t = 30\,\text{s} \) \( E = P \times t = 5 \times 30 = 150\,\text{J} \) 2. **Toaster**: \( E = 20\,000\,\text{J} \), \( t = 100\,\text{s} \) \( P = E \div t = 20\,000 \div 100 = 200\,\text{W} \) ### Kilowatts - **1 kW = 1000 W** - To convert **kW → W**: multiply by 1000 - To convert **W → kW**: divide by 1000 ### Tips for Calculations - Always write the equation first. - Substitute values with units. - Calculate and include units in answer. - For time in minutes, convert to seconds (×60). - For power in kW, convert to W (×1000). ### Quick Practice - Lamp: \( E = 2400\,\text{J}, t = 40\,\text{s} \) → \( P = 60\,\text{W} \) - Motor: \( E = 4000\,\text{J}, t = 200\,\text{s} \) → \( P = 20\,\text{W} \) - Microwave: \( P = 900\,\text{W}, t = 15\,\text{s} \) → \( E = 13\,500\,\text{J} \) - Radiator: \( P = 2000\,\text{W}, t = 90\,\text{s} \) → \( E = 180\,000\,\text{J} \)

Power: The rate at which energy is transferred. Measured in watts (W).
Watt (W): The unit of power. One watt equals one joule per second.
Energy: Measured in joules (J). The capacity to do work.
Kilowatt (kW): A unit of power equal to 1000 watts.
P = E ÷ t: Equation linking power (P) in watts, energy (E) in joules, and time (t) in seconds.
E = P × t: Rearranged equation to calculate energy from power and time.

字卡

What is power?

Power is the rate at which energy is transferred.

What is the unit of power?

The unit of power is the Watt (W).

Write the equation linking power, energy and time.

Power = Energy ÷ Time, or P = E ÷ t.

Write the equation linking energy, power and time.

Energy = Power × Time, or E = P × t.

What is the unit of energy?

The unit of energy is the Joule (J).

What is the unit of time in power calculations?

The unit of time is the second (s).

A torch has a power of 5 W and is used for 30 s. Calculate the energy used.

E = P × t = 5 W × 30 s = 150 J.

A toaster uses 20,000 J of energy in 100 s. Calculate its power.

P = E ÷ t = 20,000 J ÷ 100 s = 200 W.

How many watts are in one kilowatt?

One kilowatt (kW) equals 1000 watts (W).

Convert 2 kW to watts.

2 kW × 1000 = 2000 W.

Convert 500 W to kilowatts.

500 W ÷ 1000 = 0.5 kW.

A lamp uses 2400 J in 40 s. Calculate its power.

P = E ÷ t = 2400 J ÷ 40 s = 60 W.

An electric motor uses 4000 J in 200 s. Calculate its power.

P = E ÷ t = 4000 J ÷ 200 s = 20 W.

A microwave has a power of 900 W. Calculate energy used in 15 s.

E = P × t = 900 W × 15 s = 13,500 J.

An electric radiator has a power of 2000 W. Calculate energy used in 90 s.

E = P × t = 2000 W × 90 s = 180,000 J.

A washing machine has a power of 2.2 kW. Convert to watts.

2.2 kW × 1000 = 2200 W.

Convert 20 minutes to seconds.

20 minutes × 60 = 1200 seconds.

A washing machine has a power of 2.2 kW. Calculate energy used in 20 minutes.

First convert: 2.2 kW = 2200 W, 20 min = 1200 s. Then E = P × t = 2200 W × 1200 s = 2,640,000 J.

What is the formula to convert kilowatts to watts?

Multiply the number of kilowatts by 1000: kW × 1000 → W.

What is the formula to convert watts to kilowatts?

Divide the number of watts by 1000: W ÷ 1000 → kW.

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