Lesson 3 – V: Voyage into Volt: Measuring Power & Energy
V: Voyage Into Volt
Why Voltage Alone Does Not Tell the Whole Story
Many electrical devices have labels that list voltage and current. A phone charger, laptop charger, fan, or light may show values such as 5 V and 2 A. These numbers are clues, but one number alone does not tell you how much power the device uses.
Power depends on both voltage and current. Voltage describes the electrical push. Current describes how much charge is flowing. A device with a high voltage but low current may use less power than a device with a lower voltage but higher current.
Power Detective Prediction Sort
From what you see in the examples below, rank the device labels based on which ones you think will use the least to the most power. At first, use only what you notice on the labels.
|
Adapter Example |
Sample Output Label |
|---|---|
|
Small USB charger |
Output: 5 V ⎓ 0.5 A |
|
USB wall charger |
Output: 5 V ⎓ 2.0 A |
|
LED-strip adapter |
Output: 12 V ⎓ 1.0 A |
|
Laptop adapter |
Output: 20 V ⎓ 3.25 A |
Key Formula
Power = Voltage × Current, or P = V × I.
Looking at the label, you may assume voltage is the most important number, and the higher the voltage the higher the power. In reality, Power is more than voltage. In fact, power is determined by using both voltage and current using the formula Power = Voltage x Current.
For example, a 5 V charger that supplies 2 A has a power of 10 W because 5 × 2 = 10. A 20 V laptop charger that supplies 3.25 A has a power of 65 W because 20 × 3.25 = 65.
|
Device Example |
Voltage (V) |
Current (I) |
Power Calculation (P = V X I) |
Power |
|---|---|---|---|---|
|
Phone charger |
5 |
2 |
5 × 2 |
10 |
|
Laptop charger |
20 |
3.25 |
20 × 3.25 |
65 |
|
USB fan |
5 |
0.5 |
5 × 0.5 |
2.5 |
Check Your Understanding
Why is it possible for two devices with the same voltage to use different amounts of power?