4.4 Power and energy

• physics
• Middelbare school
• havo, vwo
• Leerjaar 2

4.4 Power and energy

ELECTRICAL POWER

The power indicates how much elektrical energy a device uses per second.

Which device uses most power?

1. Mobile phone
2. Calculator
3. Hair dryer
4. Dish washer

When is your Phone using most energy?

1. reading in night modus
2. reading in regular modus
3. watching You Tube
4. sending an app

Power depends on:

VOLTAGE:

Voltage tells you how much energy a single particle supplies to the device

CURRENT:

Current tells you how many particles are delivering their energy every second

In formula:

P = U x I 

U = voltage in Volts (V)

I = current in Amps (A)

P = power in Watts (W)

Example

A toy train works on 12 Volts and 1,5 Amps. Calculate the power.

Solution

Given: U = 12 V

               I = 1,5 A

Asked: P

Solution: P = U x I

               P = 12 V x 1,5 A 

               P = 18 W

Example

A hairdryer is working on a current of 6 A.

Calculate the power of the hairdrayer.

Solution

Given: I = 6 A

The hairdryer is plugged into the socket, so U = 230 V

Asked: P

Solution: P = U x I

P = 230 x 6 

P = 1380 W

The power of a lamp is 75 Watts. The voltage is 230 Volts. What is the current (2 decimals) through the lamp?

Example

A LED flash light has an power of 4 Watts. The current through the LEDs is 900 mA. Calculate the voltage of the batteries.

Solution

Given: P = 4 W

I = 900 mA = 0.9 A

Asked: U

Solution: P = U x I

U = P : I

U = 4 : 0.9 

U = 4,4 V

Power, time and energy consumption

The greater the power of a device, the more quickly the battery will run out . 

Therefore it is important to keep the power consumption of a device low: lower the volume, use energy saving lights (e.g. LEDs). 

This is an interesting factor for designers :-)