5.4 Power and energy

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

4.4 Power and Energy

Planning

• Revision: Power
  
• (You will need a calculator)

• Explanation: Energy consumption (Extra)
• (You will need a calculator for this part too)

• Let's work

ELECTRICAL POWER

The power indicates how much electrical energy a device uses per second

Which device uses the most power?

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

Which device uses the most power?

1. Television
2. Microwave
3. Computer
4. Sound system

Which device uses the most power?

1. Washing machine
2. Dryer
3. Vacuum cleaner
4. Iron

Which device uses the most power?

1. Fridge
2. Airconditioner
3. Underfloor heating
4. Water boiler

Which device uses the most power?

1. Television
2. Desktop computer
3. Laptop
4. Sound system

Power

The power indicates how much electrical energy a device uses per second

• So on which variables does the power depends on?

Power

The power indicates how much electrical energy a device uses per second

• VOLTAGE:
• = much energy a single particle supplies to the device

• CURRENT:
• =many particles are delivering their energy every second
  

In formula:

P  = U x I 

P = power in Watts (W)

U  = voltage in Volts (V)

 I   = current in Amps (A)

Always put the variables in the right, standard units!

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

Formula:         P = U x I

Calculation:   P = 12 V x 1.5 A 

                            P = 18 W

Example

A hairdryer is working on a current of 6000 mA.

Calculate the power of the hairdryer.

Solution

Given:             I = 6000 mA => 6 A

                          U = 230 V  (hairdryer is plugged into mainsocket) 

Asked:            P

Formula:        P = U x I

Calculation: P = 230 x 6 

                           P = 1380 W

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

Solution

g: P = 75 W         V = 230 V

a: Current I

f:  P=U * I             I =     P / U

c:                            I = 75 / 230

                                I = 0.33 A

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

g:  P = 4 W            I = 900 mA = 0.9 A

a:  U = ?

f.:  P = U x I           U = P / I

c.                             U = 4 / 0.9 

                                 U = 4.44 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, uses energy saving lights (e.g. LEDs). 

This is an interesting factor for designers :-)

Total amount of Energy

Divices are 'consuming' electrical energy:

It may require a lot of power, but if you do not use it much, its energy consumption will not be high. 

Conversely, a device that uses very little power may have an unexpectedly high energy consumption, if it works all day 

Total amount of Energy

You can calculate the energy consumption by multiplying the power by the period of time. Expressed as a formula: 

E = P x t

E = Energy concumpion (in kWh)

P = Power (in kW)

t = time (in h)

Example

A radiant heater has a power of 2000 W. The radiant heater is switched on for 3 hours. The price per kWh is 22 cents. 

• Calculate the total energy concumpion in kWh
• Calculate the energy costs of the electric heater

Solution

g:   P = 2000 W          t = 3 hours                        Price per kWh = € 0.22 

a:   E =? (in kWh)

f:    E = P x t

c.   E = 2 kW x 3 h = 6 kWh

Energy costs = 6 kWh x € 0.22 per 1 kWh = € 1.32

A hairdryer has a power of 1400 W. The hairdryer is been used for 15 minute.

A. Calculate the total energy concumpion (in kWh)
B. Calculate the energy costs of the hairdryer (1 kWh is 22 cents)

Solution

g:   P = 1400 W => 1,4 kW          t = 15 minutes = 0.25 h       Price per kWh = € 0.22 

a:   E =? (in kWh)

f:    E = P x t

c.   E = 1.4 kW x 0.25 h = 0.35 kWh

Energy costs = 0.35 kWh x € 0.22 per 1 kWh = € 0.08

A lamp is been turned on for 15 minutes. The lamp converts a total of 0.0125 kWh of electrical energy.

Calculate the power of this lamp (in Watts).

Solution

g:  E = 0.0125 kWh               t = 15 min. = 0.25 h 

a:  P =? (in W)

f:   E = P x t         ->    P = E / t

c:                                   P = 0.0125 / 0.25 = 0.050 kW = 50 W

HW

Read the (PDF)-file: "2tV_ Electrical Energy " (ELO)

Make the following exercises: 12, 13, 14, 15, 17, 19, 20 (out of the same file)