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Natural sciences

Grade 8

Energy: electricity, heat and light

Module 9

Units of electricity

Activity:

To calculate electricity costs [lo 1.3, lo 2.4]

  • You may know that the unit in which electricity is sold is known as kilowatt per hour – kW/hour

1 kWh is the amount of electric energy that an appliance with a power rating of 1 kW uses in one hour.

We use the following formula to calculate the kWh of any appliance:

KWh = power x time

Say you want to know how much energy your hair drier (1 500 W = 1,5 kW) uses in 20 minutes (20 minutes = 1 3 size 12{ { {1} over {3} } } {} of an hour):

KWh = power rating x time

= 1 500 W x 0,333

= 1,5 kW x 0,333

= 0,5 kWh

You use the following formula to calculate the cost of the use of an electrical appliance:

Cost = electrical energy in kWh x cost of 1 kWh

Supposing that 1 kWh costs 50c, the above hair drier will cost:

Cost = 0,5 kWh x 50c

= 25c

= R0,25

Assignment 1:

Examine the pictures and calculate the cost of using each appliance. Electricity costs R2, 50 per kW.

Light bulb in use: 100 w for 5 hours

TV in use: 80W for 5 hours

Iron in use: 1500 W for 2 hours

Assessment of Calculations

Were you able to do the calculations correctly?

[LO 1.3; LO 2.4]

Assessment

LO 1: Scientific investigations:

The learner will be able to act confidently on curiosity about natural phenomena, and to investigate relationships and solve problems in scientific, technological and environmental contexts.

This is evident when the learner:

  • evaluates data and communicates findings.

LO 2: Constructing Science Knowledge:

The learner will know and be able to interpret and apply scientific, technological and environmental knowledge.

This is evident when the learner:

2.4 applies knowledge.

Memorandum

Assignment 1

UNITS of ELECTRICITY

The amount of electricity that is used by an appliance depends on the type of appliance and the length of time for which it is used.

Appliances used for generating heat, like stoves (ovens), kettles, irons and hot water cylinders general use much more electricity and are more costly. Very powerful halogen light bulbs (spotlights) also consume much electricity, while appliances fitted with microchips or small motors, like electric toothbrushes and razors, will use less.

Each appliance bears an indication of its value - the watt value . This is the appliance's power rating . It is usually indicated on the outer cover of the appliance, like in the case of a light bulb. A 100-Watt light bulb will burn more brightly than a 60-Watt light bulb.

Watt actually indicates the amount of electricity per second that the appliance will use. As the watt unit is relatively small, we usually speak in terms of kilowatt (kW)

  • 1 kW = 1000 W

A unit of electricity is read in terms of the amount of electricity that a 1 kW appliance will use in one hour or a 100 W appliance will use in 10 hours. This is the unit in which electricity is sold to consumers.

Your 100-W light bulb will therefore burn for 10 hours on 1 kW of electricity. Your father will be able to operate his electric drill (500W) for two hours only.

We know that electricity is sold units of kilowatt per hour – kW/hour

  • 1 kWh is the amount of electrical energy that an appliance with a rating of 1 kW uses in one hour.

To determine the Wh rating of an appliance, we use the following formula:

  • kWh = power rating x time

If you, for instance, want to know how much energy your hairdryer (1500W) uses in 20 minutes, you work it out like this:

  • kWh = power rating x time
  • 1 500w /1000w x 0.3
  • = 0.45 kWh

The cost of using an electrical appliance can be determined by means of the following formula:

  • Cost = electrical energy in kWh x cost of 1 kWh

Supposing that 1 kWh costs 50c, the use of your hairdryer (above) will cost:

  • Cost = 0.45 kWh x 50c
  • = R 2,25

Questions & Answers

the diagram of the digestive system
Assiatu Reply
How does twins formed
William Reply
They formed in two ways first when one sperm and one egg are splited by mitosis or two sperm and two eggs join together
Oluwatobi
what is genetics
Josephine Reply
Genetics is the study of heredity
Misack
how does twins formed?
Misack
What is manual
Hassan Reply
discuss biological phenomenon and provide pieces of evidence to show that it was responsible for the formation of eukaryotic organelles
Joseph Reply
what is biology
Yousuf Reply
the study of living organisms and their interactions with one another and their environments
AI-Robot
the study of living organisms and their interactions with one another and their environment.
Wine
discuss the biological phenomenon and provide pieces of evidence to show that it was responsible for the formation of eukaryotic organelles in an essay form
Joseph Reply
what is the blood cells
Shaker Reply
list any five characteristics of the blood cells
Shaker
lack electricity and its more savely than electronic microscope because its naturally by using of light
Abdullahi Reply
advantage of electronic microscope is easily and clearly while disadvantage is dangerous because its electronic. advantage of light microscope is savely and naturally by sun while disadvantage is not easily,means its not sharp and not clear
Abdullahi
cell theory state that every organisms composed of one or more cell,cell is the basic unit of life
Abdullahi
is like gone fail us
DENG
cells is the basic structure and functions of all living things
Ramadan
What is classification
ISCONT Reply
is organisms that are similar into groups called tara
Yamosa
in what situation (s) would be the use of a scanning electron microscope be ideal and why?
Kenna Reply
A scanning electron microscope (SEM) is ideal for situations requiring high-resolution imaging of surfaces. It is commonly used in materials science, biology, and geology to examine the topography and composition of samples at a nanoscale level. SEM is particularly useful for studying fine details,
Hilary
cell is the building block of life.
Condoleezza Reply
what is cell divisoin?
Aron Reply
Diversity of living thing
ISCONT
what is cell division
Aron Reply
Cell division is the process by which a single cell divides into two or more daughter cells. It is a fundamental process in all living organisms and is essential for growth, development, and reproduction. Cell division can occur through either mitosis or meiosis.
AI-Robot
What is life?
Allison Reply
life is defined as any system capable of performing functions such as eating, metabolizing,excreting,breathing,moving,Growing,reproducing,and responding to external stimuli.
Mohamed
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Source:  OpenStax, Natural sciences grade 8. OpenStax CNX. Sep 12, 2009 Download for free at http://cnx.org/content/col11050/1.1
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