الأسبوع 17 · المحور الثالث، الوحدة الثالثة، المفهوم 3.1 «الأجهزة والطاقة» (ب): سلاسل الطاقة وحفظ الطاقة · 3 دروس
3
Energy never disappears
Discovery Education Science P4: Term 2, Unit 3, pp. 11–25
GoalExplain conservation of energy, build an energy chain model and write a claim, evidence and reasoning about a solar phone.
New words
conservation of energy · energy is never created or destroyed, it only changes form or movestransferred · moved from one place to another in the same formsolar cell · a part that turns sunlight into electrical energy
Explanation
Energy can change forms, but it is never created or destroyed. This is the law of conservation of energy. When you switch on a lamp, electrical energy becomes light and heat. When you ride a bicycle, the chemical energy from breakfast becomes motion and heat. It may seem that a phone battery “loses” energy when it goes flat. In fact the stored chemical energy has changed into light on the screen, sound and heat. It has not vanished. To use the phone again we put energy in again. In an energy chain model we label every link and say whether energy is transferred (moved in the same form) or transformed (changed into a different form). To answer how sunlight can power a phone, we write a claim, evidence and reasoning.
A phone goes flat. The 100 units of chemical energy in the battery became 40 units of light and sound on the screen and speaker and 60 units of heat: 40 + 60 = 100. All the energy is still there.
wire: transferred→motor: transformed→blades: moving air
Model chain for a fan: electricity from the socket (transferred along the wire) to the motor (transformed into motion) to the blades (moving air). Each link is labelled.
Claim
Evidence
Reasoning
light becomes electricity
charges in the sun, not in the dark
energy changes form
Claim: sunlight can power a phone because the solar cell changes light into electrical energy. Evidence: a solar charger charges a phone in the sun but not in the dark. Reasoning: light energy is transformed into electrical energy, stored as chemical energy in the battery, and then used.
Class activity
Build an energy chain. With a grown-up, choose a device and draw its chain on a long strip of paper or a row of cards, with pictures and arrows. Label each link with its form of energy and with the word transferred or transformed. Show it to your family and let them find a missing link.
Practice
1
Can energy be created or destroyed?
No. It only changes form or moves.
2
A battery had 50 units of energy. It gave 10 units of light and 5 of sound. The rest was heat. How many units were heat?
50 − 10 − 5 = 35 units.
3
In a solar phone chain, which form of energy does the solar cell make from light?
Electrical energy.
4
The wire carries electricity to the motor without changing its form. Is that transferred or transformed? Say the letter.
