8 mistakes learners make with energy stores and transfers, each one named and explained.
Energy is never made or destroyed. It moves between stores, and some of it always spreads out.
Current is described where voltage belongs, or mass where weight belongs.
Physics words come in close pairs that describe different parts of the same picture, and the nearer word arrives first.
Question. What does an ammeter measure?
A common answer. The push of the battery
The answer. The current flowing through the circuit
Why. An ammeter counts the flow of charge, which is current. The push is the potential difference, and a voltmeter measures that.
Tie each word to its picture: push across, flow through.
A device is given 100 per cent efficiency because energy is conserved.
If no energy is lost, it sounds as though nothing can be wasted.
Question. A lamp giving 20 J of light from 100 J
A common answer. 100 per cent efficient
The answer. 20 per cent efficient
Why. All 100 J is still there, but 80 J of it is heat nobody wanted.
Energy is always conserved and almost nothing is ever fully efficient. Both at once.
Mitochondria or a power station is said to make or produce energy.
Everyday language says exactly this, including on electricity bills.
Question. What mitochondria do
A common answer. they make energy
The answer. they release energy from glucose by respiration
Why. The energy was already in the glucose; respiration transfers it into a usable form.
Never write makes energy in science. Write releases, transfers or stores.
Conservation of energy is stated as energy being used up or running out.
Batteries go flat and fuel runs out, which is exactly what being used up looks like.
Question. What conservation of energy says
A common answer. energy gets used up
The answer. energy is never created or destroyed, only transferred
Why. The energy in a flat battery has moved into heat and light, not vanished.
Nothing is used up. It is dissipated, which means spread out and harder to use.
Gravitational potential energy is worked out as mass times height, with g never used.
Three numbers in a row is a lot to hold, and the one that is a constant looks optional.
Question. 2 kg lifted 3 m, g = 9.8
A common answer. 2 x 3 = 6 J
The answer. 2 x 9.8 x 3 = 58.8 J
Why. Every letter in the formula is there because the answer depends on it.
Write the formula out first, then tick each letter off as you substitute it.
Kinetic energy comes out at exactly double, because the 0.5 was never applied.
The half looks like decoration next to the mass and the speed squared.
Question. 1000 kg at 20 m/s
A common answer. 1000 x 400 = 400000 J
The answer. 0.5 x 1000 x 400 = 200000 J
Why. The half is part of the formula, not a rounding.
If your answer is exactly double the expected size, look for a missing half.
A lamp giving 20 J of light from 100 J is called 80 per cent efficient.
Eighty is the larger number and larger sounds better for an efficiency.
Question. 100 J in, 20 J of useful light out
A common answer. 80 per cent
The answer. 20 per cent
Why. Efficiency counts the useful output, and the useful output is the light.
Efficiency is useful over total. Find the useful output before you divide anything.
A store is chosen that does not change in the situation described, such as chemical for a lifted book.
Several stores change a little in any real situation, so the one the question is about is not obvious.
Question. Lifting a book onto a shelf
A common answer. the chemical store increases
The answer. the gravitational potential store increases
Why. Chemical energy in your muscles falls; the book gains gravitational potential energy.
Name the object first, then ask what is different about it now.
WAJD spots these patterns in your child's answers and names the one behind their wrong answers, instead of just marking them wrong.