Electricity, Magnetism & Light — Print · Science 6: Matter, Energy & Earth Systems, Grade 6

That charged objects sometimes repel and sometimes attract. Two plastic rods rubbed with the same cloth push apart, while a rubbed glass rod pulls a rubbed plastic rod towards it. A single kind of charge could only ever produce one of those behaviours, so at least two kinds must exist, with like repelling and unlike attracting.

The glass has lost electrons to the silk. Positive charge means a shortage of electrons, not an arrival of protons, which stay locked in the nuclei throughout. The silk has gained exactly the electrons the glass lost, so the total charge of the two together is unchanged, as conservation of charge requires.

Because polarization separates the paper's charges slightly. The balloon's charge pushes like charges in the paper away and pulls unlike charges nearer, so the side facing the balloon carries the opposite charge and the far side the same charge. The attracting charges are closer than the repelling ones, and the force is stronger at shorter distance, so attraction wins.

Because damp air leaves a thin film of moisture on surfaces, and that film conducts charge away as fast as rubbing can build it up. In cold dry conditions the air and surfaces are effectively insulating, so electrons transferred by friction stay in place and accumulate until there is enough to jump as a spark.

Because the wire is already full of electrons and the push travels through it at nearly the speed of light. Closing the switch starts every electron in the loop moving almost at once, including those already inside the filament, so energy is delivered immediately even though no individual electron has travelled from the battery to the lamp.

Energy. The number of electrons arriving each second must equal the number leaving, since they cannot accumulate or disappear inside the filament, so the current is unchanged. What each unit of charge loses as it passes through is energy, which appears as the light and heat the filament gives out.

So that every appliance receives the full supply voltage and works independently. In parallel each device sits on its own branch across the supply, gets the whole 230 V, and can be switched off without breaking the path to anything else. In series the voltage would be shared, everything would run weakly, and one failure would stop the lot.

Because the filament has far greater resistance. Resistance measures how strongly a component opposes the flow, and energy is given up where that opposition is greatest, so almost all of the energy per unit of charge is delivered in the filament rather than in the wire. The current is identical in both, since they are in the same loop.

Bring a magnet to a copper coin, aluminium foil and a brass object alongside an iron nail; only the nail is attracted. Copper, aluminium and brass are all metals and none of them respond, so being a metal is not what matters. The magnetic materials are iron, nickel and cobalt and alloys containing them, because only in those can the atoms line up so their effects add.

The force falls to roughly an eighth, not a half, because for a small bar magnet it drops with the cube of the distance. That is why magnets have to be in contact or nearly so to be useful, why a fridge magnet slides off with card behind it, and why magnetic catches are made to close the gap completely rather than nearly.

Because a compass points to the magnetic pole, which is neither at the geographic North Pole nor fixed in place. The magnetic pole is produced by moving liquid iron in Earth's core and drifts by tens of kilometres a year, so the angle between magnetic north and true north differs by location and changes over time, and navigation needs the correction for that place and date.

Because it has to let go. A permanent magnet's field cannot be turned off, so the steel could not be released at the destination, while an electromagnet's field exists only while current flows and disappears when a switch is opened. The strength can also be adjusted by changing the current, which a permanent magnet does not allow.

That you see nothing in a completely dark room however long you wait. If the eye emitted something that made seeing possible, darkness would be no obstacle, since the emitted beam would still reach the object. Seeing stops the moment there is no light to bounce off objects and enter the eye, which is what a receiver would do.

Because the size of the source decides the sharpness. The Sun is small in the sky, so from most points behind an object it is either fully visible or fully blocked, giving a crisp edge. On an overcast day the whole sky scatters light and acts as an enormous source, so light reaches almost everywhere behind the object and the shadow nearly disappears.

Because of surface roughness rather than the law. A mirror is smooth enough that parallel rays leave parallel and keep the pattern they arrived with, producing an image. A wall is microscopically rough, so each patch faces a slightly different direction and scatters rays everywhere; the law still holds at each patch, but the pattern is destroyed, which is also why the wall is visible from any angle.

Because no light ever reaches that position. Every ray bounces off the front surface and travels back into the room, and none passes through to the far side. The eye assumes light has arrived in a straight line and traces those rays backwards, placing what it sees where the extended lines cross, which is a location the light never actually occupied.

Because bending comes from one part of the beam slowing before another. Arriving along the normal, every part of the beam crosses the boundary at the same moment and slows together, so the direction is unchanged. At an angle, one edge reaches the water first and slows to about 225,000 km/s while the rest is still at 300,000, and the beam pivots.

Because light from the fish bends as it leaves the water, and the brain traces it back in a straight line. That places the image higher and nearer the surface than the fish actually is. Aiming at the apparent position sends the spear above the real one, so the aim has to be corrected downward.

By using a second prism to recombine the spectrum into white light again. If the glass were adding colour, passing the light through more glass should have added more, not removed it. Recovering the original white light shows the colours were present in it all along and the prism only spreads them apart by bending each one differently.

Because the tomato reflects only red light and absorbs the rest, and under green light there is no red present to reflect, so almost nothing reaches the eye and it appears black. A white page reflects all colours roughly equally, so under green light it reflects green and looks green rather than white.