Electricity and magnetism, Year 9

Resistance

提示The same word describes people pushing back against something.

为什么Every component holds the current back to some extent. For the same push from the battery, a component that opposes the current more lets less of it flow. A bulb filament does this far more than the copper wires leading to it.

It gets smaller

提示The charge now has two obstacles to get through, and no extra push to help.

为什么Each bulb opposes the current, and in a single loop those effects add together. The cell pushes exactly as hard as before, so with more in its way less current flows. That is why both bulbs glow more dimly than one did alone.

Bulb A

提示The push is identical both times, so ask where the flow was held back more.

为什么With the same potential difference each time, the only thing that can change the current is the component. The bulb that lets less current through is the one opposing it more, so the smaller ammeter reading goes with the larger resistance.

The ohm

提示It is named after a German physicist, and its symbol is the Greek letter omega.

为什么One of these is the resistance of a component that lets 1 amp flow when there is 1 volt across it. A torch bulb has a few of them; the plastic coating on a wire has many millions.

It is extremely high

提示Compare how strongly plastic and copper hold back the flow of charge.

为什么Conductors and insulators are not two different kinds of thing so much as two ends of one scale. A metal opposes the current very little; plastic opposes it so much that the cell cannot push a measurable current through.

18

提示Dividing by a half does not make a number smaller.

为什么Resistance = potential difference ÷ current, so 9 ÷ 0.5. Dividing by 0.5 is the same as asking how many halves fit into 9. A small current for a given potential difference always means a large resistance.

5 ohms; 4 A

提示Find the ratio of the two readings first, then keep that ratio fixed when the push changes.

为什么Resistance is 10 ÷ 2. The resistor itself has not changed, so its resistance is the same at 20 V, and the current is 20 divided by that resistance. Doubling the push across a fixed resistor doubles the current.

12 V

提示Rearrange the resistance equation so the quantity you want stands alone.

为什么R = V ÷ I rearranges to V = I × R, so 3 × 4. Check it by putting the answer back: 12 ÷ 3 gives the 4 ohms you started with.

current

提示It is the reading you would take with the meter placed in the loop itself.

为什么Resistance is a ratio: how many volts of push it takes for each amp that flows. Measure the push across a component with a voltmeter and the flow through it with an ammeter, divide, and you have its resistance in ohms.

Count how many paperclips it lifts

提示You need lots of small, identical steel objects.

为什么Strength cannot be seen, so you measure something it does. A stronger field holds more steel, so the number picked up gives a figure you can compare from one trial to the next. Repeating each trial and averaging makes the result more trustworthy.

The current

提示Think of the three things that affect the strength. One is being changed and one has been dealt with.

为什么A fair test changes one factor and holds the others steady. Winding on more wire can change how much flows, so she should check the ammeter and adjust the supply each time. Otherwise she cannot tell whether the turns or the flow made the difference.

Adding the core

提示Compare each of the last two results with the first one.

为什么Doubling the turns doubled the number lifted, from 2 to 4. Adding the iron core, with the turns back at 10, took it from 2 to 30. Iron concentrates the field so strongly that it usually matters far more than a few extra turns.

Iron loses its magnetism when the current stops

提示Think about what the device is supposed to do the instant you switch it off.

为什么Both metals are pulled into the field and strengthen it. Steel, though, stays magnetised afterwards, which would leave a scrapyard crane unable to let go of its load. Iron gives the boost without the memory.

complete

提示Charge needs an unbroken route all the way round and back to the cell.

为什么A series circuit is a single loop, so a broken filament anywhere is a gap in the only path there is. This is why finding one dead bulb in an old string of fairy lights meant testing them one by one.

A broken bulb

提示Ask which part you have now shown is not to blame.

为什么Good fault-finding is a process of elimination. Swapping in a part you trust tests one suspect at a time. Here the fault must lie elsewhere: the cell, a wire, a connection or the switch.

In the switch

提示Ask which part the charge no longer has to pass through.

为什么The spare wire gives the current a route around one component. If the circuit comes to life, everything else must have been working all along, and the part you bypassed is the one that was blocking the loop. Bypassing each part in turn finds a break without guesswork.

Ammeter

提示Its name is built from the unit that current is measured in.

为什么It goes in the loop itself, so that everything flowing passes through it. A reading of zero anywhere in a series loop tells you the loop is broken somewhere. A voltmeter is the other meter, and it is connected across a component instead.

Nearly all the current takes the wire instead

提示Charge now has two routes past that point. Compare how easy each one is.

为什么The wire has almost no resistance and the filament has a lot, so nearly all the current goes through the wire and too little is left to make the filament glow. This is a short circuit. It can make the wires and the cell dangerously hot, because the current is now much larger.