---
title: "Electric Current and Its Effects"
book: "Primary & Middle School Physics"
subject: physics
language: en
chapter: 54
exercises: 12
source: https://one-course.com/books/physics/1/en/chapter/54-electric-current-and-its-effects
---

# Chapter 54 — Electric Current and Its Effects

The current has marched namelessly useful through five years of circuits: lighting threads, spinning [motors](https://one-course.com/books/physics/1/en/chapter/41-electric-circuits-and-safety#ex-g6-circuits-and-safety-motor), sounding [buzzers](https://one-course.com/books/physics/1/en/chapter/41-electric-circuits-and-safety#ex-g6-circuits-and-safety-motor). This year electricity turns quantitative — meters, [units](https://one-course.com/books/physics/1/en/chapter/37-measurement-in-science-units-and-instruments#def-g6-measurement-in-science-measuring) and laws — and we open by taking inventory: what, exactly, can a current *do*? Three effects, it turns out, and the whole electrical world is built from them.

## 54.1 The three effects

**Proposition 54.1 (The effects of the electric current).**

A current flowing through matter shows itself by three effects:

1. the *heating effect* : every current warms the [conductor](https://one-course.com/books/physics/1/en/chapter/22-conductors-and-insulators#def-g4-conductors-insulators-conductor) it crosses — gently in good wires, fiercely in thin or overloaded ones, white-hot on purpose in filaments and toasters;
2. the *magnetic effect* : a current turns its surroundings magnetic — a wire deflects a nearby [compass](https://one-course.com/books/physics/1/en/chapter/25-magnets-and-the-compass#def-g4-magnets-and-compass-compass) needle, and a coiled wire becomes a true [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) with poles, switchable at will;
3. the *chemical effect* : crossing certain [liquids](https://one-course.com/books/physics/1/en/chapter/14-solids-liquids-gases#def-g3-solids-liquids-gases-liquid) , a current drives chemical change — splitting water into two gases, plating spoons with silver, charging and discharging every [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) .

**Example 54.2 (The heating effect at work).**

Deliberate: the toaster’s glowing bars, the kettle’s hidden coil, the old lamp’s white-hot filament, the [fuse](https://one-course.com/books/physics/1/en/chapter/48-short-circuits-and-electrical-safety#def-g7-short-circuits-safety-fuse) built to [melt](https://one-course.com/books/physics/1/en/chapter/9-ice-water-steam#def-g2-ice-water-steam-meltfreeze) first. Unwanted: warm phone chargers, hot laptop undersides — and the [short circuit](https://one-course.com/books/physics/1/en/chapter/48-short-circuits-and-electrical-safety#def-g7-short-circuits-safety-device)’s menace, which you studied as the villain’s signature. One effect, servant and danger, dosed by how hard the current runs — a dose we learn to *[measure](https://one-course.com/books/physics/1/en/chapter/37-measurement-in-science-units-and-instruments#def-g6-measurement-in-science-measuring)* in the next chapter.

![The thermal effect, glowing: current forced through the toaster’s resistive coil heats it red-hot.](https://one-course.com/images/onecourse/chapters/physics-1/g8-electric-current-effects/img-b251f7c1cdb9.jpg)

*The thermal effect, glowing: current forced through the toaster’s resistive coil [heats](https://one-course.com/books/physics/1/en/chapter/34-heat-and-insulation#def-g5-heat-and-insulation-heat) it red-hot.*

## 54.2 The current makes magnets

**Example 54.3 (The compass betrays the wire).**

Two centuries ago, during a lecture, a physicist noticed a [compass](https://one-course.com/books/physics/1/en/chapter/25-magnets-and-the-compass#def-g4-magnets-and-compass-compass) needle twitch each time he closed a circuit on the bench beside it. That twitch — barely a trembling of a needle — was the first bridge ever seen between electricity and magnetism, two sciences until then as separate as weather and music. Lay a [compass](https://one-course.com/books/physics/1/en/chapter/25-magnets-and-the-compass#def-g4-magnets-and-compass-compass) beside a wire and switch on: the needle swings aside; reverse the current, and it swings the other way. The current creates magnetism around itself, with a direction.

**Definition 54.4 (Electromagnet).**

An *electromagnet* is a coil of insulated wire — often wound on an iron core — that becomes a [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) while current flows through it: [north pole](https://one-course.com/books/physics/1/en/chapter/25-magnets-and-the-compass#def-g4-magnets-and-compass-poles), [south pole](https://one-course.com/books/physics/1/en/chapter/25-magnets-and-the-compass#def-g4-magnets-and-compass-poles), iron-grabbing power and all. Unlike any stone [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet), it obeys a [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch): current on, [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) on; current off, [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) gone; current reversed, poles exchanged.

![The electromagnet: a coil and a current make a magnet that a switch can create, destroy, or flip.](https://one-course.com/images/onecourse/chapters/physics-1/g8-electric-current-effects/fig-14b7f955abdd.svg)

*The [electromagnet](#def-g8-electric-current-effects-electromagnet): a coil and a current make a [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) that a [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) can create, destroy, or flip.*

**Example 54.5 (Switchable magnetism runs the world).**

The scrapyard crane grips a car with an [electromagnet](#def-g8-electric-current-effects-electromagnet) and drops it by opening a [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) — try that with a stone [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet). Electric door-locks, the clack of a relay, the doorbell’s hammer (an [electromagnet](#def-g8-electric-current-effects-electromagnet) snatching it against the bell, cutting its own current, springing back: rattle made law), the reading head of old telephones, and — supremely — every electric [motor](https://one-course.com/books/physics/1/en/chapter/41-electric-circuits-and-safety#ex-g6-circuits-and-safety-motor): current-made magnetism pushing against [magnets](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet), thousands of times a second. The [magnetic effect](#prop-g8-electric-current-effects-three) is the muscle of the electrical age.

**Method 54.6 (Winding an electromagnet).**

1. [wind](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-wind) a [metre](https://one-course.com/books/physics/1/en/chapter/7-measuring-length#def-g2-measuring-length-units) or two of thin insulated wire in tight, tidy turns around a large iron nail or bolt, leaving two free ends;
2. connect the ends to a [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) through a [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) (never directly for long — the coil is nearly a [short circuit](https://one-course.com/books/physics/1/en/chapter/48-short-circuits-and-electrical-safety#def-g7-short-circuits-safety-device) and warms: the [heating effect](#prop-g8-electric-current-effects-three) chaperones the experiment);
3. close the [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) briefly: the nail-head grabs paperclips; open it: they drop;
4. test with a [compass](https://one-course.com/books/physics/1/en/chapter/25-magnets-and-the-compass#def-g4-magnets-and-compass-compass) : the coil’s two ends behave as N and S poles — and swap when you reverse the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) .

More turns, stronger grip: dose the magnetism by winding.

## 54.3 The chemical effect

**Example 54.7 (Current through water).**

Dip two pencil-lead electrodes wired to a [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) into water made slightly conducting (a spoon of washing soda helps), and watch: tiny bubbles stream from both leads — two different gases, born of the water itself, twice the [volume](https://one-course.com/books/physics/1/en/chapter/38-mass-and-volume#def-g6-mass-and-volume-volume) at one lead as at the other. The current is dismantling water into its two invisible components. This door between electricity and chemistry — *electrolysis* — plates cheap spoons with silver, wins aluminium from ore, and belongs mostly to your chemistry course; physics salutes it and moves on.

**Example 54.8 (Every battery is the effect run backward).**

Inside every [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery), chemistry pushes the current: stored chemical [energy](https://one-course.com/books/physics/1/en/chapter/29-energy-in-everyday-life#def-g5-everyday-energy-energy) drives the march, spending itself as it serves — the emptying you have known since the flashlight dimmed. Rechargeable cells close the circle: the charger [forces](https://one-course.com/books/physics/1/en/chapter/12-pushes-and-pulls-forces#def-g2-pushes-and-pulls-force) current through *backward*, and the [chemical effect](#prop-g8-electric-current-effects-three) rebuilds the store. Chemical to electrical, electrical to chemical: one effect, both directions, the whole [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) industry.

**Remark 54.9 (Effects as detectors).**

The three effects answer a question that stumped you as the tester’s apprentice: how do you *see* a current — an invisible march in an [opaque](https://one-course.com/books/physics/1/en/chapter/49-sources-of-light-and-propagation#def-g7-light-sources-propagation-coats) wire? By its deeds. A warming wire, a twitching [compass](https://one-course.com/books/physics/1/en/chapter/25-magnets-and-the-compass#def-g4-magnets-and-compass-compass), a streaming bubble: each betrays the march. Every current-measuring instrument ever built reads one of the three effects — and the one waiting in the next chapter reads the magnetic twitch, refined into a numbered dial.

## 54.4 Exercises

**Exercise 54.1 ★.**

Name the three effects of the [electric current](https://one-course.com/books/physics/1/en/chapter/47-circuits-loops-series-parallel#def-g7-circuits-series-parallel-current), each with one servant use.

**Solution of Exercise 54.1.**

Heating (the toaster, the kettle, the filament); magnetic (the [electromagnet](#def-g8-electric-current-effects-electromagnet), the [motor](https://one-course.com/books/physics/1/en/chapter/41-electric-circuits-and-safety#ex-g6-circuits-and-safety-motor), the doorbell); chemical (electroplating, splitting water, charging batteries).

**Exercise 54.2 ★.**

Which effect, in each scene: a [fuse](https://one-course.com/books/physics/1/en/chapter/48-short-circuits-and-electrical-safety#def-g7-short-circuits-safety-fuse) [melting](https://one-course.com/books/physics/1/en/chapter/39-water-in-all-its-states#def-g6-water-states-changes); a doorbell hammering; bubbles at the pencil leads; a toaster’s glow; a scrapyard crane’s grip?

**Solution of Exercise 54.2.**

Heating; magnetic; chemical; heating; magnetic.

**Exercise 54.3 ★.**

What did the twitching [compass](https://one-course.com/books/physics/1/en/chapter/25-magnets-and-the-compass#def-g4-magnets-and-compass-compass) beside the lecture bench reveal? What happens to the twitch when the current reverses?

**Solution of Exercise 54.3.**

That a current creates magnetism around its wire — the first bridge between electricity and magnetism. Reversing the current reverses the twitch.

**Exercise 54.4 ★.**

Give three powers an [electromagnet](#def-g8-electric-current-effects-electromagnet) has that no stone [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) can match.

**Solution of Exercise 54.4.**

It can be switched off, switched on, and have its poles flipped at will — and its strength can be dosed (more turns, more current). A stone [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) can do none of these.

**Exercise 54.5 ★.**

In the winding method, why must the wire be insulated? And why must the coil not stay connected long without a working load?

**Solution of Exercise 54.5.**

Bare turns touching would let the current shortcut across the windings instead of touring the coil — insulation [forces](https://one-course.com/books/physics/1/en/chapter/12-pushes-and-pulls-forces#def-g2-pushes-and-pulls-force) the long magnetizing path. And the coil alone opposes the march very little: left connected it is nearly a [short circuit](https://one-course.com/books/physics/1/en/chapter/48-short-circuits-and-electrical-safety#def-g7-short-circuits-safety-device), warming coil and [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) — the [heating effect](#prop-g8-electric-current-effects-three) chaperones.

**Exercise 54.6 ★.**

How does the crane operator release the car? Contrast with what a stone [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) of equal strength would demand.

**Solution of Exercise 54.6.**

By opening a [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch): current gone, magnetism gone, car dropped. A stone [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) of such strength would hold forever — releasing the car would demand machines to tear it free.

**Exercise 54.7 ★.**

What [energy](https://one-course.com/books/physics/1/en/chapter/29-energy-in-everyday-life#def-g5-everyday-energy-energy) conversion happens inside a [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) in use? And inside a rechargeable [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) on its charger?

**Solution of Exercise 54.7.**

In use: stored chemical [energy](https://one-course.com/books/physics/1/en/chapter/29-energy-in-everyday-life#def-g5-everyday-energy-energy) becomes electrical [energy](https://one-course.com/books/physics/1/en/chapter/29-energy-in-everyday-life#def-g5-everyday-energy-energy) driving the march (ending as [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source), motion, warmth). On the charger: electrical [energy](https://one-course.com/books/physics/1/en/chapter/29-energy-in-everyday-life#def-g5-everyday-energy-energy) forced backward through the cell rebuilds the chemical store.

**Exercise 54.8 ★.**

Why are the three effects precious as *detectors*? Which effect will the next chapter’s instrument refine into a number?

**Solution of Exercise 54.8.**

Because the march itself is invisible: only its deeds betray it. Warming, twitching, bubbling are the current’s only signatures — and next chapter’s instrument refines the magnetic twitch into a numbered reading.

**Exercise 54.9 ★★.**

The doorbell: an [electromagnet](#def-g8-electric-current-effects-electromagnet) snatches the hammer toward the bell, but the moving hammer breaks its own circuit; a spring returns it, remaking the contact — and so on. Walk through one full cycle and explain why the design *must* rattle rather than hold.

**Solution of Exercise 54.9.**

Button pressed: current flows, the [electromagnet](#def-g8-electric-current-effects-electromagnet) snatches the hammer, ding — but the hammer’s swing opens a contact in its own supply; magnetism dies, the spring returns the hammer, remaking the contact; current returns, snatch again — and so on, many times a second. Holding is impossible by design: the snatch always cuts its own cause — and the rattle *is* the ring.

**Exercise 54.10 ★★.**

A relay is a [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) closed by an [electromagnet](#def-g8-electric-current-effects-electromagnet): a small current in the coil closes contacts that carry a big current in another circuit. Explain why this lets a delicate thermostat safely command a hungry electric heater — and which two circuits never touch.

**Solution of Exercise 54.10.**

The thermostat’s frail contacts carry only the coil’s small current; the coil’s magnetism closes stout contacts that carry the heater’s hungry current in a separate loop. Small commands large, and the two circuits never touch — the delicate hand pulls a [lever](https://one-course.com/books/physics/1/en/chapter/17-levers-and-balance-scales#def-g3-levers-and-scales-lever), not the load.

**Exercise 54.11 ★★.**

Your [compass](https://one-course.com/books/physics/1/en/chapter/25-magnets-and-the-compass#def-g4-magnets-and-compass-compass) needle twitches whenever a hidden wire in the wall carries current. Design a wall-wire finder from this fact: protocol, what a strong twitch means, and one honest limit of the instrument (think of what else moves [compass](https://one-course.com/books/physics/1/en/chapter/25-magnets-and-the-compass#def-g4-magnets-and-compass-compass) needles).

**Solution of Exercise 54.11.**

Protocol: hold the [compass](https://one-course.com/books/physics/1/en/chapter/25-magnets-and-the-compass#def-g4-magnets-and-compass-compass) flat against the wall, far from iron furniture; sweep slowly; where the needle twitches as the suspect appliance is switched on and off, a current-carrying wire runs. A strong twitch means a close or strong current. Honest limit: needles also answer iron pipes, [magnets](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) and the Earth itself — so only a twitch that follows the switch on–off proves a current.

**Exercise 54.12 ★★★.**

An electric [motor](https://one-course.com/books/physics/1/en/chapter/41-electric-circuits-and-safety#ex-g6-circuits-and-safety-motor), in outline: a coil on an axle sits between the poles of a [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet); current makes the coil a [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet); the push-and-pull of poles turns the axle — and just as the coil would settle aligned, the current through it is reversed, so it must chase alignment forever. Explain, pole by pole, why the reversal is indispensable — what would the [motor](https://one-course.com/books/physics/1/en/chapter/41-electric-circuits-and-safety#ex-g6-circuits-and-safety-motor) do without it? (You have just explained why [motors](https://one-course.com/books/physics/1/en/chapter/41-electric-circuits-and-safety#ex-g6-circuits-and-safety-motor) contain a reversing contact — and half of how the electrical age turns its wheels.)

**Solution of Exercise 54.12.**

Current makes the coil a [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet); unlike poles of coil and frame [attract](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet), like poles repel, and the coil turns toward alignment. Without reversal it would reach alignment and stop — one half-turn, then a shudder into rest: a door-closer, not a [motor](https://one-course.com/books/physics/1/en/chapter/41-electric-circuits-and-safety#ex-g6-circuits-and-safety-motor). Reversing the current at the aligned moment swaps the coil’s poles: yesterday’s attraction is now repulsion, alignment becomes the worst place to be, and the coil must keep chasing a goal that flips away each half-turn — perpetual pursuit, which is to say: rotation.

## 54.5 Problem: The Inventor’s Workshop

**Problem 54.1.**

Weekend problem — one battery, three effects, five inventions; an evening in the inventor’s workshop

The workshop bench holds batteries, wire, iron nails, a [compass](https://one-course.com/books/physics/1/en/chapter/25-magnets-and-the-compass#def-g4-magnets-and-compass-compass), two pencil leads, a jar of soda water — and a notebook of half-built inventions to assess.

**Part I — Taking inventory.**

1. Sketch the workshop’s three-effect test bench: for one [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) and a [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) , describe a setup that shows all three effects at once (a thin wire to warm, a [compass](https://one-course.com/books/physics/1/en/chapter/25-magnets-and-the-compass#def-g4-magnets-and-compass-compass) beside a lead, the jar in the loop). In what arrangement must the three demonstrations sit — series or parallel — for one [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) to run them all?
2. The thin demonstration wire grows warm but the thick supply wires stay cool, in the same loop. Which chapter-one fact about the series loop makes this *more* interesting than it looks — what is equal in both wires, and what differs?
3. The [compass](https://one-course.com/books/physics/1/en/chapter/25-magnets-and-the-compass#def-g4-magnets-and-compass-compass) twitches east when the [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) closes. What two changes, made together, would leave the twitch exactly as it was?
4. Bubbles stream twice as fast from one pencil lead as the other. What does this hint about water’s two components? (Chemistry will confirm the ratio.)

**Part II — Assessing the inventions.**

5. Invention: “the silent doorbell” — an [electromagnet](#def-g8-electric-current-effects-electromagnet) that pulls a soft felt hammer once and holds it until the button is released. Which piece of the classic doorbell’s mechanism must the inventor *remove* , and what will the visitor hear?
6. Invention: “the magnetic sorter” — scrap on a conveyor passes under an [electromagnet](#def-g8-electric-current-effects-electromagnet) that lifts out iron and steel. What happens at the drop bin, and why does the sorter leave aluminium cans on the belt (two [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) facts from earlier years answer)?
7. Invention: “the pole-flipper” — a [compass](https://one-course.com/books/physics/1/en/chapter/25-magnets-and-the-compass#def-g4-magnets-and-compass-compass) mounted over a coil, flipping each time a hidden [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) reverses the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) . Which property of current-made magnetism is being exploited?
8. Invention: “the eternal electroplater” — silver plating powered by a [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) that “recharges itself from the plating”. Veto it, citing the chapter’s two [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) directions and an older law about [energy](https://one-course.com/books/physics/1/en/chapter/29-energy-in-everyday-life#def-g5-everyday-energy-energy) .

**Part III — The night’s masterpiece.** The inventor closes with a crane game for the fair: a joystick, a small [electromagnet](#def-g8-electric-current-effects-electromagnet) on strings, a bin of iron trinkets.

9. Script the winning move in effect-language: what happens at “grab”, during the swing, and at “release”?
10. Fairgoers complain the crane drops prizes mid-swing whenever the old [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) sags. Which property of the [electromagnet](#def-g8-electric-current-effects-electromagnet) — virtue and weakness in one — is on display?
11. The inventor doubles the coil’s turns to help. Predict the effect on grip, and name the other dose (from the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) side) that next chapter will let us number.
12. Notebook’s last page: write the workshop’s summary — one sentence per effect, each naming its finest servant invention of the evening.

**Solution of Problem 54.1.**

**1.** All three demonstrations in *series* on one loop with the [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch): one march then serves warm wire, [compass](https://one-course.com/books/physics/1/en/chapter/25-magnets-and-the-compass#def-g4-magnets-and-compass-compass) wire and jar in turn — one [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch), three effects. **2.** The series law: the *same* current crosses thick and thin wire alike. What differs is only the road — the thin wire suffers the march more and warms: same dose, different patient. **3.** Reverse the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) *and* pass the wire on the [compass](https://one-course.com/books/physics/1/en/chapter/25-magnets-and-the-compass#def-g4-magnets-and-compass-compass)’s other side (or flip the [compass](https://one-course.com/books/physics/1/en/chapter/25-magnets-and-the-compass#def-g4-magnets-and-compass-compass)): two reversals cancel, and the twitch stays east. **4.** That water’s two components are present in a two-to-one proportion — twice the [gas](https://one-course.com/books/physics/1/en/chapter/14-solids-liquids-gases#def-g3-solids-liquids-gases-gas) from one lead as the other. **5.** Remove the self-interrupting contact (the piece the hammer’s swing opens): the [electromagnet](#def-g8-electric-current-effects-electromagnet) then holds the felt hammer against the bell in silence — the visitor hears one soft “dum” and nothing more. **6.** At the bin the operator opens the coil’s [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch): magnetism vanishes, iron drops. Aluminium rides on because [magnets](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) — stone or current-made — grip only iron’s family, and aluminium never answered them. **7.** That the poles of a current-made [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) follow the current’s direction: reverse one, reverse the other. **8.** A [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) drives plating by *spending* its chemical store; rebuilding a store requires [energy](https://one-course.com/books/physics/1/en/chapter/29-energy-in-everyday-life#def-g5-everyday-energy-energy) forced in from outside. A [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) that refills itself from the very work it pays for is the forever-machine again — the [energy](https://one-course.com/books/physics/1/en/chapter/29-energy-in-everyday-life#def-g5-everyday-energy-energy) bill cannot pay itself. **9.** Grab: [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) closes, coil magnetizes, trinket held. Swing: current maintained, grip maintained — the [magnetic effect](#prop-g8-electric-current-effects-three) on duty. Release: [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) opens over the chute, magnetism dies, prize falls. **10.** That the [electromagnet](#def-g8-electric-current-effects-electromagnet) lives by its current: sagging [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery), weaker march, weaker grip — switchability’s price is dependence. (The stone [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) never sags — and never lets go.) **11.** More turns, stronger [magnet](https://one-course.com/books/physics/1/en/chapter/6-magnets#def-g1-magnets-magnet) at the same current: firmer grip. The other dose is the current’s own strength — the *intensity*, measured in the next chapter. **12.** For example: “Heating: the [fuse](https://one-course.com/books/physics/1/en/chapter/48-short-circuits-and-electrical-safety#def-g7-short-circuits-safety-fuse), dying first and faithfully. Magnetic: the crane game, gripping and releasing on command. Chemical: the plating jar, silvering spoons with a [battery’s](https://one-course.com/books/physics/1/en/chapter/48-short-circuits-and-electrical-safety#def-g7-short-circuits-safety-battery) patience.”