Lesson 4 of 4

Batteries and electrolysis.

About 40 minutes · 10 practice questions

Read the full article: Batteries

A battery is a galvanic cell you can run backwards: what runs down as it discharges, what charging does, and how electrolysis makes metals and plates objects with them, gram by gram.

Step 1 of 10 · A battery is a cell

Step 1 of 10

A battery is a cell

The Scene shows a lithium-ion battery, like the one in a phone. Zoom to the ions is at Ions: on one side layers of graphite, on the other a metal oxide, and between them an electrolyte in which lithium ions move. When the battery discharges, lithium in the graphite is oxidised and gives an electron to the wire, and the $\mathrm{Li^+}$ ⁠ ion crosses inside the battery to the oxide. It is a galvanic cell: the reaction runs by itself and drives a current.

To the question

Step 2 of 10

Charging

A lithium-ion battery can be charged: an outside supply applies a voltage higher than the battery’s, the other way round, and forces the reaction to run backwards. Current is now fixed on discharging; guess what the ions will do when you switch to charging.

To the question

Step 3 of 10

Electrolysis

Running a redox reaction by force, with an electric current, is called electrolysis. That is how reactive metals such as sodium and aluminium are made, since no chemical reduces them easily. In electrolysis too, oxidation happens at the anode and reduction at the cathode, but here the supply sets the poles: the cathode is connected to the negative terminal.

To the question

Step 4 of 10

How much metal

In electrolysis every electron that passes through the wire reduces one ion, or part of one. So the amount of substance made is proportional to the charge that passed. The charge in coulombs is $Q = It$ ⁠, and the charge of a mole of electrons is Faraday’s constant, $F \approx 96\,500\ \mathrm{C/mol}$ ⁠. That is how objects are plated with copper, silver or gold, to a thickness set in advance.

To the question

Step 5 of 10

Counting charge

The same working describes a battery. A battery’s capacity is measured in ampere-hours: how long it can supply a given current. Now the battery is discharging at a gentle current.

To the question

Step 6 of 10

A faster current

A battery’s current is measured against its capacity: 1C is the current that empties it in an hour.

To the question

Step 7 of 10

At the terminals

The current flows inside the battery too, from electrode to electrode, and the battery has a resistance of its own.

To the question

Step 8 of 10

The power out

How much energy each second does the battery give the bulb when it discharges fast?

To the question

Step 9 of 10

Heat inside

Under Zoom to the ions you see the ions pushing through the battery. That push heats it.

To the question

Step 10 of 10

How full it is

A battery’s voltage at rest depends on how much lithium is left on the graphite side.

To the question

Interactive scene

Loading the interactive version…

Each step sets up the Scene for you, and a control marked “fixed” stays put for that step.

To the experiment

Practice · Choose

When a phone runs on its battery all day, what does the battery lose?

To the experiment

Practice · Predict, then look

The battery is discharging: lithium ions cross inside it from the graphite to the oxide. What will the ions do when you plug in a charger?

To the experiment

Practice · Choose

In the electrolysis of molten sodium chloride a current is passed between two electrodes. What forms at the negative electrode?

To the experiment

Practice · Calculate

A key is copper-plated: a current of 2.0 A passes through a $\mathrm{Cu^{2+}}$ ⁠ solution for 965 s. How many grams of copper build up on the key? Faraday’s constant $F = 96\,500\ \mathrm{C/mol}$ ⁠, and copper’s molar mass is 63.5 g/mol.

To the experiment

Practice · Calculate

A phone battery supplies a current of 0.5 A for two hours. How much charge has passed round the circuit?

To the experiment

Practice · Calculate

The battery’s capacity is 3 Ah. A Current of 1C means a current that empties it in one hour. The current is now locked at 2C. How many amperes flow?

To the experiment

Practice · Calculate

With no current flowing, the battery at 60% reads 3.80 V. Now it supplies 3 A, and its internal resistance is 0.05 Ω. What voltage reaches its terminals?

To the experiment

Practice · Calculate

The Current is locked at 2C, 6 A. The battery reads 3.80 V with no current, and its internal resistance is 0.05 Ω. What power does it deliver to the bulb?

To the experiment

Practice · Predict, then look

At a Current of 1C, 0.45 W of heat stays inside the battery. Predict: how much heat will stay in it when the current doubles to 2C?

To the experiment

Practice · Reach the target

With no current flowing, a lithium battery’s voltage tells how charged it is. Lower the Charge level until the voltage at rest is 3.60 V, and press Check.