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Ohm's Law 1

Take me to the 'Year 8 Microcontrollers' main course page.

Learning aim

To understand the relationships between voltage, current and resistance
(known as 'Ohm's Law').

(Why am I doing this?)

So that I can begin to make predictions about how electrical circuits function.
So in future, I may be able to fix, repair or even design my own simple circuits.

Experimenting with electrical properties

You will be learning how to build simple DC circuits, which will help build your understanding of how electricity behaves and how understanding Ohm's Law helps us control the current flow in the circuit to make it 'work for us'.

You will need to take screenshots periodically. To do this, press 'PRINT SCREEN' and then 'CTRL + V' to paste the image into your work.

Starter - Have a look at the object one the right. What is it? What do you know about it? Turn and talk with a partner. Use your 'show- me' boards to write down your answers.

Lesson vocabulary

Here are the meanings of some of the most important words we will be discussing in today's lesson.

Anode (when emptying, discharging)

noun: anode; plural noun: anodes

the sacrificial electrode of an electrochemical cell, where oxidation occurs.
.the electrode through which current flows into a device or cell (the negative terminal of a battery).

NB. OXIDATION is when an atom LOSES ELECTRONS. These are usually EXOTHERMIC (outside heat) processes whereby energy is given off as heat. This can be very dangerous.

Cathode (when emptying, discharging)

noun: cathode; plural noun: cathodes

the beneficial electrode of an electrochemical cell, where 'reduction' occurs.
the electrode through which current flows out of a device or cell (the positive terminal of a battery).

NB. It is called a REDUCTION because even though it GAINS ELECTRONS, it LOSES net charge because electrons are negatively charged themselves.

Electrolyte

noun: resistance; plural noun: resistances

A chemical compound that ionizes when dissolved or molten to produce an ionically conductive 'medium'. That is, that it will allow positively charged ions to move towards the cathode freely. The movement of positive ions from the anode to the cathode allows the overall charge to maintain balance in the battery cell.

NB. A 'medium' is a word given to describe A MATERIAL that SOMETHING ELSE travels through.

An ION is an ATOM that has GAINED ELECTRONS (Anions) OR LOST ELECTRONS (Cations).

Task 1 - In the simulation below, click the switch icon
or the 'connect' button and watch closely what happens.
You may need to do this several times, clicking 'reset' in-between.
What do you think this simulator is showing?
You may be asked questions by your teacher to describe the behaviour in the circuit.

Your teacher will afterwards do a
practical demonstration.

Each potato in our demonstration is known as a 'cell'. Lots of cells make up a 'battery'.

Potatoes are acidic and can form a battery cell because they have a high pH ('potential of hydrogen'), a measure of the concentration of hydrogen atoms within it. Other acidic vegetables that you can make batteries from are: tomatoes, cabbage, onion and lemon. Alkali vegetables include: spinach, cucumber and courgette. Anything that is acidic or alkali can form a battery cell if the right metallic elements are used as the anode and cathode.

Click to view a BBC Bitesize article about the electric battery.

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What is going on in a battery?

How is electrical potential energy created?

A typical battery has two different metals called electrodes, separated by a chemical called the electrolyte. Inside the battery, a chemical reaction happens at the negative electrode (the anode) that releases electrons. These electrons are tiny, negatively charged particles. They cannot travel through the electrolyte, so they build up a push or electrical potential energy at the anode. When you connect a wire from the negative to the positive electrode, the electrons flow through the wire—this is the electric current that can power a device. At the positive electrode (the cathode), another chemical reaction accepts the electrons, completing the circuit.

Why does a battery stop producing electrical potential energy?

Because the chemical reaction that generates the build up of electrons at the anode, runs out. The anode is no longer able to oxidise and release electrons.

I want to know more! This is for the more scientifically inquisitive among us. 😁

What goes on in the battery terminals? (the 'ends' of the battery)

🪫 In a Discharging Battery (e.g. an AA battery in use)

The battery is supplying electrical energy to a circuit.

Anode (−): Oxidation happens here — the chemical reaction produces electrons.
- Electrons leave the anode and travel through the external circuit.
Cathode (+): Reduction happens here — the chemical reaction consumes electrons arriving from the circuit.

So, in a discharging (galvanic) cell,

🔹 The anode is negative.
🔹 The cathode is positive.

🔋 In a Charging Battery (e.g. rechargeable cell being charged)

The current is being forced into the cell — the reactions reverse.

Anode (+): Now receives electrons (reduction).
Cathode (−): Now loses electrons (oxidation).

So, during charging,

🔹 The anode becomes positive.
🔹 The cathode becomes negative.

What is the chemical reaction that causes the negative electron build up?

E.g. A zinc–carbon (zinc oxide) battery has a zinc outer case (the anode) and a carbon rod (the cathode) in the middle, with ammonium chloride paste as the electrolyte.

At the zinc anode, a chemical reaction happens: zinc metal reacts with ammonium ions and loses electrons. This is called oxidation. The freed electrons cannot pass through the paste, so they build up at the zinc.

At the carbon cathode, another reaction happens: manganese dioxide reacts with hydrogen ions and gains electrons. This is called reduction.

When a wire connects the zinc to the carbon, the electrons flow through the wire, creating an electric current. The chemical reactions keep happening as long as zinc is left to oxidise and manganese dioxide can reduce, producing a steady flow of electrons to power devices.

⚙️ At the Anode (Negative Electrode — Oxidation)

Word equation:

Zinc + Hydroxide ions → Zinc oxide + Water + Electrons

Chemical equation:

Zn (s) + 2OH⁻ (aq) → ZnO (s) + H₂O (l) + 2e⁻

Description of the chemical process at the anode:

Zinc metal is oxidised to zinc oxide.
Hydroxide ions from the potassium hydroxide (KOH) electrolyte participate.
Electrons are released into the external circuit.

⚡ At the Cathode (Positive Electrode — Reduction)

Word equation:

Manganese dioxide + Water + Electrons → Manganese(III) oxyhydroxide

Chemical equation:

2MnO₂ (s) + H₂O (l) + 2e⁻ → 2MnOOH (s)

Description of the chemical process at the cathode:

Manganese dioxide is reduced to manganese(III) oxyhydroxide.
Water molecules provide hydrogen and oxygen atoms for the new compound.
Electrons arriving from the external circuit complete the reduction.

Safety warning -
Not all batteries can be re-charged!
If you try to do this, when you're not supposed to, you can cause the battery to explode, spilling out the dangerous acid or alkali electrolyte (the 'medium' that allows the current to flow more easily between regions inside the battery). This could even cause a fire.

Only ever recharge the correct batteries according to the manufacturers instructions.

Oxidation can be a very violent process!

Did you know?

In disposable batteries, the anode breaks down irreversibly and cannot be used again, but in rechargeable batteries, when recharging, the anode and cathode 'swap' places until the battery is charge and ready to be used again.

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Task 2

a). Open your knowledge slideshow from the
Google Classroom.

b). Add a new slide and take before/after screenshots of the battery simulation.

c). In your own words, try to describe what you think is going on both inside and outside of the battery.

Note - If you are stuck, click the 'model' icon on the left to view a literacy helper page. Also, remember your slideshow helper tools are there to make life easier for you if you need a little nudge in the right direction. If you frame your question thoughtfully, you will get a useful answer.

Steps to understanding 1 - Know that..

A battery converts chemical energy into electrical potential energy.
Batteries in an electronic circuit provide a difference in electrical potential. We call this 'voltage'.
This creates a kind of 'force' (called electromotive force, EMF) that pushes electrons around a circuit.
The electricity is converted into a different form of energy at the 'load'.

It can be turned into heat.

It can be turned into light.

It can be turned into sound.

It can be turned into motion.

Steps to understanding 2 - Know that

Current is 'tied' to voltage.
Voltage and current are directly proportional to one another.
If one goes up, so does the other (and vice versa).

Voltage is the energy potential difference (between one side of a circuit or a component and the other).
Current is the amount of flow of electrons (between one side of a circuit or a component and the other).
There can be no current if there is no voltage to 'push' it.
Battery voltages are fixed. To add more voltage to an electronic device, you need more batteries or to add them 'in-series' (we will cover this idea later).

These batteries are both the same but connected differently. The right is connected in 'series' whilst the left is connected in 'parallel'.

You can learn more about this concept here.

Task 3

a). Select different materials from the ones in the simulator below and observe how the current changes.

Pay attention to the names of the materials and their type.

b). Take a screenshot and insert it into your slideshow.

c). Define the words 'conductor' and 'insulator' a short paragraph describing the behaviour of the electron flow in different materials.

If you have done this already, make sure that you have a clear slide dedicated just to this concept. Don't muddle it up with other ideas.

Copper is a conductor
(Low resistivity)

Iron is a conductor
(Low resistivity)

Rubber is an insulator
(High resistivity)

Wood is an insulator
(High resistivity)

Steps to understanding 3 - Know that..

Resistance is a measure of the opposition to current flow.
Resistance exists because different materials have different resistivity.
High resistivity materials are called insulators & low resistivity materials are called conductors.
Current DOES NOT affect resistance. Resistance DOES affect current.

Task 4

a). Experiment with the 'Visualising Ohm's Law in Action' simulator below.

b). Expand the 'Consider these questions' panel by clicking the + button.

c). Copy and paste the questions at the bottom and attempt to answer them in your presentation. Writing in full sentences wherever possible.

Did you know?

Since the unit of electrical charge is called the 'Coulomb', Amps measures Coulombs per second. A single electron carries 1.60 x 10-19 coulombs of negative electrical charge. 1 Amp is equal to 1 Coulomb per second. This means that 6.24 × 10¹⁸ electrons moving past a point for every amp! If you can't visualise that number, it is: six quintillion, two hundred and forty quadrillion electrons per second, pass through a wire - for every amp of current. That's how small electrons are!

Whilst Amps (Amperes) measures the flow of electrical current, it is fundamentally a measure of the amount of electrical charge being passed between two points.

Now we know this, we can formulate 'Ohm's Law'

Here is a visual showing the effect of resistance on a wire. Because the wire is narrow, resistance causes a friction build-up and the wire to become very hot - hot enough to cut through an object.

Consider the animation, below...

Watch how the properties interact with each other by clicking the sliders in the simulator below. You can see that changing one value affects another - but how? What's going on?

The graphic below is a nice way of showing that resistance is inversely proportional to the cross-sectional area of the wire (imagine the circular area if you chopped a piece off).

It is also affected by the length of the wire and the 'resistivity' of the material being used.

Resistivity can be thought of as how much of an 'insulator' the material is. A high resistivity material is a poor conductor and vice versa.

Steps to understanding 4 - Know that current is 'tied' to resistance...

Resistance and current are inversely proportional to one another. If resistance goes up, current goes down.

Resistance exists because different materials have different resistivity.
We can change the resistance in a circuit by:
- - Changing the material the current flows through.
  - Changing the size of the conductor, e.g. the length or the thickness/diameter.
  - Adding a special component (part) called a resistor.

Task 5

a). Experiment with the 'PhET Circuit Lab' to create 3 circuits, side by side:

A circuit with 1 battery, 1 switch & 1 bulb
A circuit with 1 battery, 1 switch, and 3 bulbs
A circuit with 3 batteries, 1 switch and 3 bulbs

b). Screenshot your circuits, individually.

c). Describe: how do they compare?

Click here to view FULL SCREEN

Calculating 'Ohm's Law'

Consider the Ohm's Law triangle below. To use it, we 'cover up' the thing we want to find. What is left behind represents the formula we must use to calculate it.

Task 6

a). View the 'Ohm's Law Triangle' below.