Nour’s lesson
Magnetic interaction
Today, the whole chapter: "Magnetic interaction", Physics, Year 3, Mathematics section.
What Nour says
1. Magnets and magnetic field
Today, the whole chapter: "Magnetic interaction", Physics, Year 3, Mathematics section.
We will study how magnets, currents, and the Earth act on each other through magnetism.
We finish with magnetic levitation, a surprising result of these interactions.
Let's start with magnets and the magnetic field they create.
A magnet is an object that attracts iron and some other metals.
Every magnet has two ends, called poles: a north pole and a south pole.
We mark the north pole in red and the south pole in blue on our drawing.
A magnet creates a magnetic field all around it, even without touching anything.
This magnetic field is a region of space where a magnet or a compass feels a force.
We write the magnetic field with the symbol B, and its unit is the tesla, symbol T.
The field has a direction: it points from the north pole, around, back to the south pole outside the magnet.
We can see this shape using small iron filings sprinkled around the magnet.
The filings line up along curved lines called field lines, showing the field's direction.
Field lines are closer together near the poles, where the field is stronger.
Far from the magnet, the lines spread out, and the field becomes weak.
Now let's work through an example, like one you may see in the exam.
A straight bar magnet lies on a table, with north on the left, south on the right.
A small compass is placed near the north pole, outside the magnet.
We ask: which way does the compass needle point at that location.
The needle always aligns with the magnetic field direction at its position.
Outside the magnet, the field goes from the north pole toward the south pole.
So near the north pole, the field points away from the magnet, to the left.
The compass needle's north end turns to point in that same direction, to the left.
This is the key method: outside a magnet, field lines leave north and enter south.
Inside the magnet itself, the field lines continue from south back to north.
This makes the field lines close up into full loops, from north, around, to south, and back inside.
So the magnetic field is a vector: it has a size, a direction, and a unit, the tesla.
Near strong magnets, B can reach values like 0{,}1 T or more.
Remember: field lines point from north to south outside, and south to north inside.
This simple rule helps us find the field direction anywhere around a magnet.
Outside a magnet, which pole do magnetic field lines point toward?
2. Interaction magnet-magnet
Now let's move to interaction magnet-magnet.
That is, the force that two magnets apply on each other, without touching.
Every magnet has a north pole and a south pole, as we saw before.
When we bring two magnets close, each one feels the other's magnetic field.
Two like poles, north facing north, push each other away: this is repulsion.
Two unlike poles, north facing south, pull each other together: this is attraction.
Let's draw two magnets with their north poles facing each other.
Each magnet pushes the other one away, along the line joining them.
The two forces are equal in size, opposite in direction, one on each magnet.
This comes from Newton's third law, which you already know from mechanics.
Now, picture turning one magnet around, so north faces south instead.
The force now pulls the magnets together instead of pushing them apart.
Let's solve an exam-style problem about this interaction.
Two bar magnets lie on the same line, facing each other with south and north poles.
Magnet A has south on the right, magnet B has north on the left.
We ask: do the magnets attract or repel each other, and why.
The facing poles are south and north, so they are unlike poles.
Unlike poles always attract, so the two magnets pull toward each other.
We draw the force on magnet A pointing right, toward magnet B.
And the force on magnet B pointing left, toward magnet A, equal in size.
These two forces form an action-reaction pair, as Newton's third law tells us.
So the final answer is: the magnets attract, because unlike poles face each other.
This method always works: look at the facing poles, then decide attract or repel.
Like poles repel, unlike poles attract: this is the whole rule for magnet-magnet interaction.
Keep this rule in mind, since we will reuse it for currents too.
Do two magnets with north facing north attract or repel?
3. Earth's magnetic field and the teslameter
Now let's study the Earth's magnetic field and the teslameter.
The Earth itself acts like a giant magnet, creating a field around the whole planet.
We call this the Earth's magnetic field, that is, the magnetic field produced by the Earth everywhere on its surface.
This field is why a compass needle always points north.
Near the surface, the Earth's field has a horizontal part, called the horizontal component.
This horizontal component always points toward the geographic north direction.
Its value on Earth's surface is small, around 2 times ten to the power minus 5 tesla.
To measure a magnetic field precisely, we use a device called a teslameter.
That is, an instrument that gives a direct reading of a magnetic field's size, in teslas.
It has a small sensor called a probe, placed at the point we want to measure.
The teslameter display directly shows the field's value in tesla, symbol T.
We often combine it with coils that create a known, controllable magnetic field.
Let's now work through a typical exam problem using the teslameter.
A student places a teslameter's probe at a point, far from any magnet.
The teslameter reads a value of 2 times ten to the power minus 5 tesla.
This value matches the Earth's horizontal field, since no magnet is nearby.
Next, the student brings a bar magnet close to the same point.
The teslameter now reads a new value of 8 times ten to the power minus 5 tesla.
We ask: what is the value of the magnetic field created by the magnet alone, at that point.
At that point, both fields add together, since they point in a similar direction here.
So the magnet's field equals the total reading minus the Earth's field.
We substitute: 8 times ten to the power minus 5, minus 2 times ten to the power minus 5.
This gives 6 times ten to the power minus 5 tesla for the magnet's field.
We frame this result, since it is the final answer to the problem.
This method is common in bac problems: measure before and after, then subtract.
Always check if fields add or subtract, depending on their relative directions.
In this case, both pointed the same way, so we simply added them together.
If they pointed opposite ways, we would need to subtract them instead.
The teslameter makes this kind of measurement fast, direct, and precise.
Remember: the Earth's horizontal field is small, about 2 times ten to the power minus 5 tesla.
A nearby magnet's field is usually much larger than the Earth's own field.
This is why compasses can be disturbed by magnets or electric currents nearby.
The teslameter lets us measure this disturbance and separate the two contributions.
To summarize: the Earth acts as a magnet, and the teslameter measures field values directly.
We now know how to measure fields and combine them in a simple calculation.
What SI unit does a teslameter display its reading in?
4. Interaction magnet-current
Now let's study interaction magnet-current.
That is, the force a magnet and an electric current apply on each other.
A wire carrying an electric current also creates its own magnetic field around it.
We already know electric current from an earlier chapter, as moving electric charge.
Around the wire, the current creates circular magnetic field lines.
If we place a magnet near this wire, it feels the wire's magnetic field.
At the same time, the wire feels the magnetic field created by the magnet.
So each one exerts a force on the other, just like two magnets did before.
A free compass needle placed near the wire will turn and line up with the field.
This shows that a current-carrying wire really behaves like a small magnet nearby.
If we reverse the current's direction, the force on the magnet also reverses.
This means the direction of current controls the direction of the magnetic interaction.
Let's now solve an exam-style exercise about this interaction.
A straight wire is placed horizontally, carrying a current of 5 amperes, symbol A.
A small magnetic compass is placed just above the wire, at rest initially.
Before the current flows, the compass points north, along the Earth's field.
We ask: what happens to the compass needle once the current starts flowing.
Once current flows, the wire creates its own magnetic field at the compass location.
This new field adds to the Earth's field already present at that point.
The compass needle turns to align with this new total magnetic field.
So the final answer: the needle deflects away from north, toward the wire's field direction.
We frame this conclusion, since it answers the exam question directly.
This effect is called the Oersted effect, first observed with a compass near a wire.
It was the first proof that electricity and magnetism are deeply connected.
The bigger the current, the bigger the deflection of the compass needle.
This matches our earlier rule: current creates a magnetic field, like a magnet does.
So a wire with current and a magnet interact exactly like two magnets do.
The force follows the same idea: fields interact, and interacting fields create forces.
In exams, always state that current creates a magnetic field around the wire.
Then compare it with the Earth's field or a nearby magnet's field.
To summarize: a current-carrying wire acts like a magnet, and reacts to magnets too.
This interaction magnet-current is the base for many devices, like electric motors.
Next, we will see what happens when two current-carrying wires interact together.
What happens to a compass needle near a wire once current starts flowing?
5. Interaction current-current
Now let's study interaction current-current.
That is, the force two current-carrying wires apply on each other, through their magnetic fields.
Each wire creates its own magnetic field, as we saw in the last section.
When we place two wires close together, each one feels the other's field.
So each wire exerts a magnetic force on the other wire, without any contact.
If the two currents flow in the same direction, the wires attract each other.
If the currents flow in opposite directions, the wires push each other away.
Let's draw both currents flowing upward, in the same direction.
Here the two forces pull the wires toward each other: this is attraction.
Now imagine reversing current 2, so it flows downward instead.
The forces would then push the wires apart instead of pulling them together.
Let's now solve an exam-style problem on this interaction.
Two long straight wires are parallel, placed a few centimeters apart in air.
Wire A carries a current of 3 amperes, symbol A, flowing upward.
Wire B carries a current of 3 amperes too, also flowing upward.
We ask: do the wires attract or repel, and which force acts on wire A.
Both currents flow in the same direction, upward, as shown in the figure.
Same direction currents always attract, so the two wires pull toward each other.
We draw the force on wire A pointing right, toward wire B.
By Newton's third law, wire B feels an equal force pointing left, toward A.
So the final answer: the wires attract, since their currents flow the same way.
We frame this result, since it directly answers the exam question.
This rule is simple: same direction attracts, opposite direction repels, for parallel wires.
It mirrors the rule we saw for interacting magnets earlier in the lesson.
In both cases, it's the magnetic fields of each source that create the force.
A current loop can even act like a small magnet, with a north and south side.
This connects all three interactions: magnet-magnet, magnet-current, and current-current, under one idea.
That shared idea is: moving charges and magnets both create magnetic fields.
And magnetic fields always produce a force on other magnets or currents nearby.
To summarize: parallel wires attract with same-direction currents, repel with opposite ones.
This rule appears often in bac exams, so remember it well.
Next, we will use these ideas to explain magnetic levitation.
Do two parallel wires with currents in the same direction attract or repel?
6. Magnetic levitation
Now let's finish with magnetic levitation.
That is, making an object float in the air, held up only by magnetic forces.
We just saw that like poles repel, and currents can repel each other too.
If a repelling force is strong enough, it can balance an object's weight completely.
Let's place a magnet on a table, with its north pole facing up.
Above it, we place a second magnet, also with north facing down.
The two north poles face each other, so the magnets repel strongly.
This upward push can cancel the floating magnet's weight, pulling it down.
When these two forces balance exactly, the top magnet floats in the air.
This floating state is unstable without extra support, so real devices add a guide.
Let's check the balance condition with a short exam-style calculation.
A small magnet floats above a fixed magnet, in equilibrium, with no other support.
Its mass is 5 grams, symbol g, so its weight pulls it down.
The weight equals mass times the gravity strength, giving its value in newtons, symbol N.
We convert the mass to kilograms first: 5 grams equals 0{,}005 kilograms, symbol kg.
We ask: what magnetic force must the lower magnet apply for the system to float.
At equilibrium, the magnetic repulsion force must equal the weight exactly.
We substitute: 0{,}005 kilograms times 10 newtons per kilogram.
This gives a force of 0{,}05 newtons, symbol N, needed to hold the magnet up.
We frame this result, since it answers the levitation condition directly.
This is the whole idea of magnetic levitation: repulsion force balances weight.
Engineers use this principle in high-speed trains, floating above their tracks.
It is also used in some frictionless bearings and demonstration toys.
To summarize the chapter: magnets, currents, and the Earth all create magnetic fields.
These fields interact, producing attraction, repulsion, and even levitation in special setups.
In magnetic levitation, which two forces must balance exactly?
That completes our chapter on magnetic interaction, from magnets to levitation.
We saw how magnets, currents, and the Earth all create and share magnetic fields.
Practice the two exercises to master the attraction, repulsion, and measurement methods.