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Every magnet has two poles — a north pole and a south pole. These aren't just labels; they describe how the magnet interacts with other magnets and with magnetic materials.
Here's the key rule: opposite poles attract, like poles repel. North attracts south, but north pushes north away. This is the same idea as positive and negative charges in electricity.
A magnetic field is the invisible area of force around a magnet. You can visualize it using iron filings — they line up along the "field lines" that curve from the north pole to the south pole. The closer the field lines are together, the stronger the field.
"Opposites attract" — this works for magnets AND for charges. If you remember one, you remember both. Think of it like relationships: opposites attract, and two people who are too similar butt heads.
Field lines ALWAYS go from North to South outside the magnet. They never cross each other. The density of the lines tells you the field strength — bunched up = strong, spread out = weak.
Here's the big idea: any time current flows through a wire, it creates a magnetic field around that wire. That's electromagnetism in one sentence.
If you wrap that wire into a coil (called a solenoid), you concentrate the field and make it much stronger. Add an iron core inside the coil and you've got an electromagnet — a magnet you can turn on and off with a switch.
The strength of the electromagnet depends on three things: the amount of current, the number of turns in the coil, and the core material. More current = stronger. More turns = stronger. Iron core = way stronger than air.
Electromagnets are everywhere: doorbells, relays, contactors, motors, transformers. As an electrician you'll work with these daily. Understanding how they work helps you troubleshoot when they don't.
Wrap your right hand around the wire with your fingers pointing in the direction of current flow. Your thumb points to the North pole of the magnetic field. This works every time and it's a classic exam question.
Lenz's Law trips up a lot of students, but here's the simple version:
When a magnetic field changes near a conductor, it induces a current. That induced current creates its OWN magnetic field that OPPOSES the change that caused it.
Think of it like nature's pushback. If you shove a magnet toward a coil, the coil says "no thanks" and creates a field that pushes back against the magnet. If you pull the magnet away, the coil says "wait, come back" and creates a field that tries to hold onto it.
The key word is opposes. Whatever change is happening to the magnetic field, the induced current will always fight against that change.
Think of Lenz's Law as "nature is lazy and hates change." Whenever something in the magnetic field changes, nature generates a current that tries to keep things the way they were. Magnet coming closer? Nature pushes it away. Magnet leaving? Nature pulls it back. It always OPPOSES the change.
The exam loves asking about the DIRECTION of induced current. Always ask yourself: "What is changing?" then "What would oppose that change?" The answer is always the opposite direction.
Lenz's Law is the reason transformers work the way they do. It's also why motors have "back-EMF" — the motor generates a voltage that opposes the supply voltage as it spins. This is critical for understanding motor circuits.
Faraday's Law puts numbers to what Lenz's Law describes in words. It tells you exactly how much voltage (EMF) gets induced when a magnetic field changes.
The formula says: the induced voltage equals the number of turns in the coil multiplied by how fast the magnetic field is changing.
Where:
EMF = induced voltage (volts)
N = number of turns in the coil
ΔΦ = change in magnetic flux (webers)
Δt = time it takes for the change (seconds)
Negative sign = Lenz's Law (the opposition)
So if you want more voltage: use more coil turns, use a stronger magnet, or move the magnet faster. All three increase the induced EMF.
"NFL" — N (turns) × Flux change / time (L... okay it's a stretch, but "Number of turns × Flux over time" is the formula). The negative sign is just Lenz's Law reminding you it opposes.