Unit 12: Magnetism and Electromagnetic Induction
How a moving charge makes a magnetic field, and how a changing magnetic field pushes back and makes a current of its own.
6 topics · 18 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Magnets and Magnetic Fields
- How it worksHow do magnetic field lines loop continuously from a magnet's north pole to its south pole?
- Common mistakeWhy doesn't every metal object get attracted to a magnet?
- Going furtherWhat happens to the magnetic field pattern when two magnets are placed north pole to north pole?
Your turn1 of 2
Cutting a bar magnet exactly in half down the middle produces two smaller but complete magnets, each with its own north and south pole, rather than one piece holding an isolated north pole and the other an isolated south pole. Why does cutting a bar magnet in half give you two smaller magnets instead of an isolated north and south pole?Magnetic Force on Moving Charges and Currents
- How it worksHow does the magnetic force on a moving charge depend on both its velocity and the field direction, always at a right angle to both?
- Worked exampleFind the magnetic force on a wire carrying 5 amps through a 0.2 meter length inside a 0.3 tesla field.
- Common mistakeWhy doesn't a magnetic field exert any force on a charge that isn't moving?
Your turn1 of 2
Find the magnetic force on a wire carrying 21.04 amps through a 3.26 meter length inside a 2.8 tesla magnetic field, with the wire perpendicular to the field.Electromagnetism
- How it worksHow does a moving electric current generate its own magnetic field circling around the wire?
- Common mistakeWhy isn't magnetism a completely separate phenomenon from electricity?
- Going furtherWhat happens to the strength of a wire's magnetic field when the current through it is doubled?
Your turn1 of 2
Placing an ordinary compass near a wire that's currently carrying electric current makes the compass needle swing and settle pointing in a new direction, exactly the way it would respond to a nearby magnet, even though the wire is just a plain piece of copper. Why does a compass needle swing the moment it's placed near a current-carrying wire?Electromagnetic Induction
- How it worksHow does a changing magnetic field through a loop of wire push electrons into motion as an induced current?
- Common mistakeWhy doesn't a magnet sitting perfectly still inside a coil induce any current at all?
- Going furtherWhat happens to the induced current when the magnet is pushed into the coil twice as fast?
Your turn1 of 2
Pushing a bar magnet into a coil of wire connected to a light bulb makes the bulb flash on briefly, generating real electricity, even though there's no battery anywhere in the entire setup. Why does shoving a magnet into a coil of wire generate electricity with no battery anywhere in sight?Generators and Motors
- How it worksHow does spinning a coil of wire inside a magnetic field convert mechanical motion into electric current?
- Common mistakeWhy doesn't a generator create energy out of nothing rather than converting it from another form?
- Going furtherWhat happens to the electricity a generator produces when its coil is spun twice as fast?
Your turn1 of 2
The exact same basic device, a coil of wire spinning inside a magnetic field, can be used either as a generator that produces electricity from mechanical spinning, or as a motor that produces mechanical spinning from supplied electricity, just by reversing which quantity is the input. Why can the exact same machine work as either a generator or a motor depending on which way you run it?Earth's Magnetic Field
- How it worksHow does churning molten iron deep in Earth's core generate a magnetic field that wraps the whole planet?
- Common mistakeWhy isn't Earth's geographic North Pole the exact same location as its magnetic north pole?
- Going furtherWhat happens to compass needles and auroras when a burst of charged particles from the Sun reaches Earth's magnetic field?
Your turn1 of 2
A hiker anywhere on Earth, from a dense forest to an open desert to a mountain summit, can pull out an ordinary compass and watch its needle consistently swing to point roughly toward the north. Why does a compass needle always point roughly toward the north no matter where you stand on Earth?