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Magnetic Fields & Electromagnetism
Field Lines, Force on Charges, and Faraday's Law
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Magnetic fields and electromagnetism explain how moving electric charges create forces and how electricity and magnetism are linked. These ideas power many technologies, including motors, generators, speakers, MRI machines, and transformers. A magnetic field fills the space around a magnet or a current carrying wire and can push on other moving charges or magnetic materials. Understanding these fields helps students connect invisible forces to real devices.
An electromagnet is made by sending current through a coil of wire, often wrapped around an iron core to strengthen the field. The magnetic field around each loop adds together, producing a stronger overall field with clear north and south poles. The direction of the field depends on the direction of current, which can be predicted with the right hand rule. Electromagnetic induction shows the reverse connection: changing magnetic flux can create an electric voltage in a wire.
Key Facts
- A current carrying wire produces circular magnetic field lines around the wire.
- For a long straight wire, B = mu0 I / (2 pi r).
- For a solenoid, B = mu0 n I, and an iron core increases the field further.
- Magnetic force on a moving charge is F = qvB sin(theta).
- Magnetic force on a wire is F = BIL sin(theta).
- Induced voltage follows Faraday's law: emf = -dPhiB/dt.
Vocabulary
- Magnetic field
- A magnetic field is the region around a magnet or moving charge where magnetic forces can act.
- Electromagnet
- An electromagnet is a magnet produced by electric current, usually using a coil of wire and often an iron core.
- Solenoid
- A solenoid is a long coil of wire that creates a nearly uniform magnetic field inside when current flows through it.
- Magnetic flux
- Magnetic flux measures how much magnetic field passes through a given area.
- Electromagnetic induction
- Electromagnetic induction is the production of voltage in a conductor caused by a changing magnetic field or changing magnetic flux.
Common Mistakes to Avoid
- Confusing magnetic field direction with force direction, because the field points from north to south outside a magnet but the force on a charge also depends on the charge sign and velocity direction.
- Using the right hand rule incorrectly, because for current and field direction the thumb and fingers represent different quantities and reversing them gives the wrong pole or force direction.
- Assuming a stationary charge feels magnetic force, because magnetic force requires motion relative to the magnetic field so v = 0 gives F = 0.
- Thinking stronger current always means induction, because induction depends on changing magnetic flux rather than just having a magnetic field present.
Practice Questions
- 1 A straight wire carries a current of 6.0 A. Find the magnetic field 0.020 m from the wire. Use mu0 = 4 pi x 10^-7 T m/A.
- 2 A solenoid has 800 turns per meter and carries a current of 2.5 A. Estimate the magnetic field inside it using B = mu0 n I.
- 3 A bar magnet is pushed toward a loop of wire, then held still inside the loop. Explain when an induced current appears and why it changes.