Electromagnetism MCQ 2

Magnetism

• Magnetism is a phenomenon where certain materials exert attractive or repulsive forces on other materials.
• Every magnet has two poles: the north-seeking pole (north pole) and the south-seeking pole (south pole).
• A magnetic field is a region around a magnet where the magnet has an effect.

Magnetic Field

• A magnetic field line indicates the direction a 'magnetic north' would travel in a magnetic field if free to do so.
• Magnetic field lines can be plotted by placing a bar magnet on a blank sheet of paper and using a plotting compass.
• The Earth has a magnetic field around it, which behaves as though there was a giant bar magnet going through the centre of the Earth.

Magnetic Field of a Current-Carrying Conductor

• Every current-carrying conductor has a magnetic field around it caused by the current.
• The right-hand rule states that if the right hand clasps a conductor with the thumb pointing in the direction of the current, then the fingers give the direction of the magnetic field around the conductor.

Types of Magnetic Fields

• For a long straight wire, the field is a series of circles around the wire.
• For a solenoid, the field looks the same as a bar magnet.
• For a loop of wire, the field lines form circles around the loop.

Electromagnets

• An electromagnet is a magnet constructed from a soft iron core with a coil of wire wound around it.
• A magnetic field is set up when an electric current is passed through the wire and disappears when the current stops.
• Electromagnets are used in scrapyards, particle accelerators, and loudspeakers.

Magnetic Flux Density

• Magnetic flux density (B) is a measure of the strength of the magnetic field.
• It is a vector quantity with an SI unit of tesla (T).
• Its direction is along the field lines from north to south.

Force on a Current-Carrying Conductor

• The force on a current-carrying conductor in a magnetic field is given by F = IlB, where F is the force, I is the current, l is the length, and B is the magnetic field.
• The direction of the force can be determined using Fleming's left-hand rule.

Electromagnetic Induction

• Electromagnetic induction is where an emf is induced by a change in the magnetic flux linking a circuit.
• Magnetic flux (Φ) is the total amount of magnetic field in a given region; it is the product of magnetic flux density (B) and area (A).
• Faraday's law states that the magnitude of the induced emf is proportional to the rate of change of flux linking a circuit.

Faraday's Law and Lenz's Law

• Faraday's law states that induced emf = -N(dΦ/dt), where N is the number of turns in the coil.
• Lenz's law states that the direction of the induced emf is always such as to oppose the change causing it.

Applications of Electromagnetic Induction

• Generators convert mechanical energy into electrical energy by rotating a coil of wire within a magnetic field, causing flux to constantly change, thus inducing an EMF.
• Transformers are used to change the voltage of a.c. power supplies.
• Mutual induction is where a changing magnetic field in one coil induces an emf in another coil.