Magnetism

What is the strength of the magnetic field produced by solenoids controlled by?

The strength of the magnetic field produced by a solenoid is controlled by varying the current in the wire. This makes them extremely useful in many technologies.

What is a solenoid?

A solenoid is a type of electromagnet formed by passing an electric current through a coil of wire with many loops.

What is the direction of the magnetic field lines?

Magnetic field lines are directed from the North to the South pole.

What technologies are solenoids useful in?

Solenoids are useful in generators, motors, headphones, induction cook tops, MRI machines, cyclotrons.

What is the main cause of magnetic field in the bar magnet?

In a bar magnet, electron spin is the main cause of the magnetic field.

What is the form of the magnetic field around a solenoid?

The magnetic field around a solenoid has the same form as a bar magnet; within the solenoid, the field is uniform.

What is a magnetic field?

Magnetic fields are produced by moving charges. The magnetic field is in addition to the electric field at the charge.

How does the magnetic field at points change when the current triples?

The magnetic field strength is tripled. Magnetic field strength is directly proportional to the current when the radial distance from the centre of the wire is constant.

Where is the magnetic field strength stronger?

The magnetic field is stronger where there is a greater number of magnetic field lines per unit area at a point.

What is the final KE of a particle dependent on?

The final KE of the particle is determined by the radius of the cyclotron (which limits the maximum radius and therefore speed of the particle) and the strength of the field (which affects the radius of the path for a given speed).

What happens to particles as their speed increases?

Particles spiral outwards as their speed increases because the radius of the path depends on the speed.

What is the movement of particles in a cyclotron?

The magnetic field within the dees of the cyclotron causes the particles to travel in a circular path within the dees.

Describe the movement of a particle moving at right-angles to a uniform magnetic field:

For a particle moving at right-angles to a uniform magnetic field, the force is constant in magnitude, but perpendicular to the particle’s velocity at all times. This means the magnetic force is a centripetal force and the result is uniform circular motion.

Why does the period of the particle not depend on its speed?

When the particle is travelling faster, it follows a path of larger radius, therefore travelling a greater distance each lap. The effect of the increase in speed is cancelled by the increases in radius, resulting in a constant period.

What is the direction of force on a positive particle given by?

Direction of force on a positive particle is given by the right-hand force rule. The direction is opposite for negative particles.

How does the magnetic field within coils change when current is increased?

The magnetic field strength will increase as current is directly proportional to magnetic field strength.

Why doesn’t the final KE of the particle depend on the potential difference between the dees?

If the potential difference is smaller, the particle crosses the gap many times before ‘maxing out’ it’s radius. If the potential difference is larger, the max radius is reached after fewer crossings. Either way the final KE is the same.

What is Magnetic Flux?

Magnetic Flux is the measure of the ‘amount of magnetic field’ through an area. It depends on the strength of the field and the area perpendicular to the field.

What does Faraday’s Law state?

Faraday’s Law states that for a single loop of wire, the potential difference (emf) is equal to the rate of change of magnetic flux.

State and explain the effect on the magnetic flux if the loop is rotated so it is not perpendicular to the magnetic field:

The magnetic flux decreases because the density of magnetic field lines passing through the loop is reduced.

Explain how the vibration of a string over the pickup produces an emf that is transmitted to the amplifier and speaker:

The permanent magnet of the pickup has a magnetic field surrounding it. The magnetic field of the permanent magnet changes as the string vibrates over the pickup, resulting in a corresponding change in magnetic flux. The change in magnetic flux induces an emf in the coil of the pickup.

Explain why alternating current is transmitted from power stations ti homes rather than direct current?

Alternating voltages and currents can be increased and decreased by transformers which enable efficient transmission over vast distance. Direct current fails to produce a changing magnetic flux which is required to increase or decrease voltages using a transformer.

Describe and explain how a transformer reduces the voltage of electricity entering Adelaide homes:

By ensuring the secondary coil has fewer turns than the primary coil. The voltage transmitted into the home from the secondary coil is directly proportional to the number of turns in the secondary coil.

State the advantage of increasing the voltage of electricity being transmitted along power lines to Adelaide:

A higher voltage reduces power losses in the transmission lines.

State the advantage in laminating the iron core:

Laminating the iron core minimises eddy current which reduces power loss between the primary and secondary coils.

State two similarities in the functions of step-up and step-down transformers:

• Both use alternating potential difference/alternating voltage.

• Both use electromagnetic induction to induce an alternating voltage in the secondary coil.

State three differences between step-up and step-down transformers:

• Step-up transformers have more turns in the secondary coil.

• Step-up transformers have a lower current in the secondary coil.

• Step-up transformers increase the voltage.

How are trandformers used in the power network?

Transformers are used in the power network to step the voltage up for electricity transmissions through powerlines. This is done to reduce current in the powerlines which reduces energy losses due to heat from the internal resistance of the wires. For safety, the voltage is then stepped down before being delivered to households.

What is a transformer?

A transformer is a device used to change voltage levels in an AC electrical circuit. They consist of two coils that are connected to a magnetic core.

How do metal detectors detect a gold ring below the Earth’s surface?

The metal detector produces a change in magnetic flux when a pulse of current travels through the transmitter coil. The change in magnetic flux induces eddy currents in the gold ring. The eddy currents produce a magnetic field that induces a current in the receiver coil, and the current producers an audible tone, alerting the user.

State and explain whether a magnet would move more or less rapidly through an aluminium tube with lower conductivity than a copper tube:

Less rapidly. Smaller eddy currents are induced in the aluminium due to the lower conductivity. Smaller eddy currents produce a weaker magnetic field and magnetic force resulting in the object falling with greater vertical acceleration inside the tube.

Why does a magnet fall slower through a metal tube than a non-magnet?

The magnetic flux through the tube changes as the magnet falls through the tube. The change in magnetic flux induces eddy currents in the tube that produce a magnetic field. The magnetic field produced by the eddy currents exerts a magnetic force on the magnet, which reduces its vertical acceleration.

How do generators work?

In a generator, a coil is made to rotate in a magnetic field. The rotation is caused by mechanical means and results in a continuous change of flux through the coil. The changing flux induces an electric current which reverses direction every half rotation.

What do eddy currents produce?

Eddy currents produce magnetic fields which oppose the primary magnetic field as per Lenz’s Law.

What is an eddy current?

When the magnetic flux through a 2D conducting sheet or slab changes, the induced emf causes current to flow in complex closed loops within the conductor.

What is Lenz’s Law a result of?

Lenz’s Law is a result of energy conservation as the emf induced by the changing flux is a source of electric potential energy which is possible due to the work done in moving the magnetic against the induced magnetic field of the coil.

What is Lenz’s Law?

An induced emf always gives rise to a current whos magnetic field opposes the original change in flux producing the emf.