Topic 2 2/2

Question 1

Define and give units for:
Magnetic field.
Visual appearance of magnetic fields.

Answer 1

The region surrounding a magnetic material or a charge in motion. Such as a current carrying-conductor.
This is a vector quantity, appearing as concentric circles around the conductor. The number of field lines indicate the strength of the field at a point.

The unit is the Tesla (T) with the symbol B

Question 2

Describe the right hand rule:

Answer 2

The thumb points in the direction of conventional current ant the curl of the fingers indicate the direction of the magnetic field produced.

Question 3

Describe:
Visual representation of current with “x”
Visual representation of current with “o”

Answer 3

The x refers to a current of magnetic field traveling into the page, and the dot refers to the current or magnetic field travelling out of the page.

Question 4

Describe:
Solenoids
Factors affecting strength of magnetic field::

Answer 4

Many coils of wire. Acts like a bar magnet, having a north and south pole.
This is also referred to an electromagnet. The magnetic field inside the loops are stronger than outside.
Factors that may change the magnetic field strength are:
The number of coils
The magnitude of current traveling through
Presence of an iron or conductive core

Question 5

Express:

magnetic field strength, in relation to gravitational and electric field strength-

Answer 5

Magnitude is directly proportional to the current.
As gravitational field strength is proportional to the mass
Electric field is directly proportional to charge.

Magnetic field strength is inversely proportional to the radial distance from the conductor.
Gravitational field strength is inversely proportional to the distance from a mass.
Electric field strength is inversely proportional to the distance from a charge.
B=F/il

Question 6

describe and illustrate:

Motion of magnets in magnetic fields-

Answer 6

Anything that produces a magnetic field will experience a force within a magnetic field, like a compass which is essentially a small magnet.

Question 7

Express-
Force on a straight current carrying conductor is Proportional to:
And On a charged particle

Answer 7

Current flowing through the conductor
Length of conductor
Magnetic field strength
Sin/Cos of angle to magnetic field

On a charged particle, it is dependant on the magnitude and direction of the velocity.

Question 8

Describe-

Direction of the force on a current-carrying conductor and individual charges:

Answer 8

Using the right palm rule The thumb points in the direction of conventional current and the fingers point in the direction of magnetic field.

The force acts perpendicularly away from the palm of the hand. A positive charge will travel like this, a negative charge will travel in the opposite direction.
The force on a current element that is parallel or antiparallel to a magnetic field is zero.

Question 9

Describe-

Motion of charged particle in uniform magnetic field:

Answer 9

Since a charged particle, moving at right angles to a uniform magnetic field, experiences a force of constant magnitude at right angles to the velocity. The charge changes direction without a change in speed. The magnetic force provides the centripetal acceleration required for uniform circular motion.

Question 10

Derive-

Radius of the circular path of a charged particle moving at right angles to a uniform magnetic field of magnitude B.

Answer 10

F(centripetal)=F(magnetic)
(MV^2)/r=qvB
r=MV/qB

Question 11

Describe-

Cyclotron components:

Answer 11

An electromagnet is positioned above and below two D-shaped hollow conductors, Producing a uniform magnetic field inside the dees.

Question 12

Derive and explain meaning of-

Period of circular motion of an ion in a cyclotron:

Answer 12

T=(2piR)/v
But r=(mv)/qB
T=(2pim)/qB
The period does not depend on speed of the ions. Meaning the period of all ions in cyclotron are the same, meaning they reach the gap at the same time interval, regardless of the speed, thus the potential difference can be reversed on a regular basis.

Question 13

Derive and explain meaning of-

Kinetic energy of ions emerging from cyclotron:

Answer 13

Ek=(Mv^2)1/2 
But r=mv/qB, so v=(rqB)/m
K=(M(rqB/m)^2)½
K=(rqB)^2/(2m)
The kinetic energy is independent of the potential difference across the dees. A larger potential difference does not mean more kinetic energy to the ions, it means the ions will cross the electric field fewer times before they emerge from the cyclotron.

Question 14

Describe and express with units:

magnetic flux-

Answer 14

Is a measure of the number magnetic field lines passing through an area. Being the product of magnetic field strength B and the area A.
Flux=BAcos(theta)
Where theta is the angles between the area and a line normal to the magnetic field.
The units for magnetic flux is the Weber (Wb)

Question 15

Express

Units of area conversion, mm, cm, m, km

Answer 15

cm^2 -> x10^-4 -> m^2
mm^2-> x10^ -6 -> m^2
mm^2 -> x10^-2 -> cm^2
km^2 -> x10^-6 -> m^2

Question 16

Describe

Electromagnetic induction:

Answer 16

A changing magnetic flux induces a potential difference in a conductor.
This induced emf (electromotive force) causes a current to flow.
Electrons in the conductor are forced to move given by F=qvB sin(theta). This causes one end of the conductor to become more negative or positive.

Question 17

Describe and Express

Faraday’s law:

Answer 17

The induced emf is equal to the rate of change of the magnetic flux.
e=Change in flux / change in time
The SI unit of emf is the volt (V)
This may be continued to a situation where there are several loops, giving the equation:
emf=(n) Change in flux / change in time.

Question 18

Express:

Emf for a changing area or magnetic field:

Answer 18

For a changing area
emf= (n) (B) (A2-A1)/ change in time
For a changing magnitude of magnetic field strength.
emf=(n) (A) (B2-B1)/ Change in time

Question 19

Describe:
Lenz’s law
application of lenz’s law to solenoid

Answer 19

Lenz’s Law states that the induced emf creates a current in a direction that opposes the change in magnetic flux.

This may occur through the motion of a magnet and a solenoid, causing the poles to reverse depending on the motion of a magnet, to repel or attract the magnet in motion.

Question 20

Describe:

Lenz’s law in terms of the conservation of energy

Answer 20

If the induced poles of the solenoid were to be opposite, then, the magnetic force of attraction between the two unlike poles would cause the magnet to accelerate towards the solenoid. Inducing a greater current in the solenoid, the magnet would further accelerate. This would continue to speed up, increasing the kinetic energy. The energy created would violate the law of conservation of energy. The induced current must therefore flow in a direction to oppose the change in magnetic flux.

Question 21

Describe:

Eddy current:

Answer 21

Is the current induced in little swirls on a large solid conductor due to a changing magnetic field. These occur in accordance with Lenz’s law so that they have a direction that will flow to oppose the change in magnetic flux.

Question 22

Describe:
Generators:

Answer 22

A generator uses a fixed magnet to generate emfs in rotating conducting loops for electricity production. A generator therefore converts rotational mechanical energy into electrical energy.

Question 23

Describe:
Lamentation:

Answer 23

To avoid large eddy currents being produced in a solid conductor, slits can be cut into the metal.

Question 24

Describe and explain:

Transformers:

Answer 24

Allow generated voltages to be either increased or decreased before they are used.
This is done through primary and secondary coils, each coil with it’s own number of loops. Given the relationship:
VpNs=VsNp
These are wrapped around an iron core to increase the strength of the magnetic field.
Since the same magnetic flux is passing through both coils, the emf for both are the same. Given that the number of loops vary, then the potential difference will change, changing the volts.
Though energy is conserved, so it is transferred from current.

Question 25

Describe:

Step-up and step-down transformers.

Answer 25

A step up transformer.
When the number of coils in the secondary coil is greater than the primary. Resulting in a greater potential difference in the secondary coil, increasing voltage. The output current is lower than the input current
A step down transformer.
When the number of coils in the secondary coil is lesser than the primary. Resulting in a lesser potential difference in the secondary coil, decreasing voltage. The output current is greater than the input current.

Question 26

Explain
Why cyclotrons are positioned near the place where they need to be used?
What if they were not in the same place?

Answer 26

Used to create medical isotopes or in the industry.
Isotopes have short half lives, thus they need to be kept in the hospital where they will be used, so that the isotopes do not decay before used.

If there was not a readily available cyclotron, then the isotope would have to be made elsewhere and transported. The time would require more of the isotope to be made to compensate for the radioactive decay.
Having cyclotron on site reduces costs and increases the number of scans available to patients in time.
Some isotopes have short half lives, making them impractical to ship

Question 27

Describe:

Components of generators:

Answer 27

Magnets: Produce a uniform magnetic field
Iron core: A soft iron that serves to maximise the magnetic field. It is usually laminated to reduce effect of eddy currents which are bad.
Rotator coils: Loops of wire around the iron core. A current is induced in the rotor coils as they rotate in the magnetic field.
Split ring communicator: 2 metal half rings connect either end of coils to brushes.
Brushes: 2 blocks on either side of communicator. Provide electrical contact to an external circuit.
Axle: A rod passing through iron core and coils to provide an axis or rotation for the rotator coils.
AC: Induced current is not reversed every half rotation of the coils.
DC: Induced current is reversed every half rotation of the coils.

Question 28

Describe:

Why the high voltage is used in transmission lines from generator to consumer.

Answer 28

This is to keep the current low, as some energy is lost to the surroundings as heat due to resistance. To minimise the energy loss, the current is kept low by using a high transmission voltage.

Question 29

you’re given a she question, what you do?

Answer 29

Identify the key points
Explain the points relation to physics
Impact on society

Question 30

Describe:
function of cyclotron

Answer 30

Charged particles are accelerated by a uniform electric field in the. The electric field does work on the ions and transfers kinetic energy to the ions (W=qV=Ek=(mV^2)/2).
The uniform magnetic field causes the charged particles to move in a circular path so that they return to the electric field by providing centripetal acceleration.
The current passing through each dee, alternates, reversing the electric field before the particle returns, accelerating it further.