Chapter 6: Fields and forces

Question 1

Newton´s universal law of gravitation

Answer 1

• Every point masses attract each other point mass. The force is proportional to the product of two masses and inversely proportion to the square of the distance of 2 point F
• Any 2 point objects exert a force of attraction on each other. This force is proportional to the product of the masses of the 2 objects and inversely proportional to the square of the distance between the centers of the same objects

Question 2

Gravitational field strength

Answer 2

gravitational field strength or acceleration due to gravity at a point is the force acting on a 1kg mass placed at that point

Question 3

State abd apply the law of censervation of charge

Answer 3

The law states that the net electric charge in an isolated system remains constant

Question 4

Coulomb´s law

Answer 4

The magnitude of the electrostatic force between 2 point electric charges is directly proportional to the product of the magnitudes of each charge and inversely proportional to the square of the distance between the charges

Question 5

Electric field strength

Answer 5

The force per unit charge in a region of space or the electric field stength is equal to the force experienced by a small test charge placed at a point divided by the charge itself

Question 6

Magnitude and direction of a magnetic field

Answer 6

a magnetic field is defined as the space around a magnet in which another will feel force, and is measured in Tesla (T). Magnetic field lines have no beginning and no end , but generally are drawn from the north pole of a magnet to its south pole

Question 7

Describe the induced of an emf by relative motion between a conductor and a magnetic field

Answer 7

the relative motion of a conductor in a magnetic field causes current to flow. This is because the motion changes the flux. These changes increases the current flowing throught the conductor and create an emf. Thus the generated potential difference increases as the rate of change of flux in the circuit increases

Question 8

Magnetic flux and magnetic flux linkage

Answer 8

• Magnetic flux is the magnetic field lines that pass through a taken area
• Magnetic flux linkage occurs in magnetic coils passing through magnetic fields.
• This allows us to state that the magnitude od an induced emf is proportional to the rate of change of flux linkage

Question 9

Faraday´s law

Answer 9

any change in the magnetic environment of a coil of wire will cause a voltage (emf) to be induced in the coild

Question 10

Lentz´s law

Answer 10

the polarity of the induced emf results from a change in magnetic flux and induces a current whose direction is such that the induced magnetic field opposes the original flux change

Question 11

If the coild is rotating with a constant speed, the emf generator is sinusiudal and effected by 4 factors:

Answer 11

• the number of turns
• the angular frequency of rotation
• the cross-sectional area of the coil
• the magnetic field strength

Question 12

Discuss what is meant by the root measn squared (rms) value of an alternating current or voltage

Answer 12

-the value of the direct current (or vltage) that dissipates power in a resistor at the same rate. The rms value is also knwon as the reating

Question 13

Describe the opperation in an ideal transformer

Answer 13

• the primary winding is connected to a 60Hz ac voltage source. The magnetic field (flux) builds up (expands) and collapses (contracts) about the primary winding.
• the expanding and contracting magnetic field around the primary winding cuts the secondary winding and induces an alternating voltage into the winding.This voltage causes alternating current to flow through the load.
• The voltage may be stepped up or down depending on the design of the primary and secondary windings

Question 14

Outline the reason for power losses in transmission lines and real transformers

Answer 14

• In power lines, energy is dissipated when they warm up, since they have a resistance. Resistance can decrease through having a thicker wire.
• Flux losses are caused by magnetic leakage. A ransformer is only 100% efficient if all of the magnetic flux that is produced by the primary links with the secondary

Question 15

Explain the use in hight-voltage step-up and step-down transformers in the transmission of electrical power

Answer 15

Step-up transformers can be used to lower the current without losinh power so as to minimize dissipation. The hight voltage created however is not safe for use at home; therefore the voltage is at the ned stepped down for safe consumer use

Question 16

Suggest how extra-low-frequency electromagnetic fields, such as those created by electrical appliances and power lines, induce currents within a human body

Answer 16

Photons are send out from electromagnetic fields, these are too low to ionize anything but can induce currents in the human body

Question 17

Discuss some of the possible risks involved in living and working near high-voltage power supplies

Answer 17

Current experiments suggests that low-frequencu fields do not harm genetic material in adults but there is some evidence that there could be a link to infant cancer and leukemia rates due to the low frequency fields. These risks are likely to be dependent on current (density), frequency and length of exposure