Workbook 4

Question 1

What are the two different ways of charging?

Answer 1

1. Charging by friction.
   Perspex becomes positive.
   Polythene becomes negative.
2. Charging by induction: To charge a metal sphere negatively, a positive test charge is brought close to the metal sphere which divides the charged inside. An Earth connection is added, which gives free electrons from Earth the opportunity to come up and closer towards the positive test charge. This gives the sphere an overall negative charge and if the Earth connection is removed before the test charge, the sphere will remain positively charged. If a negative test charge is brought close to a sphere, the charges will divide inside. If Earthed, the electrons will move down towards Earth, away from the negative test charge. If the Earth connection is removed before the negative test charge, the sphere will have become charged positively.

Question 2

State Coulomb’s Law of Electrostatics

Answer 2

Coulomb’s Law of Electrostatics states that the force between any two charges is directly proportional to the product of the two charges and inversely proportional to the square of the distance between them.

Question 3

Define the Permittivity of a Substance

Answer 3

The Permittivity of a substance is the ability to stop the formation of an electric field within it.

Question 4

Define electric field

Answer 4

An electric field is a region around a charge where a force is felt be another charge.

Question 5

Define line of force

Answer 5

A line of force is defined as being the path taken by a unit positive charge if free to move in an electric field.

Question 6

Define electric field strength

Answer 6

Electric field strength is the force felt by a unit charge when in an electric field.

Question 7

Distinguish between the electric field strength in a radial field and a uniform field.

Answer 7

In a radial field, the force varies with distance so this equation only gives the strength at particular points. In a uniform field, the force is constant all throughout so the intensity is the same anywhere in the field.

Question 8

Define electric potential

Answer 8

Electric potential is defined as the electric potential energy per unit charge.

Question 9

What is a capacitor?

Answer 9

A capacitor is a device used to store charge. It is made of two conducting plates separated with an insulator in between known as the dielectric.

Question 10

Define capacitance

Answer 10

Capacitance is defined as the charge that a capacitor can store per unit potential difference.

Question 11

Describe dielectrics

Answer 11

Dielectric materials in capacitors are essentially insulators, therefore, no current will flow through the material when a voltage is applied across the plates. Molecules in the dielectrics are said to be polarised meaning each molecule has a slightly positive side whilst the opposite side is a bit more negatively charged.

Question 12

Define dielectric strength

Answer 12

Dielectric strength is about the maximum amount of voltage that a dielectric can allow across it without breaking down and allowing current to flow through it.

Question 13

Describe the electrolytic capacitor

Answer 13

This capacitor uses an electrolyte (a liquid or gel containing a high concentration of ions) to achieve a higher capacitance than other capacitors, They are made of two pieces of aluminium foil separated by a paper spaced soaked in electrolyte. One of the two foils is covered with an oxide layer and that foil acts as the positive plate (anode) while the other acts as the negative plate (cathode). The anode, electrolyte soaked paper and cathode are stacked, rolled and placed into a cylindrical enclosure and connected to the circuit using pins.

Question 14

What happens when charging a capacitor?

Answer 14

Electrons leave the negative terminal of the battery towards one plate of the capacitor as there is a potential difference between the negative terminal and the capacitor’s plate. As one plate becomes more negative, it induces a positive charge on the opposite plate as it repels electrons from it. The volage across the capacitor increases as more charge is stored on the capacitor’s plates. The voltage will keep on increasing until it reaches the same potential as that of the battery.

Question 15

What happens to a capacitor during discharging?

Answer 15

Upon, changing the switch to let current flow through the resistor, electrons rush away from the negative plate towards the positive causing a current flow. Current will keep flowing until the capacitor reaches zero.

Question 16

Define the time constant of a capacitor.

Answer 16

The time constant of a capacitor is the time take to charge a capacitor to 63 percent of the total charge or the time take to discharge a capacitor to 37 percent of the full charge.

Question 17

Define electrical current

Answer 17

Current refers to the rate of flow of charge.

Question 18

What occurs when a potential difference is applied across a conductor?

Answer 18

When a potential difference is applied across a conductor, the free electrons will begin to flow in an ordered manner depending on the polarity (towards the positive potential) with an average velocity known as the drift velocity.

Question 19

Describe the simple band theory.

Answer 19

The simple band theory takes the discrete energy levels or orbitals found in singular atoms and applies them to a condition that they are found in solids. Therefore, their orbitals would overlap and start to form continuous bands. Therefore, simple band theory discusses these bands as whole ranges of energy levels that an electron may occupy. In between these bands, there are forbidden gaps which are areas that can never be occupied by electrons. Only if enough energy is given will the electron be able to jump the gap.

Question 20

Distinguish between the fermi level, valence band and conduction band.

Answer 20

The fermi level is a measure of the energy of the highest occupied electron energy level. The valence band is the outermost band in a material that holds electrons. The conduction band is the band that the electrons must jump to, for them to be able to move freely during conduction.

Question 21

What is the difference between conductors, semi-conductors and insulators?

Answer 21

In conductors, the valence band and conduction band overlap, therefore, it is easy for free electrons to start conducting. In semi-conductors, there is a small gap between the valence band and conduction band. The free electrons need to be given enough energy to jump this gap before conduction can start. In insulators the gap is far too wide. It is unlikely that any free electron will be given enough energy to be able to jump the gap for conduction to occur.

Question 22

Distinguish between the two categories

Answer 22

Intrinsic semi-conductors have a material structure that allows for them to act as semi-conductors naturally whilst extrinsic semi-conductors have a material structure that is modified to respond differently to temperature.

Question 23

Describe intrinsic semi-conductors

Answer 23

These are made from a pure material, therefore the semi-conducting property comes from the structure of the material itself, as in silicon. At temperatures above absolute zero, there will be some electrons which are excited across the band gap into the conduction band, producing current. When an electron crosses the gap it will leave a vacancy or hole in the regular lattice. Under the influence of external voltage, both the electron and the hole can move across the material. Current flows when the electrons move into the conduction band but also hole current occurs. This happens when electrons hop between the lattice positions to fill the holes left by other electron. Whilst this occurs, it is as if holes are migrating across the material in the opposite direction to the free electron movement.

Question 24

Describe N-type semi-conductors

Answer 24

When impurity atoms with 5 valence electrons are added (pentavalent) , n-type semi-conductors are produced by having extra electrons that can make the jump to the conduction band. The majority of charge carriers in this case would be electrons in the conduction band whilst the minority would be the holes in the valence band.

Question 25

Describe P-type semi-conductors

Answer 25

When impurity atoms with 3 valence electrons are added, p-type semiconductors are produced by producing a hole or electron deficiency. The majority charge carriers in this case would be the holes in the valence band whilst the minority of charge carriers are electrons in the conduction band.

Question 26

State Ohm’s Law

Answer 26

Ohm’s Law states that the current in a circuit is directly proportional to the potential difference applied to it as long as the resistance remains constant.

Question 27

Distinguish between PTC and NTC Thermistors

Answer 27

Thermistors vary their resistance depending on temperature. Positive temperature coefficient thermistors have their resistance directly proportional to temperature whilst negative temperature coefficient thermistors have their resistance inversely proportional to temperature.

Question 28

Define electrical resistivity

Answer 28

Electrical resistivity quantifies how strongly a material opposes the flow of electric current.

Question 29

Define electrical conductivity

Answer 29

Electrical conductivity is a measure of the material’s ability to conduct an electrical current.

Question 30

What is the resistance coefficient of a material?

Answer 30

The resistance coefficient of a material is an indication of the sensitivity of a material’s resistance to temperature.

Question 31

How does the battery move electrons round a circuit?

Answer 31

A battery creates a potential difference by having one terminal at a higher potential than the other. Electrons at the negative terminal have a higher potential than those at the positive, therefore, electrons flow from the negative terminal to the positive. Once at the positive terminal, the battery gives energy to the electrons and the cycle continues. Each time the electrons get to the positive terminal with zero joules of electrical potential energy. The work done by the battery per unit charge that flows through it is called the electric potential.

Question 32

Define Potential Difference

Answer 32

Potential difference is the difference in electric potential between two points in a circuit.

Question 33

State Kirchoff’s first law

Answer 33

Kirchoff’s first law states that the sum of current entering a junction is equal to the sum of currents flowing out.

Question 34

State Kirchoff’s second law

Answer 34

Kirchoff’s second law states that the summation of all the potential difference across the individual components must add up to the EMF of the cell.

Question 35

Define the EMF of a Power Source

Answer 35

The EMF of a Power Source is the energy transformed within it per Coulomb of charge that it delivers,

Question 36

Define lost volts and terminal pd

Answer 36

Lost volts is the term for the voltage lost inside the battery itself due to internal resistance whilst the terminal pd is the potential difference available for use in the circuit.