Topic 3.1-Electrical Quantities

Question 1

Define electric current:

Answer 1

A flow of negatively charged particles (electrons) or “charge carriers”. Electric current flows from negative to positive.

Question 2

What direction does conventional current flow in?

Answer 2

+ to -

Question 3

What is the charge of one electron?

Answer 3

1.6 x10^-19 C

Question 4

What is the difference between DC and AC?

Answer 4

Direct current only flows in one direction

Alternating current changes direction 50 times every second (50Hz)

Question 5

What is the equation for charge?

Answer 5

Q= It

Charge= current x time

Question 6

Define voltage:

Answer 6

A measure of the amount of energy a component transfers per unit of charge passing through it. V=E/Q

The force that pushes electrons around a circuit (requiring a p.d)

Question 7

What is the equation for voltage (in terms of energy):

Answer 7

V= E/Q

Voltage= Energy/ Charge

Question 8

Define Electromotive force (EMF):

Answer 8

The total work done by a cel per coulomb of charge. V= W/Q

This is the correct term for supply voltage.

Question 9

Define potential difference:

Answer 9

The voltage across components in a circuit (the work done to them by the current). V=W/Q

The correct term for voltage.

Question 10

What is an electron volt?

Answer 10

A unit of energy that is used with sub-atomic particles. It is a derivation from the V=E/Q equation and states that:

“if an electron is accelerated by a p.d of 1V then the energy it will gain is E=V x e

Question 11

Define resistance:

Answer 11

The opposition to the flow of current within a conductor.

Question 12

What is the equation for resistance?

Answer 12

Resistance= p.d / current

R= V/I

Question 13

What is Ohm’s Law?

Answer 13

The current through a component is directly proportional to the voltage.

Question 14

Describe the relationship between current and voltage for a filament lamp (and the IV graph):

Answer 14

A filament lamp is an ohmic conductor until the current becomes too large, then it heats up and increases resistance (curving the graph)

Question 15

Describe the relationship between current and voltage for a fixed resistor (and the IV graph):

Answer 15

Resistors are ohmic conductors so I is directly proportional to V.

Question 16

Describe the relationship between current and voltage for a diode (and the IV graph):

Answer 16

A diode only conducts in the forward direction.

Question 17

Describe the relationship between current and voltage for a thermistor (and the IV graph):

Answer 17

A thermistor alters resistance with temperature (in the reverse manner to a filament bulb). Resistance reduces with temperature.

Question 18

Define resistivity:

Answer 18

The general property of a material to resist the flow of electric current.

Question 19

Give the equation for resistivity:

Answer 19

Resistance= (Resistivity x Sample length) / Cross sectional area

R= pl / A

Question 20

Define carrier density:

Answer 20

Assuming each atom has one free electron, the carrier density is the number of free charges per unit volume (n).

Question 21

Define drift speed:

Answer 21

The speed with which electrons will move down a wire.

Question 22

What is a semiconductor?

Answer 22

A solid material (generally) with small numbers of delocalised electrons that are free to conduct.

Question 23

Give the equation for current in terms of drift velocity:

Answer 23

Current= number of electrons x cross sectional area x average drift velocity x electron charge

I= nAve

Question 24

How would increasing the temperature of a wire impact the resistance, current and drift velocity in a conductor?

Answer 24

The vibrations would increase so the resistance would increase, decreasing the current. This would decrease the drift velocity as I=nAve.

Question 25

How would increasing the temperature impact the resistance, current and carrier density of a semiconductor?

Answer 25

Increasing the temperature of a semiconductor increases the already small carrier density, so the resistance decreases and the current increases (as I=nAve). This is known as a negative temperature coefficient.

Question 26

What are 3 properties of free atoms which are not found in solid materials?

Answer 26

• Series of discrete energy levels
• Number of energy levels depends on the electrons recieved energy
• Electrons can leave their atoms if they have enough energy

Question 27

Describe the layout of energy levels in solid materials:

Answer 27

As there are many atoms close together, energy levels become wider and form energy bands. These bands are the valence band and the conduction band. Electrons of many different energy values are in the valence band but if they get enough energy they can move to the conduction band and become delocalised.

Question 28

Explain the difference between the energy bands for insulators, semiconductors and metals:

Answer 28

Insulator: Empty conduction band and full valence band.

Semiconductor: Almost empty conduction band and almost full valence band.

Metal: Almost full conduction band.

Question 29

What is a conduction ‘hole’?

Answer 29

An electron hole is the lack of an electron in a certain position where one could exist in an atom or atomic lattice. As an electron is missing, a net positive charge remains.

These holes are not particles but quasiparticles.

Question 30

How do semiconductor diodes work?

Answer 30

They are made from joining together two different types of semiconductor (P and N types). This creates an energy barrier at the junction which blocks charge carriers like holes or electrons. The barrier can be overcome in the forwards but not the reverse direction

Question 31

What is superconductivity?

Answer 31

A quality of a device with zero resistance below a critical temperature. This means that it also has infinite current. They can be used to make high power electromagnets.