Chapter 14: Current of Electricity

Question 1

Define current

Answer 1

The rate of flow of charged particles

I = Q / t

Question 2

Derive I = nAvq

Answer 2

Since I = ΔQ / Δt,
I = (ΔNq) / Δt, where ΔN = number of charge carriers
= (nΔVq) / Δt, where n = number density, v = volume
= (nAqΔx) / Δt, where A = area, Δx = length
= nAvq, where v = drift velocity

Question 3

Define potential difference (V)

Answer 3

The p.d. across a device is the electrical energy converted to other forms of energy per unit charge passing through the device

(V = W/Q)

Question 4

Define electromotive force (ε)

Answer 4

The e.m.f. of a source is the electrical energy converted from other forms of energy per unit charge, transferred by the source in driving a unit charge round a complete circuit

Question 5

Define resistance (R)

Answer 5

The ratio of the potential difference across a conductor and the current flowing through it

(R = V/I)

Question 6

State Ohm’s Law

Answer 6

The current I through a metallic conductor is proportional to the potential difference V across it ends under constant physical conditions

Question 7

Explain how a filament lamp (non-ohmic conductor) changes resistance with temperature

Answer 7

1. As the p.d. across a filament lamp increases, the current increases and the energy dissipated as heat increases, thus resulting in an increase in temperature.
2. This causes the conducting electrons to collide more frequently with the lattice ions in the wire, resulting in lower mobility of the eelctrons and decreasing the mean drift velocity of the electrons, thus causing the resistance of the filament to increase

Question 8

Define a semiconductor diode

Answer 8

An electrical component which conducts electricity in only one direction

Question 9

Define a thermistor

Answer 9

A type of resistor where the resistance (conventionally) decreases when its temperature increases

Question 10

Explain how a thermistor’s resistance decreases when its temperature increases

Answer 10

1. As the temperature of the thermistor increases, more charge carriers are liberated and become mobile, increasing conductivity.
2. The amplitude of the lattice ions vibration also increases, resulting in more frequent collisions between the charge carriers and lattice ions, decreasing the mean drift speed of the charge carriers and thus decreasing conductivity.
3. As the effect of increased mobile charge carriers is greater than the effect of more frequent collisions, the electrical conduction of the thermistor improves overall and thus the resistance decreases

Question 11

Define resistivity

Answer 11

The measure of how strongly a material opposes electric current, characterising the resistance of materials at a fixed temperature regardless of the materials’ dimension

Question 12

State the equation of terminal p.d. when there is internal resistance

Answer 12

V = ε - Ir

Question 13

For an e.m.f. source to supply maximum power, the resistance of a external resistor should be:

Answer 13

The same value as the e.m.f. source’s internal resistance
(By Maximum Power Transfer Theorem)

Question 14

Define coulomb

Answer 14

One coulomb is the amount of charge which flows past a point when a steady current of one ampere flows for time of one second.