Chapter 14: Current of Electricity

Question 1

Define current

Answer 1

It is the rate of flow of charged particles with respect to time (I = dQ/dt)

Question 2

Define drift velocity

Answer 2

It is the average velocity of charge carriers along the conductor

Question 3

Define charge

Answer 3

The amount of charge that flows past a point in a point in time is the product of current and time, if the current is steady

Question 4

State the conservation of charge

Answer 4

The amount of charge entering and leaving each junction is conserved, and charges cannot be created or destroyed (Current remains constant along on single conductor)

Question 5

Define potential difference

Answer 5

It is the electrical energy converted to other forms of energy per unit charge passing through the device

Question 6

Define emf

Answer 6

It is the amount of electrical energy converted from other forms of energy per unit charge, transferred by a source in driving one unit charge around one complete circuit

Question 7

Define resistance

Answer 7

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

Question 8

State Ohm’s Law

Answer 8

It states that the current flowing through a metallic conductor is proportional to the potential difference across its ends, under constant physical conditions

Question 9

Define resistivity

Answer 9

A measure of how strongly a material opposes electric current and characterises the material’s resistance at a fixed temperature regardless of the material’s dimension

Question 10

What is the formula of the terminal pd across a battery with internal resistance?

Answer 10

V = ɛ - Ir

Question 11

What is the maximum power-transfer theorem?

Answer 11

Power transfer is maximum when R = r

Question 12

Explain both reverse and forward bias

Explain the shape of the I-V graph of an ideal semiconductor diode

Answer 12

• In the forward bias direction, the graph is an exponential curve beyond the threshold point shortly after the origin, and the diode’s resistance is insignificant since I increases exponentially with V
• In the reverse bias direction, the graph is a horizontal line from the origin followed by a near-vertical line. Since I is insignificant before the breakdown voltage, the diode’s resistance is very high

Question 13

Explain the shape of the I-V graph of a negative temperature coefficient (NTC) thermistor

Answer 13

• When temperature increases, more charge carriers are liberated and become mobile, increasing conductivity
• The amplitude of vibration of lattice ions increase, causing more frequent collision between charge carriers and lattice ions, reducing the mean drift speed of charge carriers and decreasing conductivity
• The effect of increasing mobile charge carriers is greater than the effect of more frequent collisions, causing an overall rise in conductivity and a fall in resistance

Question 14

Explain the shape of the I-V graph of an ohmic conductor

Answer 14

• As the pd across the ohmic conductor increases, the temperature of the conductor remains constant, meaning the mobility of charge carriers remains constant (reliant on temperature).
• This shows that the resistance of an ohmic conductor remains constant

Question 15

Explain the shape of the I-V graph of a filament lamp

Answer 15

• As the pd across the lamp increases from zero, the current increases and the energy dissipated as heat increases, resulting in a higher temperature
• An increase in temperature of the filament lamp will cause the amplitude of lattice ion’s vibration to increase, increasing the frequency of collision between the lattice ions and conducting electrons
• This decreases the mean drift speed of the electrons as they become less mobile, causing resistance to increase