5.9 current of electricity

Question 1

state the three formulae involving current

Answer 1

I = Q/t
I = nAvq
V = IR

Question 2

define resistance

Answer 2

resistance of a component is the ratio of potential difference V across the component to the current I passing through

Question 3

define electromotive force

Answer 3

emf is the work done per unit charge when non-electrical energy is transferred to electrical energy when the charge is moved around a complete circuit

Question 4

define potential difference

Answer 4

pd between 2 points in a circuit is the work done per unit charge when electrical energy is transferred to non-electrical energy when the chargee passes from one point to another

Question 5

compare emf and pd

Answer 5

1) emf refers only to source while pd refers to any 2 points in the circuit
2) emf is non-electric to electric, pd is electric to non-electric
3) emf exists whether or not there is current flowing, pd exists only with current flowing through

Question 6

explain how resistance is affected by increased voltage.

Answer 6

when V increase, I increase, hence temperature increases. when temp increases, 2 changes affect R:
1) number of charge carriers per unit volume increases which decreases R
2) amplitude of atomic vibrations of the lattice atoms increases which increases R

Question 7

explain the shape of the I-V graph of an ohmic resistor

Answer 7

I increases proportionally with V, hence R remains constant

Question 8

explain the shape of the I-V graph for filament lamp

Answer 8

I increases proportionally with V initially. as V increases further, I increases less than proportionately, hence R increases.
when temp increases, number of charge particles negligible increase but amplitude of atomic vibration increases, hence R increases.

Question 9

explain the shape of the I-V graph for semiconductor diode

Answer 9

for forward-biased graph:
as V increase, temperature increases. electrons from semiconductor are more likely to have sufficient energy to escape the atoms, which significantly increases number of charged particles, dominating increase in amplitude, hence R decreases

Question 10

explain the shape of the I-V graph for NTC thermistor

Answer 10

NTC’s R decreases as temperature increases since number of charged particles increases

Question 11

state the two formulae of internal resistance

Answer 11

E = I (R + r)
VR (terminal V) = E - Ir