H214 Current of Electricity

Question 1

Define electric curent

Answer 1

Electric current is the rate of flow of charge

Question 2

What is the formula of electric current?

Answer 2

I = Q/T

Question 3

For a current I in a wire that causes the charge carriers to have a drift velocity v, derive an equation relating current to the number density of charge carriers in the wire n, the cross-sectional area of the wire A and the charge on each charge carrier q.

Answer 3

For a wire segment of length L, total charge in this volume is:
Q = (number of charge carriers in volume)(q)
= n(volume)q
=nALq

Time taken, t, for charge carriers to have net displacement of L:
L = v/t

Current is the rate of flow of charge:
I = Q/t
I = nALq / L/v
= nAvq

Question 4

What is the formula for the drift velocity of a current-carrying conductor?

Answer 4

I = nAvq

I: current (A)
n: number density of charge carriers
A: cross-sectional area
V: drift velocity
q: charge

Question 5

Define electromotive force.

Answer 5

Electromotive force (emf) the energy transformed from chemical to electrical per unit charge when charge is driven round a complete circuit.

Question 6

Define potential difference.

Answer 6

Potential difference is the energy transformed from electrical to other forms per unit charge when charge passes through an electrical component.

Question 7

What is the formula for potential difference?

Answer 7

V = W/Q

Question 8

State the formulas relating power output of electrical component.

Answer 8

P = IV
= I^2R
= V^2/R

Question 9

State the formulas relating power output of electrical component.

Answer 9

P = Iemf
= I^2R
= emf^2/R

Question 10

Define resistance.

Answer 10

Resistance is the ratio of potential difference across a component to the current passing through it.

Question 11

What does the resistance of a component depend on?

Answer 11

a) The number density of charge carriers
b) the extent of vibration of the atomic lattice

Question 12

Using these components, explain why the resistance of metals increases with increasing temperatures.

Answer 12

1. the valence electrons of metal atoms make up the seat of delocalised electrons responsible for conduction - the number density is therefore fixed
2. however higher temperatures result in greater vibration of the giant metallic lattice, and more frequent collisions with delocalised electrons disrupt charge flow.

Question 13

Using these components, explain why the resistance of thermistors decreases with increasing temperatures

Answer 13

1. at higher temperatures, more electrons are set free to carry charge. the freed electrons also create holes that can act as mobile positive charge carriers.
2. the increased number density of charge carriers is greater in effect than the increased vibration of lattice causing more frequent collisions with charge carriers

Question 14

What is the formula relating geometry and resistance?

Answer 14

R = pL/A

Question 15

What is the formula for terminal pd?

Answer 15

Vt = emf - Ir