Chapter 9. Electricity

Question 1

What is current?

Answer 1

Rate of flow of charge
Current, I = charge, Q(C)/time,t(s)

Question 2

What determines the conductivity of a material?

Answer 2

Its electron density. The ammount of electrons per unit volume. If its around 10²⁰ per mm³ then it is a good conductor around 10¹⁰ per mm³ then it is a semiconductor and if its around 1 its an insulator.

Question 3

Note

Answer 3

1 amp is a flow of about 6 x 10¹⁸ per each second
1 electron is only 1.6 x 10^-19 C(coulomb)
1 coulomb is the amount of charge that passes a point when a current of 1 ampere flows for one second

Question 4

What is drift velocity?

Answer 4

The average velocity attained by charged particles, (eg. electrons) in a material due to an electric field

Question 5

Note

Answer 5

I = nAve
where I is current
n is the electron density
A is the cross sectional area of the wire
v is the drift velocity(L/t)
and e is the charge on each electron
Derived from a wire of length L with cross sectional area, A, its volume is AL and n is the electron density of the wire, so nAL is the total number of electrons in the wire, e is the charge on each electron so nALe is the total charge, Q, if it takes time, t for an electron to travel the length of the wire then after time, t all electrons would have traveled L. Since I = Q/t and Q/t = nALe/t then I = nALe/t. But L/t is the drift velocity, v so I = nAve

Question 6

Note

Answer 6

Charge on charge carriers is quantised meaning it can only have integeral values, Q = ne where n=1,2,3,… therefore Q cannot have arbitrary values

Question 7

Define potential difference

Answer 7

The energy transefered per unit charge.
p.d.,V(volts) = energy transfered, W(joules)/charge, Q(coulombs)

Question 8

Note

Answer 8

P=VI, P=I²R, P=V²/R
all from W = VQ, P = W/t = VQ/t = VI
and V = IR so substitute V or I into the equation above.

Question 9

Define resistance

Answer 9

The ratio of the p.d. applied acroos it to the current passing through it.
R(ohms) = V/I

Question 10

State Ohm’s law

Answer 10

The current through a metal wire is directly proportional to the p.d. accross it (provided the temperature is constant)
Ohmic conductors are conductors that obey Georg Ohm’s law

Question 11

Factors that affect resistance

Answer 11

A long wire has more resistance than a short wire.
A thin wire has more resistance than a thick wire
A hot wire has more resistance than a cold wire

Question 12

Note

Answer 12

resistance, R(ohms) = resistivity x length/cross sectional area

Question 13

What is resistivity?

Answer 13

The value numerically equal to the resistance of a sample of the material of unit length and cross-sectional area.

Question 14

Note

Answer 14

For a metallic conductor at constant temperature I is directly proportional to V and resistance is constant
For a filament lamp Resistance increases as current increases as temperature increases so the graph is curved(take note of the axes and the formulae)
-With temperature increase the metal ions vibrate faster with greater amplitude so when it is more difficult for the electrons to pass throght the lattice, for insulators and semiconductors as temperature increases more electrons break free from their atoms and the material has a decrease in resistance as it is now able to conduct electricity
For a semiconductor diode the current is almost zero for negative voltage as there is very high resistance for the reverse diode and the grapgh shoots at V = 0.6 because the restistance becomes extremely low.

Question 15

Note

Answer 15

Make sure you know the symbols for a diode, LED, filament lamp and a thermistor and all that bash

Question 16

Note

Answer 16

thermistors have a negative temperature coefficient, as temperature increases resistance decreases(not directly proportional)

Question 17

Note

Answer 17

energy transfered(kWh) = power(kW) x time(h)

Question 18

Define an ampere

Answer 18

1A is the current in two infinitely long straight wires 1 m apart in a vacuum which produces a force of 2 x 10-7 N m-1 on each wire.