Alessandro Volta invented the electric battery in 1800, and with it, he produced the first steady flow of electric charge, i.e. current electricity. This discovery has become significant today as many household applications such as computers, televisions and air-conditioners use electricity.

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Chapte

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Current electricity

Chapter 21: Current Electricity

In an insulator, the movements of the electrons are limited to only within the electron clouds.

The free electrons in a conductor are in constant, random motion

Conductors & Insulators

Metals have many free electrons.

When a metal wire is connected to a battery such that there is a potential difference across it, the free electrons will be pulled towards the positive end.

Chapter 21: Current Electricity

Current in Metals

When a circuit is closed, the free electrons in the circuit drift towards the positive terminal of the cell, and away from the negative terminal of the cell.

Chapter 21: Current Electricity

Current I is defined as the rate of flow of charged particles. It is a measure of the amount of charges Q flowing through a point in a closed circuit per unit time t.

Chapter 21: Current Electricity

Since electrons are the only charged particles capable of flowing from one end to the other end of a circuit, the flow of charges here refer to the flow of electrons.

A single electron carries a charge of 1.6 x10-19 coulombs.

Semiconductors such as silicon and germanium have both negative (electrons) and positive (holes) charge carriers.

In silicon, all four electrons in the outer shell are shared among neighbouring atoms to form covalent bonds.

The free electrons leave behind electron holes in the atoms where neighbouring electrons can move to fill them.

In this way, the ‘holes’ seem to behave like positively-charged particles, moving in the opposite direction to the electron flow.

Chapter 21: Current Electricity

Current in Semiconductors

Electrolytes are basically ionic solutions that consist of both positive and negative ions (e.g. NaCl).

Ions are highly mobile and when oppositely charged electrodes are dipped in an ionic solution, the ions move to oppositely charged electrodes to complete the circuit.

Chapter 21: Current Electricity

Current in Electrolytes

(+) (-)

When Sir J J Thomson discovered electrons in late 19th century, the validity of the existence of conventional current became questionable.

Conventional Current vs. Electron Flow

Chapter 21: Current Electricity

Chapter 21: Current Electricity

Measuring Current

Q = It

If more charges Q flow through the circuit per unit time t, greater amount

of current I will be detected at any point in the circuit.

measured in coulombs

measured in amperes

measured in seconds

Chapter 21: Current Electricity

Chapter 21: Current Electricity

A current that has constant magnitude and flows only in one direction

D.C. A.C.

A current that reverses its direction many times and has fluctuating magnitude.

Chapter 21: Current Electricity

Chapter 21: Current Electricity

Frequency and Period of A.C. Voltage

T : Period of the a.c. voltage. The period is the time taken for onecomplete cycle. It can be measured between any two corresponding points in successive cycles.

VP : Peak voltage. This is the maximum value of voltage in either direction.

f : Frequency of the a.c. voltage. This is the number of cycles completed per second. The measurement unit for frequency is the hertz, Hz.

1 Hz = 1 cycle per second

If the period is known, the frequency of the signal can be calculated

from: f = 1/T