The Atom

An atom is a fundamental unit of matter made up of:

• Protons (positively charge)

• neutrons (neutral - no charge)

• electrons (negative charge)

Everything is made of atoms which contain positive particles called protons and negative particles called electrons.

An atom will usually have the same number of positive and neutral charges which makes the atom neutral.

Electric Charge

Electric charge is given the symbol Q. Electrons are the charge carriers that flow an electrical current - from the negative to positive terminals.

Charge is measured in coulombs which is given the symbol C.

The charge on a proton is 1.6 × 10^-19 C.

The charge on a electron is -1.6 × 10^-19 C.

Electrons

• Particles of matter possessing a negative charge and a small mass

• Net movement of electrons is an electrical current

• Movement of electrons is like a domino reaction

• Electrons will move from higher to lower potential

Charge, Current & Time

Electric current is given the symbol I. Electric current is the movement of negative charges (electrons) in a circuit.

Current is the amount of flowing per second is given the unit Amperes (A).

So a current of 1A is 1C of charge transferred in 1s is I = Q / t.

• charge (Q) transferred in coulombs (C).

• current (I) in Amps (A)

• time (t) in seconds (s)

Potential Difference (Voltage)

When two objects have different charges we say there is a potential difference between them. Potential difference cause electrons to move. Voltage is measured in volts and is given the symbol V.

What does a battery do?

• A battery uses chemical energy to move charges

• If you connect a circuit with a battery the charges flow out of the battery carrying the energy.

You can’t measure voltage:

• in a circuit

• through a circuit

• through a component

• flowing

Current Electricity

Energy or Work (W)

Energy (E) = Charge (Q) x Voltage (V)

Energy (Joules) = current (Amperes) x time (seconds) x voltage

E = QV, Therefore Q = E/V

Power is the rate of energy transfer or the rate at which work is done so:

P = Energy/Time

Formula:

• P = IV

• P= I² R

• P = V² / R

Resistance

All components in an electric circuit provide a certain amount of impediment to the flow of the current. This property is called resistance. The unit of resistance is the ohm.

The resistance of device is 1 ohm if the current through it is 1 amperes and the potential difference across it is 1 volt.

Ohm’s Law:

I = V/R

• I - current

• V - volts

• R - resistance

V = IR means that at a constant temperature, the potential difference across the ends of a conductor is directly proportional to the current through it.

Devices which do not obey ohm’s law are filament lamps and semiconductor diode.

Materials can be classified by a property called resistivity. The best conductors have the lowest resistivity values.

Resistivity (p) is related to the resistance (R) by the equation: R = pl / A, where”

• l is the length of the conductor

• A is the cross-sectional area

• p is the resistivity with the units of ohm per meter

Conductors and Insulators

Conductors allow electric currents to flow through them with minimum opposition. We say they have a low resistance.

Metals are good conductors of electricity because their atoms have one or more loosely held held electrons in their outer shells.

These electrons are shifted from atom to atom in the metal structure and are shared among all atoms.

If no potential difference is applied the electrons just drift around not moving in any one direction.

When a battery is connected across the ends of a conductor there is a net movement towards the positive end of the conductor B. At the same time electrons are moving from the negative terminal of the battery

Insulators are materials where all electrons including the outer ones are tightly bound and are not free to move.

Examples of insulators: glass porcelain ,polystyrene and paraffin wax.

Semiconductors

Semiconductors have electrical conducting properties but they are not as bad as insulators and nowhere as good as conductors.

The electrical conducting properties of semiconductors may be greatly increased by:

• Raising the temperature

• The addition of small amounts of suitable impurities .

Examples are silicon germanium and gallium arsenide.

Current Electricity

Alternate Current versus Direct Current

The simple flow of charge when electrons leave the negative terminal of a battery and flow around the circuit is known as direct current.

• Direct current flows in one direction only

• Direct current may be steady or fluctuating

Alternating current refer to the event where electrons flow backwards and forwards in the circuit. The alternating current reverses directions periodically.

Simple Circuits

1. Series circuit

   • All in a row

   • path for electricity

   • light goes out and the circuit is broken

2. Parallel circuit

   • Many paths for electricity

   • Tlight goes out and the others stay on

Measuring Current

Electric current is measured in amps (A) using an ammeter connected in series in the circuit.

The ‘electrical push’ which the cell gives to the current is called the voltage. It is measured in volts (V) on a voltmeter.

In a series circuit current is the same at all points in the circuit.

In a parallel circuit has the current being shared between the components.

Resistance in a Series Circuit → R_t = R₁ + R₂ + R_3. This means that the total resistance is equal to the sum of all the resistors in the circuit. Also the voltage in the battery can be found as:

• V = V_R1 + V_R2 + V_R3

• V = IR₁ + IR₂ + IR₃ or V = I ( R₁ + R₂ + R₃₎

Resistance in a Parallel Circuit → 1/R_t = 1/R₁ + 1/R₂ + 1/R_3. The voltage in the battery is found the same way as in the series circuit.