Chapter 9 - Current Electricity

Current

The rate of flow of charge across a cross section normal to the direction of flow of I.

Direct Current (d.c.)

A current of constant magnitude flowing in one direction.

Primary Cells

Non-rechargeable cells that provide current as a result of irreversible chemical reactions.

Example - Dry cell, Daniel Cell

Secondary Cells

Rechargeable cells where chemical reactions are reversible.

Example - Lead accumulator, Ni-Fe accumulator.

Ohm's Law

States that the current through a conductor is directly proportional to the voltage across its ends provided temperature is constant. (I = V/R).

Resistance

The obstruction to the flow of electric current by a conductor, measured in ohms.

Potential Difference (Voltage)

The work done in transferring a unit positive charge from one conductor to another, measured in volts.

Insulators

Materials that do not allow electric current to flow, such as rubber and glass.

Conductors

Materials that allow electric current to flow easily, such as copper and aluminum.

Galvanometer

An instrument used to detect the presence of very weak electric current or the direction of flow of current in the circuit.

Ammeter

An instrument used to measure the magnitude of current flowing in a circuit.

Voltmeter

An instrument used to measure the potential difference between two points in a circuit.

Resistance Wire

A wire with a specific resistance used to control the flow of current in a circuit.

Rheostat

A device by which resistance in a circuit can be varied continuously.

Load

An appliance connected to an electric circuit.

Alternating Current (a.c.)

An electric current that reverses direction periodically.

Conventional Current

The flow of positive charge, taken to flow from the positive to the negative terminal.

Electron Flow

The actual flow of electrons, which is opposite to the direction of conventional current from negative terminal to the positive terminal.

Temperature Effect on Resistance

Resistance usually increases with temperature due to greater collision of electrons.

SI unit of Current and other units

Ampere (A)

1 milli-ampere = 10^-3 A

1 micro-ampere = 10^-6 A

1 coulomb of charge

6.25 × 10¹⁸ e^-

Key

It is to put the current on or off in a circuit.

Material of Resistance Wire

Manganin

Resistance Box

It is a box provided with several resistance connected in series between two terminals.

Why is Ammeter always connected in Series?

Because the entire current of the circuit has to pass through it.

Why must ammeter have very low resistance?

SO that the resistance of the circuit is not altered

Why must a voltmeter be of high resistance?

So it does not draw appreciable current from the circuit.

SI unit of Voltage

Volt

SI unit of resistance

I ohm

How does thickness of wire affect resistance

More thickness results in lower resistance due to increased cross-sectional area. Inversely Proportional

Affect of length of wire on Resistance

The longer the wire, the higher the resistance due to increased collisions of electrons. Directly Proportional

how does material of wire affect resistance

Conducting material offers more resistance , as they have higher concentration of free electrons.