Electricity

electric circuit

A continuous and closed path of electric current.

Electrical current

Electric current is expressed by the amount of charge flowing through a particular area in unit time.

S.I unit of current

ampere, A

Current formula

I=Q/t

S.I unit of charge

Coulomb, C

Ammeter

an instrument for measuring electric current in amperes.

Charge formula

Q = It

One couloumb

charge in 6 X 10^18 electrons

One electron

-1.6x 10^-19 C

Potential difference is produced by

battery

Potential difference

Potential difference between two points in an electric circuit carrying some current is the work done to move a unit charge from one point to another.

V =

W/Q (energy/charge)

​

S.I unit of potential difference

V (Volt)

1V

1J/C

Volmeter

A device used to measure voltage, or electrical potential energy difference

Ohm's Law

The potential difference, 'V' across the ends of a given metallic wire in an electric circuit is directly proportional to the current flowing through it provided the temperature remains the same

Ohms law formula

V = IR

Resistance

The property of a conductor to resist the flow of charges through it.

S/I unit of resistance

Ω Ohm.

If the resistance is doubled

the current will be halved

Variable resistance

A component used to regulate current without changing the source of the voltage.

Rheostat

A device which can vary the resistance without opening the circuit

Factors on which resistance depends

Length, Area of cross section, Nature of its material

Motion of electrons through a conductor

is retarded by its resistance. Low resistance, good conductor. High resistance, poor conductor.

Resistor

An electrical device that resists the flow of electrical current due to its appreciable resistance.

R =

p l/A

Resistivity

The electrical resistance of a conductor of unit cross-sectional area and unit length.

S.I unit of resistivity

Ωm

Series connection- potential difference

V= V1 + V2 + V3

Series connection- resistance

R= R1 + R2 + R3

Disadvantages of series connection

Different appliances requiring different levels of current cannot be connected. If one component fails, the entire circuit breaks.

Parallel connection resistance

The reciprocal of the equivalent resistance of a group of resistances joined in parallel is equal to the sum of the reciprocals of the individual resistances. 1/R = 1/R1 + 1/R2 + 1/R3

Advantages of parallel connection

If one of the components fails, the others still work. Each component receives the required amount of current since they are in different branches.

Heating effect of electric current

If the electric current is purely resistive, the source energy gets dissipated in the form of heat.

Power

P = VI

Heat

H = I^2Rt or VIt

Joule's law of heating

The heat produced in a resistor is directly proportional to the square of current for a given resistance, directly proportional to the resistance for a given current and directly proportional to the time for which current flows through the resistor.

incandescence

the emission of visible light by a hot object

If resistance increases

temperature increases

Filament

Should have a high melting point, have high resistance and be thermally isolated

Power

The rate at which electric current is dissipated or consumed in an electric circuit.

P

V^2/R or P = VI

S.I unit of power

W

1W

1Vx 1A

Kilowatt

1000W

Wh

1 watt of power for 1 hour

kWh

1000 watt x 3600 second or 3.6 x 10^6 J

W=

VIt

Examples of filament

Tungsten - highest melting point and high resistivity

Fuse is made of

Al, Cu, Fe alloy

Metal used for electrical heating devices

Nichrome as it is an alloy, therefore has higher melting point, higher resistance and does not oxidise.

Rating of electrical appliances

The voltage at which the appliance functions and the rate of consumption (power) of that voltage.

Energy si unit

Joules (J)

Energy =

Power x time

resistance is directly proportional to

length, temperature

resistance is indirectly proportional to

area of cross section