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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
Note 
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