Electrical quantities
● Conductors
○ The materials that can pass electricity through them easily
○ Free electrons are present in conductors
○ E.g. metals → silver, copper etc.
● Insulators
○ The materials that cannot pass electricity through them
○ Free electrons are absent
○ E.g. wood, rubber, plastics etc.
● Electric current
○ It is the flow of charge through a conductor
○ Electric current = charge / time
○ I = Q / t
○ S.I unit of electric current is Ampere (A)
● 1 ampere = 1 coulomb / 1 second
● Ammeter
○ An instrument used to measure electric current
○ 1 milliamp = 1 mA = 1/1000 Ampere (.001 or 10^-3)
○ 1 microamp = 1 nA = 10^-6 Ampere
● Electric circuit
○ A closed path in which an electric current flows
● Types of current
○ Conventional current
■ Positive to negative
○ Electronic current
■ Negative to positive
● Electromotive force
○ An electrical supply (a power pack, cell or battery) provides electrical energy which drives
charge around a complete circuit
○ The electromotive force of a supply is the energy provided per coulomb of charge and is
measured in volts
● Combining electromotive force
○ E = E1 + E2 + E3
● V = W / Q
● Volts / EMF = Work (energy) / Charge in coulombs
● Voltmeter
○ A device that is used to measure the voltage / potential difference
○ Voltmeter is always connected
● Potential difference
○ The potential difference or voltage across a component in a circuit is the energy required per
coulomb of charge to drive the current through that component
○ It is the amount of electrical energy converted into other forms per coulomb of charge
○ It is measured in volts
○ Potential difference = work/Charge (C)
○ 1 volt = 1 joule / coulomb
● Ohm’s law
○ The current flowing through a wire is proportional to the potential difference across it provided
the temperature remains constant
● Resistance
○ The electrical resistance of an object is a measure of its opposition to its opposition to the flow
○ Resistance is measured in ohms (Ω)
○ Resistance = Potential difference (V) / Current (A)
○ Current is directly proportional to the voltage
Factors affecting resistance
● Length of the conductor (L)
○ Resistance of the conductor is directly proportional to the length of the conductor
● Area of cross-section of the conductor (A)
○ Resistance of the conductor is inversely proportional to the area of the cross-section
● Temperature (T)
○ Resistance of the conductor is directly proportional to the temperature
● Nature of the material
○ Copper has lower resistance than iron, the more conductive it is, the lesser resistance
● Resistivity
○ It is the property of a material that measures how strongly it resists the flow of electric
current
○ Resistivity = (Resistance x area) / Length
○ SI unit of resistivity is Ωm, Ohm meter
● The resistor follows ohm's law therefore is a
ohmic resistor
● The filament lamp is not constant and
proportional therefore does not obey the ohm's
law therefore is a non-ohmic conductor
● Electrical power
○ It is the rate at which energy is transferred from one place to another place or
transferred from one form to another
○ Electrical power = Energy transformed / time taken
○ Electrical power = Current x voltage
○ SI unit of electrical power is Watt (W)
● The amount of electrical energy used by a component depends on 3 things
○ The current
○ The potential difference
○ The amount of time the device is used for
● Energy transferred
○ Current x voltage x time ( I x V x t)
○ Power x time (P x t)
