Electric Current:
Electric current is the electric charges in motion.
An electric current in the rate of flow of electric charges.
SI is Ampere (A). Coulumb/ Second
A current of one ampere is a flow of charge at the rate of one coulomb per second.
Conventional current is a current that flows out of positive terminal.
Electro-Motive force:
A chemical change in a battery provides energy to push and drive a charge through the circuit.
The electromotive force of any electrical source is the work done by the source in driving a unit charge around the circuit.
→ S.I unit is Joule / coulomb or volt.
Batteries in series and parallel:
→ When batteries are connected in series the combined emf is the sum of all individual EMFs.
→ In parallel all batteries will give emf in equal portions.
Potential difference:
→ Due to chemical changes.
When the charge is passing through the component some of its energy will be converted from electrical to other forms.
Energy is dissipated as a unit charge passes through the components of a circuit.
The dissipation of electrical energy between two points in an electrical circuit causes a potential difference between two points.
The amount of electrical energy is converted to other forms when one unit charge passes between two points/components.
S.I unit: Volts V= J/c
→ Potential difference across a component in a circuit is given by the work done to push the charge through it.
The greater the energy changed from electrical to light and heat the greater the work done on the charge
A voltmeter is an instrument used to measure the electric potential difference between two points in an electric Circuit.
It is connected in parallel with the circuit in which the potential difference is being measured.
An Ammeter is connected in series.
An Ammeter is a highly sensitive device.
Resistance:
The degree to which a component opposes the passage of electric current through it.
Resistance of a component is the ratio of potential difference V across it to the current flowing through it.
V = IR
S. I unit Ohms (~~)
One ohm is the resistance of a material through
which a current of one ampere flows when a P.D of one volt is maintained across it.
The rheostat is a fixed resistor. We can increase the resistance by a small current passing through it.
OHM's Law:
The current passing through a conductor is directly proportional to the potential difference across it and provided the physical conditions (Temperature) remain constant.
I ∝ V
V = IR
Components that obey ohm's law are called Ohmic conductors. In non-ohmic conductors
the ratio V/I is not constant.
Resistance of material changes when temperature changes.
T ∝ R
Factors affecting resistance:
Increasing the length of the wire will increase the resistance, but will decrease the current.
Decreasing the length of the wire conductors will decrease the resistance but the current will increase.
→Increasing the cross-sectional area of a conductor decreases its resistance. The current will increase.
Decreasing the cross-sectional area of a conductor increases its resistance. The current will decrease.
Resistance is different for different materials.
R∝length
p - resistivity.
A resistor is a conductor which has a known value of resistance associated with it.
In series:
R = R₁ +R₂ + R3
In parallel:
The effective resistance in parallel will be smaller than individual resistance.
Non-ohmic conductors:
In Non-ohmic conductors, the current does not increase proportionately as a potential difference.
A filament lamp is an electric light with a wire filament heated until it glows.
Current through a filament lamp is not directly proportional to the potential difference. Because at high temperatures, the atoms in the filament vibrate more and collide frequently with the moving electrons.
As current passes, temperature also increases.
More energy is needed to push the current through the filament.
As the P.D increases, the current does not increase.
A filament lamp is a non-ohmic conductor.
Light dependent resistors:
It is used to detect the intensity of light in the circuit.
Resistance decreases as the light intensity increases.
In bright light the resistance of a 10 ohm is low, and more current flow will take place.
