3. Current Electricity
Potential and potential difference :
- The flow of electric charge between two points depends on a kind of electric level at those points. This level is called electric potential.
- A positive charge flows from a point of higher potential to a point of lower potential.
- Electricity flows due to the conduction of negatively charged electrons.
- Electrons flow from the point of lower potential to a point of higher potential.
- A lightning strike is the flow of electrons from point of lower (negative) potential on the clouds to the point of higher (zero) potential on the earth.
- Work has to be done against the electric field to take a positive charge from a point of lower potential to a point of higher potential.
Potential difference of a cell :
- The difference in potential between the positive and negative terminals of a cell is the potential difference of that cell.
- This potential difference is caused by chemical reactions occurring inside the cell.
- The potential difference sets the electrons in motion and results in the flow of electricity through a conducting wire connected to the two ends of the cell.
- The amount of work done to carry a unit positive charge from point A to point B is called the electric potential difference between the two points.
An introduction to scientists:
- The Italian scientist Alessandro Volta constructed the first electric cell.
- The unit of potential difference is named ‘volt’ in his honour.
Free electrons :
- Every atom of a metallic conductor has one or more outermost electrons which are very weakly bound to the nucleus. These are called free electrons.
- Electrons can easily move from one part of a conductor to its other parts.
- The negative charge of the electrons also gets transferred as a result of this motion.
- The free electrons in a conductor are the carriers of negative charge.
Current flowing through a wire :
- If a conducting wire is not connected to a cell, its free electrons move randomly in all directions in the space between the atoms.
- When we connect the ends of the wire to the two terminals of a cell, electric force acts on the electrons.
- Being negatively charged, they start moving from the negative (lower potential) to the positive (higher potential) terminal of the cell.
- Due to the flow of these electrons, current starts to flow through the wire.
- This motion of electrons is irregular but there is a definite, non-zero value to their average velocity.
Electric current:
- An electric current is the flow of electrons through a conductor.
- Quantitatively, current (I) is defined as the charge passing through a conductor in unit time. If charge Q is flowing through cross-section of a conductor in time t then the
- I=Q/t
- The unit of charge in SI units is Coulomb (C).
- Current is expressed in Ampere (A).
- The charge of one electron is 1.6 x10-19 C.
- Ampere : One ampere current is said to flow in a conductor if one Coulomb charge flows through it every second.
- 1A=1C/1s
Even though, electrons
move from negative end to
positive end, conventionally, the
direction of current flow is taken
to be opposite of the direction of
flow of electrons i.e. from the
+ve end to the –ve end of a cell.
Scientist Information :
- The French mathematician and scientist, Ampere, conducted a number of experiments on electricity.
- Today, we can measure the current flowing in a conductor only because of his work.
- The unit of current is called ampere in recognition of his work.
Ohm’s law:
- If the physical state of a conductor remains constant, the current (I) flowing through it is directly proportional to the potential difference (V) between its two ends.
- Potential difference and current are measured in Volts and Amperes respectively. The unit of resistance is called Ohm.
- If one Ampere current flows through a conductor when one Volt potential difference is applied between its ends, then the resistance of the conductor is one Ohm.