## Review: Resistance, EMF, Current, Potential, Potential difference and Ohm’s law

### Basic Definitions

#### Resistance

Definition: The opposition offered by a substance to the flow of electric current is called  its resistance.

Unit of resistance: Ohm and is represented by the symbol Ω.

Ohm: A wire is said to have a resistance of 1 ohm if a p.d. of 1 volt across its ends causes 1 ampere to flow through it

#### Electromotive Force (emf)

Electromotive force (emf) is a measurement of the energy that causes current to flow through a circuit. It can also be defined as the potential difference in charge between two points in a circuit. Electromotive force is also known as voltage, and it is measured in volts.

#### Electric Current

Definition: The directed flow of free electrons (or charge) is called electric current.

The flow of electric current can be explained by referring to the adjoining figure. The copper strip has a large number of free electrons. When electric pressure or voltage is applied, then free electrons, being negatively charged, will start moving towards the positive terminal around the circuit as shown in figure. This directed flow of electrons is called electric current.

#### Electric Potential

Definition: The capacity of a charged body to do work is called its electric potential.

Electric potential, V = Work done Charge = $$\frac{W}{Q}$$

S.I unit of electric potential: joules/coulomb or volt(V)

#### Potential Difference

Definition: The difference in the potentials of two charged bodies is called potential difference.

If two bodies have different electric potentials, a potential difference exists between the bodies. Consider two bodies A and B having potentials of 5 volts and 3 volts respectively as shown in adjoining figure. Each coulomb of charge on body A has an energy of 5 joules while each coulomb of charge on body B has an  energy of 3 joules. Clearly, body A is at higher potential than the body B.

If the two bodies are joined through a conductor [See Fig.  (ii)], then electrons will flow from body B to body A. When the two bodies attain the same potential, the flow of current stops. Therefore, we arrive at a very important conclusion that current will flow in a circuit if potential difference exists. No potential difference, no current flow.

Unit: Since the unit of electric potential is volt, one can expect that unit of potential difference will also be volt.

### Ohm’s Law

The ratio of potential difference (V) between the ends of a conductor to the current (I) flowing between them is constant, provided the physical conditions (e.g. temperature etc.) do not change i.e. $$V=I \ast R$$where R is the resistance of the conductor between the two points considered.