## 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.