Electric Circuit and Ohm’s law

CONTENT

• Definition and functions of electric circuit and its components
• Definition of some physical quantities in dc circuit
• Verification of ohm’s law

Before explaining electric circuit, let us define some terms

1. Conductors:  They are materials which allow electrons to pass through them easily e.g. metal, graphite, acids, salt solution etc.

2.    Semi conductors:   They are materials whose resistivity is mid way between

       good conductors and insulators e.g. germanium, silicon etc

3.    Insulators:   They are materials which do not allow electrons to pass through 

        them e.g. paper, plastic, glass, oil, cotton, dry hair, polythene etc

 Electric Circuit: An electric circuit is a complete path provided for the flow of electric current.  The circuit diagram below is a symbolic representation of such circuit.

Functions of dc circuit components

• Cells are chemical devices which produce electric force/pressure that pushes the current to flow.
• Switch / key is a device used to start or stop the current flow.
• Ammeter measures the electric current flowing in a circuit
• Voltmeter measures the potential difference across the terminal of a load
• Rheostat varies the flow of current
• Resistor is a component that limits or regulate the flow of electric current

EVALUATION

1. Define electric circuit
2. State the functions of the components that make up a circuit

DEFINITIONS OF SOME PHYSICAL QUANTITIES

Electric Current (I): it is the measure of the rate of movement (flow) of charged particles along an electrical conductor (a circuit). It is simple electric charge (Q) in motion which consists of moving electrons.

I = Q/t ____________ (1a)   where t – time (s)

Q = It ………………  (1b)

Potential Difference (V): Potential difference between two points  in a circuit is the work done (W) when one coulomb of charge moves from one point to another.

W = Q (V_B – V_A) = QV ____________ (2a)

V = W/Q   ………………………………(2b)

Electromotive Force (E): e.m.f of a cell is the p.d between the terminals of the cell when it is not delivering any current to the circuit.

Internal Resistance (r): r of a cell is the resistance offered by the electrolyte to the motion of the current.

Resistance (R): R is the ratio of the potential difference (p.d) across the conductor to the current flowing through it. Its unit is the ohm (Ω)

Ohm’s Law

Ohm’s law states that the electric current in a given metallic conductor is directly proportional to the potential difference applied provided that the temperature and other physical factors remain constant i.e V α I

EVALUATION

1. Define the following terms (a) Electric current (b) Potential difference (c) Internal resistance
   1. state ohm’s law

GENERAL EVALUATION

1. State the seven fundamental quantities and their units.
2. State three apparatus  each for measuring mass, distance and time

READING ASSIGNMENT

Read more on electric circuit – New school physics (pg 77 – 80)

WEEKEND ASSIGNMENT

1. The SI unit of electric current is (a) ampere (b) volts (c) ohm’s (d) coulomb
2. The SI unit of electric charge is (a) ampere (b) volts (c) ohm’s (d) coulomb
3. The SI unit of potential difference is (a) ampere (b) volts (c) ohm’s (d) coulomb
4. The SI unit of resistance is (a) ampere (b) volts (c) ohm’s (d) coulomb
5. Ohm’s law states that (a) V=IR (b) Q=It (c) R=IV (d) W=QV

THEORY

1. Define the following terms (a) Electric current (b) Potential difference (c) Internal resistance
   1. Show graphically how the resistance of a resistor can be obtained  from a graph.