Module 2.1 Ohm\'s Law and Electric Circuits PDF

Preview

Summary

Detailed course notes that includes formulas and tutorial work related to the topic....

Description

OHM'S LAW AND ELECTRIC CIRCUITS

ELECTRIC CIRCUIT An electric circuit is defined as a complete conducting path for current to flow. A simple circuit may consist of a battery, switch and a lamp as shown below.

Pictorial Diagram To simplify the drawing of circuits, each component is represented by a schematic symbol.

Schematic Diagram The battery supplies the electrical energy or electromotive force (E) to the circuit and causes current (I) to flow through the resistance (R) of the lamp. The lamp converts the electrical energy into heat and light energy. The electrons actually flow from the negative to the positive terminal, this is referred to as electron flow However many text books use conventional current which is taken to flow from positive to negative. This convention was established by the pioneers of electrical theory who had no knowledge of the atomic structure of matter. The basic laws of electricity were developed using this current flow and therefore it is still used today. We will use conventional current flow (from positive to negative) for the remainder of this course.

Ohm's Law This is the basic relationship between E, I, and R in an electrical circuit. In any circuit, the current flowing is directly proportional to the EMF and inversely proportional to the resistance. Current =

or

I=

Voltage Resistanc e

E R

Example

If in the circuit shown the battery has an EMF of 12V and the lamp has a resistance of 24 , find the current flowing

Example

How much voltage is necessary to send a current of 10mA through a 5k

Example

What resistance is present in a circuit when a source of 100V produces a current of 50 A?

resistance?

SERIES CIRCUIT When circuit components are connected so that there is only one path for current to flow, they are said to be in series.

Rules for the series circuit 1. Current is the same through each component. 2. R T = R1 + R2 + R 3 3. I =

E RT

Example

If in the circuit shown, E = 24V, R1 = 2 , R2 = 4 , R3 = 6 , calculate the total current and voltage across each resistor.

Note that if the voltage drops are added up the total is 24V. This proves Kirchhoff's Voltage Law which states: - The sum of the voltage drops in a series circuit equals the applied voltage. V1 + V2 + V3 + ..... = E

Voltage Division In a series circuit the voltage drops are proportional to resistance. This can be used to develop a method for directly calculating the voltage drop across a resistor without calculating the current.

R T = R1 + R 2 + R3 E I= RT

V1 = I R 1 =

E RT

x R 1=

R1 xE RT

V 2 = I R2 =

R2 x E RT

V 3 = I R3 =

R3 xE RT

In general, for any series circuit, the voltage drop across a particular resistor can be found from the following relationship:

Resistor Voltage =

Resistor Value x Total Voltage Total Resistance

Example

Calculate the voltage drops in the following circuit....