Lab 3: Resistance Measurements and Ohms Law

Lab 3: Resistance Measurements and Ohms Law

By:

Eng. Nael S. Dokhan

Eng. Malak E. Al-Ashi

The Islamic University of GazaElectrical Engineering & Smart Systems DepartmentEELE 2110Electric Circuits Laboratory

05/10/2020

• Objective:

• To measure and calculate resistors by several methods.

• Approve ohm’s law practically.

• Learn how to use the various instruments.

Theoretical Background

Instruments that will be used in this lab

1. Multimeter:

• Multimeters are designed to measure several variables, typically voltage, current and resistance.

• In this lab we will use a digital multimeter. It consists of :

• Ammeter which is used to measure A.C or D.C current passing in a branch and is connected in series with the circuit’s elements.

• Voltmeter for measuring the A.C or D.C voltage drop a cross any two point in the circuit, and is connected in parallel.

• Ohmmeter for measuring the resistance, and is connected across theresistant.

Dc Power Supply

Electrical measurements

• All basic electrical circuits consist of three separate but very much related electrical quantities called: Voltage ( V ) Current ( I ) and Resistance ( R ).

Ohms Law Relationship

• By knowing any two values of the Voltage, Current or Resistance quantities we can use Ohms Law to find the third missing value.

Electrical Power in Circuits

• Electrical Power, ( P ) in a circuit is the rate at which energy is absorbed or produced within a circuit.

• A source of energy such as a voltage will produce or deliver power while the connected load absorbs it.

• To find the Power (P):

[ P = V x I ] P (watts) = V (volts) x I (amps) Also:

[ P = V2 ÷ R ] P (watts) = V2 (volts) ÷ R (Ω) Also:

[ P = I2 x R ] P (watts) = I2 (amps) x R (Ω)

Ohms Law Pie Chart

DC measurements

• It includes the average value of a time-varying quantity.

AC measurements

• It includes the rms(root mean square)value of a time-varying(usuallyperiodic)quantity.

Multiples and Sub-multiples

Measurement of Resistance

1. Technical method: It depends on the color of resistance.

• Three bands indicate the nominal value of the resistor and the fourth band indicates the tolerance in value.

• The tolerance band is typically either gold or silver.

• A gold tolerance band indicates that the measured value will be within 5% of the nominal value. The silver tolerance band indicates that the measured value will be within 10% of the nominal value.


Example

Measurement of Resistance2. Manufactured method: Electrical resistance of an electrical conductor depends on:

• the length of the conductor.

• the material of the conductor.

• the temperature of the material.

• the cross-sectional area of the conductor.

• And can be expressed as:

R =𝜌 𝐿

𝐴

• R = resistance of the conductor (Ω).

• ρ: The resistivity of the material conductor (Ω.m).

• L = length of conductor (m).

• A = cross-sectional area of conductor (m2).

Experimentally


3. Experimental method:

I. By ohms law.

II. By using Ohmmeter.

III. By using Wheatstone Bridge.


I. By ohms law:

• Y=mx+a

• When the resistance is constant, the current is directly proportional with voltage.


a- Connect the circuit as shown.

b- Set the resistor to 1KΩ.

c- Record the value of I(ma) for the voltages in the table.

d- Compute the values of R(KΩ) using OHM Law.

Voltage(v) Current(mA) R

1 v 0.921 mA 1k Ω

2 v 1.835 mA 1k Ω

3 v 2.97 mA 1k Ω

4 v 4.04 mA 1k Ω

5 v 5.04 mA 1k Ω


0.921

1.835

2.97

4.04

5.04

Vo

ltag

e(v)

Current (mA)

Let’s try on LTspice


1 v

2 v

3 v

4 v

5 v

Plot the curve on LTspice


II. By using Ohmmeter.

1. Connect the circuit as shown.

2. Measure the value of R (KΩ) directly by connecting the digital multimeter in parallel with the resistor and using it as a ohmmeter.

HW Lab3

a- Connect the circuit as shown.b- Set the resistors to 300Ω and 1200Ω.c- Record the value of I(ma) for the voltages inthe table.(from Ltspice)d- Compute the values of R(Ω) using Ohm’s Law.e. Write your comment.


3 v

6 v

9 v

12 v

15 v

HW Lab3

a- Connect the circuit as shown.b- Set the resistors to 300Ω and 1200Ω.c- Draw the graph between V&I.d. Find the slope.e. Write your comment.

Deadline: 12/10/2020 at 11:59 PM

Good Luck

Documents

Lab 3: Resistance Measurements and Ohms Law