Ohms Law

MATERIALS REQUIRED

Resistance wire, ammeter, voltmeter, battery eliminator, rheostat, One-way plug key, and connecting wires.

THEORY

Ohm’s Law states that “If the physical conditions such as temperature, pressure, etc., remain the same during the experiment, then the current (I) flowing is directly proportional to the potential difference (V) across the ends of the circuit.”

Mathematically,

V= IR or

Where,

1. The resistance(R) is the characteristic property of the conductor which resist the flow of electric current through it.

2. The potential difference (V) is the is the potential difference across the ends of a conductor.

3. The electric current (I) is the amount of charge flowing through a particular area in a unit time.

If we plot a graph between the current (I) and the applied potential difference(V) between its ends, for an ideal resistance it will be a straight line as shown:

Circuit Diagram

In the above diagram

Apparatus Arrangement :

The actual diagram of the Ohm’s Law apparatus is shown here:

PROCEDURE

1. By using the circuit diagram or apparatus arrangement, we set up the circuit for finding the dependence of voltage on the current flowing in the circuit.

2. Clean the end of connecting wires using with sandpaper to remove the insulation.

3. Determine the least count of the ammeter and voltmeter and note them under observation section.

4. Check for any zero error in the ammeter and voltmeter and if any record it in table ‘A.’

5. Switch on the battery eliminator, plug the key and adjust the resistance offered by the rheostat by sliding its variable terminal till the ammeter and the voltmeter show a reading.

6. Write the readings of ammeter and voltmeter in the observation table. Take out the key plug out of the circuit to make the circuit open.

7. Repeat the process done in step 4 and 5 for the different values of current by varying the sliding terminal of the rheostat and note down the reading for a respective value of voltage for current in the observation table.

8. Note all the observations in the observation table ‘B’ and then find the ratio of - for each set of observations. Find the mean value of R.

9. Plot a graph by taking I along the y-axis and V along the x-axis.

Observations:

OBSERVATION TABLE

1. Least count of ammeter:

The image of the ammeter is attached here:

The range of the ammeter =500 - 0 mA = 500 mA = 0.5 A

The number of divisions in between two consecutive values= 10

Therefore, the least count =

= 0.01 A

2. Zero error of ammeter:

The needle of the ammeter points towards zero of the main scale of the ammeter.

3. Least count of voltmeter:

The range of voltmeter = 2 - 0 V= 2 V.

The number of division in small scale between two division on the main scale

= 10

Therefore, the least count

4. Zero error of voltmeter = 0 V.

Table (B) for the reading of ammeter and voltmeter

CALCULATION

1. The ratio of V and I for each corrected set of observation is given in the table:

The mean value of Resistance is calculated as:

=2.046 Ω ~ 2.05Ω

2. The graph between the potential difference ‘V’ along the x-axis and the current ‘F along the y-axis for I-Vgraph as shown or V’ on the y-axis and ‘I’ an x-axis is drawn:

3. Find the slope of the line.

(i) For I-V graph.

The slope of a line (Conductance)

Here, slope =

:. Resistance of nichrome (Or manganin) wire =1.97 Ω.

(ii) For the V-I graph,

The slope of Line Resistance of nichrome (or manganin)

RESULT

1. The linear nature of the I-V graph or V-I graph shows that potential difference across the end of the conductor is directly proportional to the current flowing through it,
i.e., V ∝ I. This proves the Ohm’s law graphically.

2. The resistance of the wire obtained from the graph is equal to the mean calculated value of R. It also verifies the Ohm’s law.

3. The resistance of the given wire = 2.05 Ω.

PRECAUTIONS

1. The ends of connecting wire should be rubbed using sandpaper.

2. All connections should be kept clean and tight.

3. The positive terminal of ammeter or voltmeter should be connected to the positive terminal of the battery or battery eliminator.

4. The ends of the resistance wire must be connected across the terminals of the voltmeter.

5. Remove the key out of the circuit when not in use to avoid the heating effect of the circuit.

The circuit diagram is a schematic representation which shows the arrangement of different devices or components by using their electrical symbol is called a circuit diagram.

The basic circuit diagram is given as:

Electric current: It is the amount of charge following through a particular area in a unit time is called an electric current.

The SI unit of current is 1A.

The 1A of current is the flow of 1 coulomb of charge per second.

Ampere is the standard unit of current. The small units of current are:

1. Milli-Ampere: 1 mA = 10^-3A

2. Micro-Ampere: 1Μa = 10^-6 A

The conventional direction of current is taken from positive terminal of the battery through the various circuit components & then to negative terminal.

The functions of the components are given as:

i. Ammeter: It is a measuring instrument used to measure the current in a circuit.

ii. Voltmeter: A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit

iii. Resistor: It is an electrical component that limits or regulates the flow of electrical current in an electronic circuit.

iv. Switch: A device for making and breaking the connection in an electric circuit.

v. Battery: A battery is an electrochemical cell that can be charged electrically to provide a static potential for power.

i. The work done in bringing a unit positive charge from infinity to a point in an electric field is called as Electric potential of a point.

ii. The potential difference between two points in an electric field or across the ends of a conductor is equal to the work done in bringing a charge from one point to another

Resistance: It is the characteristic property of a conducting wire which resists the flow of electric current through it. Its SI unit is ohms.

The factor affecting the resistance of a body are:

1. Length of the wire.

2. Cross-sectional area.

3. The material of the conductor.

4. The temperature of the conductor.

Voltmeter is a very high resistance device which is used to measure the potential difference between two points. It is connected in parallel combination with the resistance across which the voltage drop has to be found.

It is a very low resistance device which is used to measure the strength of the current in a circuit, it is always connected in series in a circuit.

A voltmeter is connected in parallel with a device to measure its voltage, while an ammeter is connected in series with a device to measure its current. The diagram is given below:

The ideal resistance of voltmeter is infinite ohms.

The ideal resistance of ammeter is 0 ohms.

it is a device which is used as a current controller in the circuit by changing its resistance. The Rheostat consists of wounds of nichrome wire in which each wound has a specific area and are connected in series with each other as we slide over this wounded wire the resistance of the rheostat changes.
A picture of rheostat is attached here:

If the physical conditions such as temperature, pressure remain the same, the current flowing through a conductor is directly proportional to the potential difference applied across it.
Mathematically,
V = IR

Where V is the potential difference
I is the current

R is the resistance

The flowing current in the circuit led to the heating effect of current. As per joule law H = I²RT, the wire will be heated up which increases the resistance of wire.

The necessary condition are:

1. The circuit must be closed.

2. There must be a source of electrical energy.

Where there is no connectivity in the medium, the current does not flow. The air present in the gap caused due to removal of plug from the key is the bad conductor of electricity. Hence, the current does no flow.

It is used in place of cell/ battery. With the help of step down transformer higher voltage of an alternating current is converted into a low voltage and then converted into direct current with the help of a rectifier.

You might find battery eliminator in your school lab, it looks like this:

The slope is a measure of the steepness of any line connecting two points.

For a line connecting any two points say P(x₁, y₁) and Q (x₂, y₂), the slope is given as :

More steep graph has more slope. Thus, A has more slope than B.

The resistance of an ideal ammeter is zero.