CBSE Class 10 Science Practical Skills – Ohm’s Law
BASIC BUILDING CONCEPTS
Circuit: A closed conducting loop in which electric current flows continuously is called an electric circuit or simple circuit.
Circuit diagram: The schematic representation which shows the arrangement of different devices or components by using their electrical symbols is called a circuit diagram.
The following figure represents an electric circuit having a cell, a resistance, a voltmeter, an ammeter and a closed key.
Electric current: Amount of charge flowing through a particular area in a unit time is called electric current, i.e., the rate of flow of electric charge is called electric current.
If net charge ‘q’ flows across any cross-sectional area in a time ‘t’, then the current flows through the conductor is
The SI unit of current is ampere(A). Flow of one coulomb of charge per second is called one ampere, i.e.
1 A = 1 coulomb per second
Small units of current are
1 mA =10-3 A (mA = milliampere)
1 μA =10-6 A (pA = microampere)
Direction of current: The charge carriers in metallic conductor are free electrons. The motion of these free electrons in a particular direction constitutes an electric current. Conventionally, the direction of electric current is taken as opposite to the direction of the flow of electrons.
In an electric circuit, the direction of conventional current is always taken from the positive terminal of the cell/battery through the various components and then to the negative terminal.
Potential: The work done in bringing a unit positive charge from infinity to a point in an electric field is called potential at that point.
Potential difference: 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.
If W is the work done in bringing a charge q from one point to another, then the potential difference between these two points is given by
V=Work Done (W)/Charge(q)
The unit of work done is joule and that of charge is coulomb. Therefore, the unit of potential difference is volt (V).
1 volt=1 joule(J)/1 coulomb(C) or 1V = 1 JC-1
Resistance: It is the characteristic property of a conducting wire which resists the flow of electric current through it.
From Ohm’s law,
V = IR or R = V/I
Thus, the ratio of potential difference to the current flowing through the conductor is equal to the resistance of the conductor.
The flow of electrons is retarded by the resistance of the conductor. It is expressed in volt/ampere or ohm(Ω).
One ohm is equal to the resistance of a conductor through which a current of one ampere flows when one volt potential difference is applied across its ends. The resistance of a conductor depends on its
- cross-sectional area and
- material of the conductor.
1 Ω = 1 VA-1
DESCRIPTION OF APPARATUS USED
Ammeter: 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. It reads the current directly in ampere or fractions of an ampere,
i. e., in milliampere or micro ampere. One sided deflection in the ammeter is to make it necessary to mark the positive and negative sign of the terminal of the ammeter. Accordingly, positive terminal of the ammeter is always connected to the positive terminal of the cell/ battery eliminator (both are red) and negative terminal to the negative terminal of the cell/battery eliminator in the circuit.
A part of circuit having ammeter and cell is shown here:
Voltmeter: It is a very high resistance device which is used to measure the potential difference between two points in the circuit. It is always placed in parallel across the two points between which the potential difference is to be measured. It is connected in such a way that the positive marked terminal is connected to the high potential end of the conductor while the negative marked terminal to the low potential end of the conductor as shown.
Sliding Rheostat: It is a device which is used as a current controller in the circuit by changing its resistances. It is always placed in series with the circuit as shown here.
To study the dependence of potential difference (V) across a resistor on the current (/) passing through it and determine its resistance. Also plot a graph between V and I.
Nichrome or manganin wire, ammeter, voltmeter, battery eliminator, rheostat, one-way plug key and connecting wires.
Ohm’s Law: If the physical conditions such as temperature, pressure, mechanical strain, etc., remain the same, the current (I) flowing through a conductor is directly proportional to the potential difference (V) across the conductor. Mathematically,
I ∝V or V∝ I or V=IR or R=V/I
The proportionality constant R is called the resistance offered by the conductor to the flow of electric current. If a graph is plotted between the current (I) flows through the conductor and the applied potential difference (V) between its ends, it will be a straight line as shown.
The graph shows that the current through conductor increases linearly as the potential difference across it increases.
- Set up the circuit arrangement as shown in circuit diagram or apparatus arrangement.
- Note the least count of the ammeter and voltmeter.
- Find, if any, zero error of the ammeter and voltmeter and record it in table ‘A’.
- Plug the key/switch on the battery eliminator and adjust the rheostat by sliding its variable terminal till the ammeter and the voltmeter show a reading.
- Note the readings of ammeter and voltmeter in table ‘B\ Take out the plug or switch off the battery eliminator for a moment.
- Repeat the step 4 and 5 for the different values of current by varying the sliding terminal of rheostat.
- Tabulate all the observations in the observation table ‘B’ and find the ratio of V/I for each set of observations. Find the mean value of R.
- Plot a graph by taking / along y-axis and V along v-axis or V along .r-axis and I along y-axis.
Table (A) for ammeter and voltmeter
Table (B) for reding ammeter and voltmeter
1. Find the ratio of V and I for each corrected set of observation.
Mean value of R = ………….Ω
2. Plot the graph between the potential difference ‘V along x-axis and the current T along y-axis for I-V graph as shown or ‘V’ on y-axis and T on x-axis. _
3. Find the slope of line.
(i) For I-V graph,
Slope of line AB =BC/AC=I2-I1/V2-V1 (Conductance)
.’. Resistance of nichrome (or manganin) wire = 1/slope of line AB =………………….Ω
(ii) For V-I graph,
Slope of line AB =BC/AC=I2-I1/V2-V1=Resistance of nichrome (or manganin) wire =………………Ω
- Straight line nature of the I-V graph or V-I graph shows that potential difference across the conductor is directly proportional to the current flowing through it, i.e., V x /. This proves the Ohm’s law graphically.
- The resistance of nichrome (or manganin) wire obtained from the graph is equal (or approximately equal) to the mean calculated value of R. It also verifies the Ohm’s law.
- The resistance of the given wire =………..Ω
- The ends of connecting wire should be neat and clean.
- All connections should be kept tight.
- Positive terminal of ammeter or voltmeter should be connected to positive terminal of the battery or battery eliminator.
- The ends of resistance wire must be connected across the terminals of voltmeter.
- Never allow the current to flow in the resistance wire for a longer time to avoid heating effect of current as R∝ T.
- Range of voltmeter should be greater than the applied voltage.
- A low resistance ranged rheostat must be used.
- The area of cross-section of the connecting wire should be more because it offers negligible resistance.
SOURCE OF ERROR
- Reading error may be possible while observing the pointer of ammeter and voltmeter.
- Thick connecting wires may not be available at the time of performing the experiment.
- Area of cross-section of resistance wire may not be uniform across the length of wire.
- There may be the use of. a high resistance rheostat.
- Current may be allowed for a longer period of time.
- The terminal screw of the instrument may not be tightened properly.
Which basic law of electricity is verified by you from this experiment?
State Ohm’s law.
If physical conditions such as temperature, pressure etc., remain the same, the current flowing through the conductor is directly proportional to the potential difference applied across it.
In which unit current is measured?
It is measured in amperes.
What are the most essential conditions for current flow through the conductor?
- There must be a closed circuit and
- source of electrical energy.
Which physical quantity is represented by the ratio of ‘V and T?
The ratio of V and I represents the resistance of the conductor,i. e. R = V/I
What is the SI unit of resistance?
The SI unit of resistance is ohm (Ω).
Why is it advised that never allow the current to flow for a long time in a resistance wire?
To avoid heating effect of current. As per Joule’s law of heating effect, (H = I2Rt) the wire will be heated up which leads to increase its resistance.
What do you mean by resistance of a conductor?
The obstruction offered by the conductor to the flow of electric current through it is called resistance.
On what factors, does the resistance of wire depend?
- The length of wire
- Area of cross-section of the conductor
- Nature of material
What is least count?
The least quantity that can be measured accurately by any instrument is called its least count.
What is zero error?
If the pointer of the meter (ammeter/voltmeter) does not coincide with zero of the scale, i.e., it reads a little more or less when the circuit is open, this type of error in reading of the scale is called zero error. It is always substracted from the observed reading.
There are two wires, thick and thin. Which one will have greater resistance?
Thin wire as R ∝ l/A
Give the composition of nichrome alloy.
68% Ni, 15% Cr, 15.5% Fe, 1.5% Mn. *
What is a battery eliminator?
It is used in place of cell/battery. With the help of step down transformer higher voltage of an alternating current is converted into low voltage and then converted into the direct current with the help of a rectifier.
Why does the current not flow in the circuit when we take out the plug from key?
The air present in the gap is the bad conductor of electricity. So, the circuit breaks.
NCERT LAB MANUAL QUESTIONS
In this experiment, it is advised to take out the key from the plug when the observations are not being taken. Why?
To avoid unnecessary heating of wire, it is advisable to take out the key from the plug when the observations are not being taken because current produces heating effect and the resistance increases with the increase in temperature.
Suppose the ammeter (or voltmeter) you are using in this experiment does not have positive (+) and negative (-) terminal markings, how will you use such ammeter (or voltmeter) in the circuit?
We will connect the ammeter or voltmeter arbitrary in the circuit and observe the deflection of the pointer. If the pointer strikes the stopper (i.e., before the zero reading of the scale) or deflection occurs in the opposite direction, then by interchanging the terminal connections, we can use these devices properly in the circuit.
If the resistor of a known resistance value is replaced with a nichrome wire of say 10 cm length, how do the values of current through the nichrome wire and potential difference across the two ends of it change? How the values will change if the replaced wire is of manganin in place of nichrome?
The resistivity of an alloy is generally higher than that of metals. Therefore, by replacing the known resistance value resistor with nichrome wire, the value of current through nichrome wire will decrease and potential difference across its two ends will increase.
The resistivity of manganin wire is 44 x10-8 Ωm while that of nichrome wire is 100 x10-8 Ωm. So, manganin wire offers less resistance as compared to the nichrome wire for the same length and same arean of cross-section. Hence, the value of current through the manganm wire will increase and the potential difference across its ends will decrease.
Suppose in this experiment you see that the deflection on ammeter (or voltmeter) scale goes beyond the full scale. What will you infer from such an observation? What will you infer if the deflection takes place in opposite direction?
If the deflection on ammeter (or voltmeter) scale goes beyond the full scale, we can infer that
- the higher range ammeter must be used for measuring the higher value of current in the circuit.
- the applied voltage will be very high so there will be a need of higher range voltmeter to measure the applied voltage.
If deflection takes place in opposite direction, the device is not properly connected in the circuit.
We must interchange the terminal connection, so that devices can be used properly in the circuit.
Why is it advised to clean the ends of connecting wires before connecting them?
To remove the insulating layer, if any, from the ends of the connecting wire.
PRACTICAL RASED QUESTIONS
Multiple Choice Questions/VSA (1 Mark)
A student sets up the circuit for studying the dependence of current (/) flowing on the applied potential difference (V) in the manner shown. The ammeter and the voltmeter in his circuit have been checked and found to be correct. On closing the key K, he observes a deflection in the ammeter but no deflection in the voltmeter. This could be due to a loose connection or break in the wire at
The best choice of open and closed time is that of student
(a)W1 or W2
(b ) W3 or W4
(c)W5 or W6
(d) W6 or W1
Three students X, Y and Z, while performing the experiment to study the dependence of current on the potential difference across a resistor, connect the ammeter (A), the battery (B), the key (K) and the resistor (R) in series, in the following three different orders.
X——>B, K, R, A, B
Y ——> B, A, K, R, B
Z ——> B, R, K, A, B
Who has connected them in the correct order?
(d) All of them
Out of the four circuits shown for studying the dependence of the current on the potential difference across a resistor, the circuit that has been correctly drawn is circuit
A voltmeter has a least count of 0.05 volt. While doing Ohm’s law experiment, a student observed that the pointer of the voltmeter coincides with 15th division, the observed reading is
(a) 0.75 V
(6) 0.075 V
(c) 7.5 V
(d). 75 V
In an experiment to study dependence of current on the potential difference across a given resistor, four students P, Q, R and S kept the plug key in the circuit closed for time t1 and then open
for time t2 as given in the table below:
The best choice of open and closed time is that of student
The following ‘precautions’ were listed by a
student in the experiment on study of ‘Dependence of current on potential difference’: [Delhi 2009]
(A) Use copper wires as thin as possible for making connections.
(B) All the connections should be kept tight.
(C) The positive and negative terminals of the voltmeter and the ammeter should be correctly connected.
(D) The ‘zero error’ in the ammeter and the voltmeter should be noted and taken into consideration while recording the measurements.
(E) The ‘key’ in the circuit, once plugged in, should not be taken out till all the observations have been completed.
The ‘precautions’ that need to be corrected and revised are
(a) (A), (C) and (E)
(b) (C) and (E)
(c) (B) and (E)
(d) (A) and (E)
A student draws the following circuit diagram
for the experiment on studying the dependence of current (/) on p.d. (V) across a resistor. The parts labelled X, Y and Z, in this diagram are, respectively,
(а) a resistor, a voltmeter and a milliammeter
(b) a voltmeter, a resistor and a milliammeter
(c) a milliammeter, a resistor and a voltmeter
(d) a resistor, a milliammeter and a voltmeter
An ammeter has 20 divisions between mark 0 and mark 2 on its scale. The least count of the ammeter is
(a) 0.02 A
(6) 0.01 A
(c) 0.2 A
(d) 0.1 A
In a voltmeter, there are 20 divisions between the 0 mark and 0.5 V mark. The least count of the voltmeter is
(a) 0.020 V
(6) 0.025 V
(c) 0.050 V
(d) 0.250 V
The rest positions of the needles in a milliammeter and voltmeter when not being used in a circuit are as shown in the figure. The ‘zero error’ and ‘least count’ of these two instruments are
(a) (+ 4 mA, – 0.2 V) and (1 mA, 0.1 V respectively
(b) (+ 4 mA, – 0.2 V) and (2 mA, 0.2 V) respectively
(c) (- 4 mA, + 0.2 V) and (2 mA, 0.2 V) respectively
(d) (- 4 mA, + 0.2 V) and (2 mA, 0.1 V) respectively
A student arranged an electric circuit as shown
He would observe
(а) no reading in either the ammeter or the voltmeter
(б) no reading in the voltmeter but a finite reading in the ammeter
(c) no reading in the ar.’. meter but a finite reading in the voltmeter
(d) a finite reading in both the ammeter and the voltmeter
Three students A, B and C carried out measurements of current and potential difference with the help of the ammeter and voltmeter, they did the following
A: viewed the divisions from the left of the pointer
B: viewed the divisions from the right of the pointer
C: viewed the divisions directly above the pointer
The correct procedure followed by the student
(d) none of them
The plane mirror is used in the meters is to
(a) avoid the error of parallax
(b) help to take more accurate observations
(c) reflected light makes the scale illuminated
(d) make the meter look good
In the experiment on studying the dependence of current (I) on the potential difference (V), three students plotted the following graphs between (V) and (I) as per their respective observations.
The observations, likely to be correct, are those of
(a) student I only.
(b) student II only.
(c) student III only,
(d) all the three students.
A student performs an experiment on studying the dependence of the current (7) flowing through a conductor on the potential difference (V) applied across it by setting up his circuit as shown. He records four values of T by keeping the sliding contact J, in the positions K, L, M and N, one by one. The corresponding points on his V-I graph are labelled as Pl; P2, P3 and P4. The point P3, would correspond to the case when the sliding contact, J, is in the position
Short Answer Questions
In an experiment to study the relation between the potential difference across a resistor and the current through it a student recorded the following observations.
On examining the above observations, the teacher asked the students to reject one set of readings as the values were out of agreement with the rest. Which one of the above sets of readings can be rejected? Calculate the mean value of resistance of the resistor based on the remaining four sets of readings.
Name and state the law that connects the electric current flowing through a metallic resistor and potential difference across its two ends. State the condition under which it is valid.
What would be the reading of ammeter and voltmeter in the given circuit?
Mention the condition under which charges can move in a conductor. Name the device which is used to maintain this condition in an electric circuit for verifying Ohm’s law.
A graph between difference (V) and current (I) is given in figure.
- What is the relation between V and I?
- Find the resistance of the conductor using the graph.
Multiple Choice Questions/VSA
Short Answer Questions
1. The third reading for V = 3.0 volt and I – 0.6 A will be rejected as it has larger deviation from the rest of readings.
The value of resistance in the other four observations will be
2. Ohm’s Law: It states that the potential difference (V) across the ends of a given metallic conductor in an electric circuit is directly proportional to the current (I) flowing through it.
Ohm’s law is valid only when the temperature of the conductor remains constant.
3. Using Ohm’s law,
I = 1 A
Therefore, ammeter reading is 1 A.
Voltmeter reading = Potential difference across 3 Q resistor
= 1 3 = 3 V.
4. Electric potential should be maintained across the ends of a conductor to move the charge through it. Cell or battery is the device used to maintain the potential difference across the conductor.
5. (t) A straight line plot shows that as the current through a wire increases, the potential difference across it increases linearly.