In the previous section we saw how a cell provides the required electromotive force. In this section, we will see combination of cells. Later in this section, we will see the methods of connecting voltmeter and ammeter.
First we will see the method for measuring current. We will write it in steps:
1. We know that electric current is the flow of charges.
• So if more charges flow, can we say that there is more current?
• To answer this question, we must introduce 'time' also into the calculations.
2. Consider two different circuits A and B. Each has a conductor connected to a cell. This is shown in fig.8.9 below:
• Let in a time of 5 seconds, QA charge flow through the conductor in the circuit A
• Let in a time of 5 seconds, QB charge flow through the conductor in the circuit B
3. Note that time is the same 5 seconds. So if QA > QB, we can say that there is more current in circuit A
• So current can be defined as the charge flowing in one second through a conductor.
4. The unit for charge is coulomb. It's symbol is C
• The unit of time is seconds. It's symbol is s
• If in a time of 5 s, 10 C charge flows through a conductor, then:
The charge which flows in one second is 10⁄5 = 2
5. So '2' is the charge which flows in one second and so it is the current.
• In other words:
Current in that conductor = charge⁄time = 10⁄5 = 2 coulombs⁄second ⟹ 2 C⁄s
■ Current is the quantity of charge that flows through a conductor in a circuit in one second
6. This 'C⁄s' is given a special name: Ampere. It's symbol is A
• So we say: The current in that conductor is 2 amperes or 2 A
7. The device used to measure current is called ammeter.
• Some images of can be seen here. The position of the needle gives the current in the circuit.
Now we can discuss about combination of cells
■ A cell is a single unit which produces electrical energy. But a battery is a combination of two or more cells.
The connection between the cells can be done in two ways:
I. Series connection
• In this method, the cells are connected one after the other
• The positive of one cell is connected to the negative of another cell
• This is shown in fig.8.10(a) below. It shows 3 cells connected in series.
• Fig.8.10(b) shows the symbolic representation of three cells connected in series
■ The fig.8.11 below explains the 'symbolic representation of cells':
The explanation can be written in steps:
1. In fig.8.11(a) we see two vertical lines.
• The longer vertical line indicates the positive terminal of the cell
• The shorter vertical line indicates the negative terminal of the cell
2. Also in fig.8.11(a), the horizontal lines indicate the leads taken from the terminals of the cell
3. When the cells are connected in series, the symbolic representation will be as shown in fig.8.11(b)
• The longer vertical lines and shorter vertical lines are drawn alternately
♦ This indicates the positive of one cell being connected to the negative of another cell
II. Parallel connection
In this method, similar poles are connected together. This is shown in fig.8.12(a) below:
• Fig.8.12(b) shows the symbolic representation
• In the discussion so far in this chapter, we have seen two devices:
♦ Voltmeter to measure the potential difference between two points in a circuit
♦ Ammeter to measure the current flowing through a circuit
• Now we will see how each of these devices are connected to a circuit
• First we will see the voltmeter. We will write the connection details in steps:
1. Consider the circuit in fig.8.13 below:
• The switch is turned on. So current will flow through the circuit. This current is indicated by the letter 'I'
• When 'I' passes through the bulb, it will glow.
2. We know that, for the current I to pass through the bulb, there must be a potential difference between the two points X and Y.
• We want to know this potential difference.
• For that, we will use the voltmeter.
3. But how do we connect the voltmeter into the circuit?
• Consider fig.8.14(a) below:
• The voltmeter is connected in series with the bulb.
• In this connection, the voltmeter will surely give us a reading.
• But the reading is useless to us. Let us see the reason:
4. The current I flows from the positive terminal of the battery to it's negative terminal.
• This direction is indicated by the arrow marks.
5. During this flow, some energy will be lost in the form of heat energy.
• So the actual energy supplied by the battery will not be available at every part of the circuit.
• As the distance from the battery increases, energy loss will be more
6. Now consider the voltmeter in the fig.a. The reading in it will correspond to the potential difference between it's own terminals.
• We want the potential difference between X and Y
7. The current leaves the voltmeter meter and has to flow some more distance to reach X and Y.
• During this flow, there will be a further 'drop in potential'.
• So we will not get the required value between X and Y.
• Thus the reading is useless to us
8. Now consider fig.8.14(b)
• The voltmeter is connected in parallel with the bulb.
• The leads from the voltmeter are connected to X and Y
• In this connection, we will get the potential difference between X and Y
• So this is the correct method. A voltmeter should always be connected in parallel.
■ Note that in fig.b, the current I splits into I1 and I2 at the point X. But they combine together to form I at Y
Now we will see how the ammeter is connected. We will write the steps:
1. Consider the same fig.8.13 that we saw previously. This time, we want to know the current flowing through the bulb.
2. Consider fig.8.15(a) below:
• We can see that the current I in the circuit is flowing through the ammeter also.
• So it will give the correct reading
3. But in fig.8.15(b), only the portion I1 is flowing through the ammeter.
• This is because, the current I splits into I1 and I2 at X.
• So this connection will not give the correct reading.
• An ammeter should always be connected in series
First we will see the method for measuring current. We will write it in steps:
1. We know that electric current is the flow of charges.
• So if more charges flow, can we say that there is more current?
• To answer this question, we must introduce 'time' also into the calculations.
2. Consider two different circuits A and B. Each has a conductor connected to a cell. This is shown in fig.8.9 below:
 |
| Fig.8.9 |
• Let in a time of 5 seconds, QB charge flow through the conductor in the circuit B
3. Note that time is the same 5 seconds. So if QA > QB, we can say that there is more current in circuit A
• So current can be defined as the charge flowing in one second through a conductor.
4. The unit for charge is coulomb. It's symbol is C
• The unit of time is seconds. It's symbol is s
• If in a time of 5 s, 10 C charge flows through a conductor, then:
The charge which flows in one second is 10⁄5 = 2
5. So '2' is the charge which flows in one second and so it is the current.
• In other words:
Current in that conductor = charge⁄time = 10⁄5 = 2 coulombs⁄second ⟹ 2 C⁄s
■ Current is the quantity of charge that flows through a conductor in a circuit in one second
6. This 'C⁄s' is given a special name: Ampere. It's symbol is A
• So we say: The current in that conductor is 2 amperes or 2 A
7. The device used to measure current is called ammeter.
• Some images of can be seen here. The position of the needle gives the current in the circuit.
Now we can discuss about combination of cells
■ A cell is a single unit which produces electrical energy. But a battery is a combination of two or more cells.
The connection between the cells can be done in two ways:
I. Series connection
• In this method, the cells are connected one after the other
• The positive of one cell is connected to the negative of another cell
• This is shown in fig.8.10(a) below. It shows 3 cells connected in series.
 |
| Fig.8.10 |
■ The fig.8.11 below explains the 'symbolic representation of cells':
 |
| Fig.8.11 |
1. In fig.8.11(a) we see two vertical lines.
• The longer vertical line indicates the positive terminal of the cell
• The shorter vertical line indicates the negative terminal of the cell
2. Also in fig.8.11(a), the horizontal lines indicate the leads taken from the terminals of the cell
3. When the cells are connected in series, the symbolic representation will be as shown in fig.8.11(b)
• The longer vertical lines and shorter vertical lines are drawn alternately
♦ This indicates the positive of one cell being connected to the negative of another cell
II. Parallel connection
In this method, similar poles are connected together. This is shown in fig.8.12(a) below:
 |
| Fig.8.12 |
• In the discussion so far in this chapter, we have seen two devices:
♦ Voltmeter to measure the potential difference between two points in a circuit
♦ Ammeter to measure the current flowing through a circuit
• Now we will see how each of these devices are connected to a circuit
• First we will see the voltmeter. We will write the connection details in steps:
1. Consider the circuit in fig.8.13 below:
 |
| Fig.8.13 |
• When 'I' passes through the bulb, it will glow.
2. We know that, for the current I to pass through the bulb, there must be a potential difference between the two points X and Y.
• We want to know this potential difference.
• For that, we will use the voltmeter.
3. But how do we connect the voltmeter into the circuit?
• Consider fig.8.14(a) below:
 |
| Fig.8.14 |
• In this connection, the voltmeter will surely give us a reading.
• But the reading is useless to us. Let us see the reason:
4. The current I flows from the positive terminal of the battery to it's negative terminal.
• This direction is indicated by the arrow marks.
5. During this flow, some energy will be lost in the form of heat energy.
• So the actual energy supplied by the battery will not be available at every part of the circuit.
• As the distance from the battery increases, energy loss will be more
6. Now consider the voltmeter in the fig.a. The reading in it will correspond to the potential difference between it's own terminals.
• We want the potential difference between X and Y
7. The current leaves the voltmeter meter and has to flow some more distance to reach X and Y.
• During this flow, there will be a further 'drop in potential'.
• So we will not get the required value between X and Y.
• Thus the reading is useless to us
8. Now consider fig.8.14(b)
• The voltmeter is connected in parallel with the bulb.
• The leads from the voltmeter are connected to X and Y
• In this connection, we will get the potential difference between X and Y
• So this is the correct method. A voltmeter should always be connected in parallel.
■ Note that in fig.b, the current I splits into I1 and I2 at the point X. But they combine together to form I at Y
Now we will see how the ammeter is connected. We will write the steps:
1. Consider the same fig.8.13 that we saw previously. This time, we want to know the current flowing through the bulb.
2. Consider fig.8.15(a) below:
 |
| Fig.8.15 |
• So it will give the correct reading
3. But in fig.8.15(b), only the portion I1 is flowing through the ammeter.
• This is because, the current I splits into I1 and I2 at X.
• So this connection will not give the correct reading.
• An ammeter should always be connected in series
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