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Electricity: Circuits and their Components - An electrical circuit

Grade 7CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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An electric circuit is a closed loop through which electric current flows, consisting of components like a cell (source), a switch (control), and a bulb (load) connected by wires.

A basic electric circuit diagram showing a cell, a switch, and a bulb connected in a loop.
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Symbols are used to represent electrical components in circuit diagrams for simplicity and consistency. For example, a single cell is represented by a long vertical line (positive terminal) and a shorter, thicker vertical line (negative terminal).

Circuit symbol for an electric cell.
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When the switch is in the 'ON' position, the circuit is 'closed' and the path is complete, allowing current to flow and the bulb to glow.

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In the 'OFF' position, the circuit is 'open' or broken, preventing the flow of electricity, and the bulb will not glow.

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A battery is formed by connecting two or more cells together, usually with the positive terminal of one cell connected to the negative terminal of the next.

A battery symbol formed by two cells in series.

📐Formulae

I=QtI = \frac{Q}{t}

Vtotal=V1+V2+V3+⋯+VnV_{total} = V_1 + V_2 + V_3 + \dots + V_n

H=I2RtH = I^2 R t

💡Examples

Problem 1:

Ravi connects four cells in a battery holder to power a toy car. If each cell provides a potential difference of 1.5V1.5 V, calculate the total voltage provided to the toy car using vertical addition.

Solution:

1.51.51.5+1.56.0\begin{array}{r} 1.5 \\ 1.5 \\ 1.5 \\ + 1.5 \\ \hline 6.0 \end{array} The total voltage is 6.0V6.0 V.

Explanation:

In a battery, cells are usually connected in series. Therefore, the total voltage is the sum of the voltages of each individual cell: 1.5V+1.5V+1.5V+1.5V=6.0V1.5 V + 1.5 V + 1.5 V + 1.5 V = 6.0 V.

Problem 2:

In a circuit containing a battery, a switch, and a bulb, the bulb does not glow even when the switch is in the 'ON' position. List two possible reasons involving the circuit components.

Solution:

  1. The bulb might be fused (the filament is broken, creating an open circuit).
  2. The cells might be connected incorrectly (e.g., positive terminal to positive terminal).

Explanation:

For current to flow, the circuit must be 'Closed' and have a continuous conducting path. A fused bulb breaks the path (R=∞R = \infty), and incorrect cell polarity prevents the flow of electrons effectively.

Problem 3:

If a charge (QQ) of 30C30 C passes through a point in a circuit in 1010 seconds (tt), calculate the electric current (II).

Solution:

Given Q=30CQ = 30 C and t=10st = 10 s. Using the formula: I=QtI = \frac{Q}{t} I=3010=3AI = \frac{30}{10} = 3 A

Explanation:

Electric current is defined as the rate of flow of electric charge. By dividing the total charge by the time taken, we find the current in Amperes (AA).

Problem 4:

Draw a circuit diagram where three cells are connected in series to form a battery that powers a single bulb. Ensure the switch is closed.

Circuit diagram with a three-cell battery and a bulb.

Solution:

The diagram represents a battery (three cells) connected to a bulb with a closed switch. The total voltage would be the sum of the three cells.

Explanation:

In a series connection, the positive terminal of one cell connects to the negative of the next. A closed switch ensures the bulb glows.

Problem 5:

A simple electric circuit consists of a battery (formed by two cells), an electric bulb, and a switch in the 'OFF' position. Represent this circuit using a circuit diagram and determine if the bulb will glow in this state.

A circuit diagram showing two cells connected to a bulb with a switch in the open (OFF) position.

Solution:

The circuit is in an 'Open' state because the switch is 'OFF'. In an open circuit, the path for the flow of current is broken.

Current (I)=0\text{Current (I)} = 0

Since no current flows through the filament, the bulb will not glow.

Explanation:

An electric circuit must be a continuous and closed path for current to flow. When the switch is in the 'OFF' position, it creates a gap in the circuit, preventing the flow of electrons from the battery to the bulb.

An electrical circuit Class 7 Notes & Examples