Practical circuits

In D.C. (direct current) circuits, electrical components are connected in various ways to control and measure the flow of electric current. Understanding practical circuits involves recognizing standard circuit symbols, drawing and interpreting circuit diagrams, and understanding how sources of e.m.f. (like batteries) behave in real circuits.

Circuit Symbols and Diagrams

• Circuit diagrams use standard symbols to represent components such as cells, batteries, resistors, switches, ammeters, voltmeters, and lamps.
• Drawing and interpreting these diagrams is essential for analyzing and constructing circuits.

Common circuit symbols include:

• Cell: two parallel lines, one longer (positive)
• Battery: multiple cells in series
• Resistor: rectangle
• Variable resistor: rectangle with diagonal arrow
• Switch: break in line with a pivot
• Lamp: circle with a cross inside
• Voltmeter: circle with 'V'
• Ammeter: circle with 'A'

Electromotive Force (e.m.f.)

• The e.m.f. of a source (such as a battery) is the energy supplied by the source per unit charge as it moves charge around a complete circuit.

• SI unit: volt (V), where $1\,\text{V} = 1\,\text{J C}^{-1}$.

Potential Difference (p.d.) vs. e.m.f.

• Potential difference (p.d.) is the energy transferred per unit charge from electrical energy to other forms (e.g., heat, light) as charge passes through a component.
• e.m.f. refers to energy supplied to charge by the source; p.d. refers to energy lost by charge as it passes through a component.

Internal Resistance and Terminal p.d.

• Real sources of e.m.f. (like batteries) have internal resistance ($r$), which causes some energy to be lost as heat inside the source.
• When current ($I$) flows, the terminal potential difference ($V$) is less than the e.m.f. ($E$) due to this internal resistance.

• As current increases, the voltage available to the external circuit decreases.

Summary

• Use correct symbols to draw and interpret circuit diagrams.
• e.m.f. is the energy supplied per coulomb by a source; p.d. is the energy lost per coulomb in a component.
• Internal resistance reduces the terminal p.d. when current flows.
• Understanding these concepts is essential for analyzing real electrical circuits.