Resistance | AP Physics C Electricity and Magnetism - Grade 9 PDF Download

Ohm's Law

• Resistance is the opposition to current. 
  • For a given potential difference, the higher the resistance, the lower the current. 
  • Therefore, resistors are used in circuits to control the current. 
  • The unit of resistance is the ohm, represented by the Greek symbol omega (Ω).
  • It can be calculated using the formula
    R = V/I, 
• where,
  • R = resistance (ohms, Ω) 
  • V = potential difference (volts, V)
  • I = current (amperes, A)
• Ohm's Law: According to Ohm's Law, the current flowing through a conductor is directly proportional to the potential difference applied across it, as long as the temperature remains constant.

Consequences of Ohm's Law

• Resistors serve the purpose of regulating either:
  • The current flowing through specific components in the circuit.
  • The potential difference (voltage) across particular components.
• Resistors are crucial due to the implications of Ohm's Law:
  • When the resistance of an electrical conductor increases (with a constant voltage), the current flowing through it decreases.
  • When the resistance of an electrical conductor increases (with a constant current), the voltage across it increases.
• Specifically, resistors help in controlling:
  • The current in branches of the circuit, directing it through specific components.
  • The potential difference across certain components, modulating the voltage they experience.

I-V Graphs for Ohmic Resistors, Filament Lamps & Diodes

• When the voltage across a component increases, the current through it also increases. This relationship is depicted by an I-V graph.
• The I-V graph illustrates the varying relationship between voltage and current for different components.

• The current-voltage (IV) graph of a resistor is straightforward: current is directly proportional to the potential difference across it.
• This proportionality arises because the resistor maintains a constant resistance.
• In contrast, the relationship for a lamp is more intricate:
  • The current's growth is proportionally subdued compared to the potential difference.
• The reason behind this lies in the following:
  • Electrical current heats the lamp's filament.
  • As the filament temperature rises, its resistance also increases.
  • This resistance increment opposes the current flow, resulting in a slower rate of increase in current.

• A diode functions as a non-ohmic conductor, permitting the flow of current exclusively in one direction. This unidirectional conductive property is visually represented by a triangular arrow within the diode symbol, denoting forward bias.
• Forward bias in a diode indicates the condition where current flows easily due to the low resistance in that direction.
• Conversely, in reverse bias, the diode presents high resistance, impeding the flow of current significantly.
• The I–V (current-voltage) graph of a diode exhibits a distinctive shape. When the diode operates in forward bias, the graph displays a sharp incline in both voltage and current on the right side. In contrast, during reverse bias, the graph maintains a flat line, signifying zero current across all voltage levels on the left side.