IB MYP 4-5 Physics- Resistance and Ohm’s law- Study Notes - New Syllabus
IB MYP 4-5 Physics-Resistance and Ohm’s law- Study Notes
Key Concepts
- Resistance and Ohm’s law
Resistance and Ohm’s Law
Resistance and Ohm’s Law
Resistance
Resistance is a measure of how much a component opposes the flow of electric current. A higher resistance means it is harder for current to flow.
Formula:
\( R = \dfrac{\rho l}{A} \)
The unit of resistance is the ohm (Ω). 1 Ω means a component allows 1 A of current when 1 V is applied across it.
Resistance depends on:
- Material – conductors (e.g., copper) have low resistance; insulators (e.g., rubber) have very high resistance.
- Length – longer wires have higher resistance.
- Thickness – thicker wires have lower resistance.
- Temperature – in metals, resistance increases with temperature.
Ohm’s Law
Ohm’s law states that:
\( V \propto I \) (at constant temperature)
This means that the potential difference across a conductor is directly proportional to the current through it, provided its temperature remains constant.
Mathematical form:
\( V = IR \)
- Graph of \( V \) vs \( I \) is a straight line for ohmic conductors (resistors, metals at constant temperature).
- Non-ohmic devices (e.g., filament lamps, diodes) do not follow a straight-line relationship.
Ohmic vs Non-Ohmic Conductors
| Feature | Ohmic Conductor | Non-Ohmic Conductor |
|---|---|---|
| Definition | Follows Ohm’s law, \( V \propto I \) | Does not follow Ohm’s law, \( V \) not proportional to \( I \) |
| Graph of \( V \) vs \( I \) | Straight line through origin | Curved graph |
| Examples | Resistor, metallic wire (constant T) | Filament lamp, diode, thermistor |
Example:
A lamp has a potential difference of 12 V across it, and the current is 3 A. Find the resistance of the lamp.
▶️ Answer/Explanation
Using \( R = \dfrac{V}{I} = \dfrac{12}{3} = 4 \, \Omega \). Final Answer: \( \boxed{4 \, \Omega} \).
Example:
A resistor has a resistance of 15 Ω. If the current through it is 0.4 A, what is the potential difference across it?
▶️ Answer/Explanation
Using \( V = IR = 0.4 \times 15 = 6 \, V \). Final Answer: \( \boxed{6 \, V} \).
Example:
A student measures the current through a wire at different voltages and finds a straight-line graph. Another student measures the same for a filament bulb and gets a curve. Explain why.
▶️ Answer/Explanation
Wire: Temperature remains constant → obeys Ohm’s law → straight-line \( V \) vs \( I \). Bulb: As current increases, filament heats → resistance increases → graph curves.Final Answer: \( \boxed{\text{Wire is ohmic; bulb is non-ohmic.}} \).