4.2.4 Resistance (3)
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1.
Question 3
Describe a method for determining the resistance of a resistor using a variable power supply and a multimeter set to measure resistance. Explain how to ensure accurate measurements.
Method: Using a Variable Power Supply and Multimeter
Apparatus:
- Resistor
- Variable power supply
- Multimeter (set to measure resistance - Ohms)
- Connecting wires
Procedure:
- Connect the resistor to the circuit with the variable power supply.
- Ensure the power supply is switched off.
- Set the multimeter to the appropriate resistance range (start with a high range and decrease if necessary).
- Connect the multimeter in series with the resistor.
- Turn on the power supply and slowly increase the voltage until the multimeter reads a stable resistance value.
- Record the voltage (V) from the power supply and the resistance (R) displayed on the multimeter.
- Calculate the resistance using Ohm's Law: R = V / I. The current (I) will be the current flowing through the resistor at the time of the measurement.
- Repeat the measurement for several different voltage settings to check for consistency.
Ensuring Accurate Measurements:
- Ensure the multimeter is in the correct resistance measurement mode.
- Use a power supply with a stable output voltage.
- Avoid any stray electrical interference.
- Ensure good quality connections between the resistor, power supply, and multimeter.
- Allow the circuit to stabilise before taking a reading.
- Use a resistor with a known value to check the accuracy of the setup.
2.
A student is designing a simple electric heater. They connect a 12V battery to a resistor of 2 ohms. Calculate the power dissipated by the resistor. Explain, in terms of the physics involved, how the electrical energy supplied by the battery is converted into heat energy in the resistor.
Calculation:
Power (P) can be calculated using the formula P = V2 / R, where V is the voltage and R is the resistance.
P = (12V)2 / 2Ω = 144V2 / 2Ω = 72W
Explanation:
When an electric current flows through a resistor, the electrical energy is converted into heat energy through a process called Joule heating. This occurs because the moving electrons in the current collide with the atoms in the resistor's material. These collisions transfer kinetic energy to the atoms, causing them to vibrate more rapidly. This increased vibration manifests as an increase in temperature – hence, heat. The electrical energy is effectively 'lost' as heat in this process. The higher the current or the resistance, the more energy is converted to heat.
3.
Sketch and explain the current-voltage (I-V) graph for a resistor of constant resistance. Clearly label the axes and indicate the physical meaning of the slope of the graph.
Sketch: The I-V graph for a resistor is a straight line passing through the origin. It has a positive slope. The graph should be labelled with 'Current (I)' on the y-axis and 'Voltage (V)' on the x-axis.
Explanation: A resistor obeys Ohm's Law, which states that the current through a resistor is directly proportional to the voltage across it. The relationship is expressed as V = IR, where I is the current and R is the resistance. Therefore, the slope of the I-V graph is equal to the resistance (R). A steeper slope indicates a higher resistance. The straight line through the origin signifies that when the voltage is zero, the current is also zero.