GCSE Physics - V = IR Equation & Current/Potential Difference Graphs #15

Summary

This video delves into Ohm's Law, expressed as V=IR, where voltage equals current times resistance. It explains how altering voltage or current proportionally affects the other in a circuit with constant resistance. The script discusses the linear relationship in graphs of current versus voltage for simple circuits with resistors or wires, and how resistance changes with temperature. It contrasts this with non-linear graphs of filament lamps due to increased resistance from heat and diodes, which only allow current flow in one direction.

Takeaways

• ⚡ v = ir represents the relationship between voltage (v), current (i), and resistance (r).
• 🔋 In a circuit with a current of 4 amps and resistance of 2 ohms, the voltage is 8 volts (4 x 2).
• 🧮 Rearranging the equation to r = v / i, resistance can be calculated, such as 24 volts / 8 amps = 3 ohms.
• 📈 As long as resistance stays constant, voltage and current increase proportionally.
• 🔄 The graph of current versus potential difference shows a straight line when resistance remains constant, even if the battery is reversed.
• 📏 Larger resistors produce less steep lines on the graph, while smaller resistors produce steeper lines.
• 🌡️ Resistance increases with temperature, which can cause deviations from the ideal straight line when wires heat up.
• 💡 Filament lamps (light bulbs) heat up, increasing resistance and causing the graph to curve at higher currents.
• 🔌 Diodes only allow current to flow in one direction, showing current only when the potential difference is positive.
• 🚫 Diodes have high resistance in the reverse direction, effectively blocking current flow in that direction.

Q & A

What does the equation V = IR represent?

The equation V = IR represents the relationship between voltage (V), current (I), and resistance (R). It shows that the potential difference (voltage) is equal to the current multiplied by the resistance.

How can you calculate voltage if the current and resistance are known?

You can calculate voltage using the equation V = IR. For example, if the current is 4 amps and the resistance is 2 ohms, the voltage would be 4 times 2, which equals 8 volts.

How do you rearrange the equation V = IR to calculate resistance?

To calculate resistance, you rearrange the equation to R = V / I. For instance, if the voltage is 24 volts and the current is 8 amps, the resistance would be 24 divided by 8, which equals 3 ohms.

What happens to current when the voltage increases in a circuit with constant resistance?

If the resistance remains constant, increasing the voltage causes the current to increase proportionally. The relationship between voltage and current is linear in this case.

What does the negative part of the current versus voltage graph represent?

The negative part of the graph represents the scenario where the battery is connected in reverse. In this case, both the voltage and current are negative, but the relationship between them remains the same.

How does the resistance affect the steepness of the line on a current vs. voltage graph?

In a circuit with only wires or resistors, a higher resistance results in a less steep line, while a lower resistance results in a steeper line. This shows how much potential difference is needed to produce a given current.

What effect does temperature have on resistance and current in a circuit?

As the temperature increases, the resistance also increases, which can cause the current to decrease. In practice, higher currents can cause wires to heat up, which may alter the ideal straight-line relationship.

What happens to the resistance of a filament lamp as it heats up?

As the filament lamp heats up, its resistance increases. This causes the current vs. voltage graph to become less steep at higher voltages, meaning less current flows as the potential difference increases.

What is the key characteristic of a diode regarding current flow?

A diode only allows current to flow in one direction. It has very high resistance in the reverse direction, which prevents current from flowing when the potential difference is negative.

Why might the lines in a current vs. voltage graph not always be perfectly straight in practice?

In practice, the lines might not be perfectly straight because real-world factors, like temperature increases due to higher currents, can change the resistance and alter the ideal linear relationship.