4.2.1 Current, Potential Difference, and Resistance

In electricity, we study the movement of charge. To understand how circuits work, you must be able to define and calculate the relationship between current, potential difference, and resistance.

1. Standard Circuit Symbols

Before analyzing circuits, you must recognize the standard symbols used by AQA. These represent components in a way that is universally understood.

2. Electrical Charge and Current

Electric current is the rate of flow of electrical charge. In a metal wire, this charge is carried by electrons.

1. Definition: Current is the flow of electrical charge per unit time.
2. Condition for Flow: For electrical current to flow, there must be a closed loop (a complete circuit) and a source of potential difference (like a cell or battery).
3. Unit: Amperes (A), measured using an ammeter connected in series.

The Charge Equation

The size of the electric current is the rate of flow of electrical charge.

Q = I * t

Where:

1. Q is the charge flow, in coulombs (C)
2. I is the current, in amperes (A)
3. t is the time, in seconds (s)

Edge Case: Always check your units! Examiners often give time in minutes. You must convert this to seconds before using the formula.

3. Potential Difference and Resistance

Potential Difference (V), often called voltage, is the energy transferred per unit charge passed between two points. It is measured in Volts (V) using a voltmeter connected in parallel across a component.

Resistance (R) is a measure of how much a component opposes the flow of current. It is measured in Ohms ($\Omega$).

The Ohm’s Law Equation

The potential difference, current, and resistance are linked by the following equation:

V = I * R

Where:

1. V is the potential difference, in volts (V)
2. I is the current, in amperes (A)
3. R is the resistance, in ohms ($\Omega$)

4. Resistors and I-V Characteristics

The resistance of a component can stay constant or change as the current through it changes.

Ohmic Conductors

For an Ohmic conductor (like a fixed resistor at a constant temperature), the current is directly proportional to the potential difference. This means the resistance remains constant.

Non-Ohmic Components

For other components, the resistance changes as the current changes:

1. Filament Lamp: As current increases, the temperature increases, which causes the resistance to increase.
2. Diode: Current only flows in one direction. It has a very high resistance in the reverse direction.

5. Mathematical Example

Question: A component has a resistance of $15\Omega$. If a potential difference of $12V$ is applied across it, calculate the current flowing through the component.

Step 1: Identify the variables

1. V = 12V
2. R = $15\Omega$

Step 2: Rearrange the formula ($V = IR$) for $I$

$I=\frac{V}{R}$

Step 3: Substitute and solve

$I=\frac{12}{15}=0.8A$

Answer: The current is 0.8 A.

6. Special Resistors (LDRs and Thermistors)

AQA requires you to know two specific types of resistors where resistance changes based on external conditions:

1. LDR (Light Dependent Resistor): In bright light, resistance falls. In darkness, resistance is highest. (Used in outdoor night lights).
2. Thermistor: In high temperatures, resistance falls. In low temperatures, resistance is highest. (Used in thermostats).

Exam Tip: Remember "LURD" for LDRs: Light Up, Resistance Down!