Welcome to Current Electricity!
Welcome! In this chapter, we are going to explore how electricity actually moves and how we can control it. This is the heart of the "Electricity" section of your AQA A Level Physics course. Understanding these basics is like learning the rules of a game; once you know them, you can understand how everything from your phone charger to a supercar's motor works!
Don't worry if some of these ideas feel a bit "invisible" at first. We will use plenty of everyday analogies to bring these abstract concepts to life. Let’s dive in!
1. The Basics: Charge, Current, and PD
To understand electricity, we need to look at three main players: Current, Potential Difference (PD), and Resistance.
Electric Current (\(I\))
Current is the rate of flow of charge. Imagine a pipe full of water; the current is how much water flows past a point every second. In a wire, those "water drops" are actually electrons (charge).
The Formula: \(I = \frac{\Delta Q}{\Delta t}\)
Where:
• \(I\) is Current (measured in Amperes, A)
• \(\Delta Q\) is the change in Charge (measured in Coulombs, C)
• \(\Delta t\) is the time interval (measured in seconds, s)
Potential Difference (\(V\))
Potential Difference (often called voltage) is the work done per unit charge. Think of it as the "pressure" pushing the charges through the circuit. If there is no PD, the charges won't want to move!
The Formula: \(V = \frac{W}{Q}\)
Where:
• \(V\) is Potential Difference (measured in Volts, V)
• \(W\) is Work Done or Energy transferred (measured in Joules, J)
• \(Q\) is Charge (C)
Resistance (\(R\))
Resistance is exactly what it sounds like—it's how much a component "resists" the flow of current. It is defined by the ratio of PD to current.
The Formula: \(R = \frac{V}{I}\)
Resistance is measured in Ohms (\(\Omega\)).
Quick Review Box:
• Current = Flow rate of charge.
• PD = Energy given to each bit of charge.
• Resistance = How hard it is for charge to flow.
2. I-V Characteristics and Ohm's Law
How do different components behave when we change the voltage? We use I-V graphs to visualize this.
Ohm’s Law
Ohm's Law states that the current through a conductor is directly proportional to the potential difference across it, provided physical conditions (like temperature) remain constant.
Analogy: If you double the "push" (voltage) on a steady pipe, you get double the water flow (current).
Important Point: Only components that follow this rule are called Ohmic Conductors. Their I-V graph is a straight line through the origin.
Key Components you need to know:
1. Filament Lamp: As the voltage increases, the current increases, which makes the wire hotter. This extra heat causes the metal ions to vibrate more, making it harder for electrons to get past. Therefore, resistance increases as temperature increases. The graph looks like a soft "S" curve.
2. Semiconductor Diode: A diode is like a one-way street for electricity. It has very high resistance in one direction (reverse bias) and very low resistance in the other (forward bias) once you reach a certain "threshold" voltage (usually around 0.6V).
3. Ideal Ammeters and Voltmeters: Unless a question tells you otherwise, assume Ammeters have ZERO resistance and Voltmeters have INFINITE resistance. This ensures they don't mess up the circuit they are measuring!
Key Takeaway: Not everything follows Ohm's Law! Always check if the temperature is changing, as that usually changes the resistance.
3. Resistivity
Resistance depends on the shape of an object (long wires have more resistance than short ones). However, Resistivity (\(\rho\)) is a property of the material itself, regardless of its shape.
The Formula: \(\rho = \frac{RA}{L}\)
Where:
• \(\rho\) (rho) is Resistivity (\(\Omega m\))
• \(R\) is Resistance (\(\Omega\))
• \(A\) is Cross-sectional area (\(m^2\))
• \(L\) is Length (\(m\))
Memory Aid: Think of REPLAY to remember the parts: \(R = \frac{\rho L}{A}\). Resistance is proportional to Length (more road to travel) and inversely proportional to Area (narrower roads are harder to drive through).
Thermistors and Temperature
For your exam, you only need to know about Negative Temperature Coefficient (NTC) thermistors.
• As temperature increases, their resistance decreases.
• Why? Heat gives electrons enough energy to break free from their atoms, increasing the number of charge carriers available to flow.
Superconductivity
Some materials have a "magic" property: if you cool them down below a Critical Temperature (\(T_c\)), their resistance drops to exactly zero!
• Uses: Making incredibly strong magnets (like in MRI scanners) and reducing energy loss in power cables.
Key Takeaway: Resistance is about the object; Resistivity is about the material. Metals get more resistive when hot; NTC thermistors get less resistive when hot.
4. Circuit Rules (Series and Parallel)
When we build circuits, we follow two big conservation laws: Conservation of Charge (Kirchhoff’s 1st Law) and Conservation of Energy (Kirchhoff’s 2nd Law).
Resistors in Series
• Current is the same everywhere.
• Total PD is shared between components.
• Total Resistance: \(R_T = R_1 + R_2 + R_3 + ...\)
Resistors in Parallel
• PD is the same across every branch.
• Total Current is split between branches.
• Total Resistance: \(\frac{1}{R_T} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + ...\)
Power and Energy
Power is the rate at which energy is transferred.
• Energy: \(E = IVt\)
• Power: \(P = IV\) or \(P = I^2R\) or \(P = \frac{V^2}{R}\)
Did you know? Using \(P = I^2R\) explains why power lines use high voltage and low current—it minimizes the energy wasted as heat in the wires!
5. Potential Dividers
A potential divider is a simple circuit that uses two or more resistors in series to "divide" the total voltage. This is how sensors work!
The Concept: The voltage is split in the same ratio as the resistances. If one resistor has double the resistance of the other, it gets double the voltage.
Example: If you replace one resistor with an LDR (Light Dependent Resistor), the output voltage will change depending on the light level. This can be used to turn on a night-light automatically when it gets dark!
Quick Tip: In a potential divider, \(V_{out} = V_{in} \times (\frac{R_{out}}{R_{total}})\).
6. EMF and Internal Resistance
Don't worry if this seems tricky; it's a common area for confusion. Every battery has some "internal resistance" (\(r\)) because the chemicals inside it aren't perfect conductors.
The Terms:
• EMF (\(\epsilon\)): The total energy the battery gives to each Coulomb of charge. It's the "theoretical" voltage printed on the side of the battery.
• Terminal PD (\(V\)): The actual voltage that makes it out of the battery to the rest of the circuit.
• Lost Volts (\(Ir\)): The voltage "wasted" inside the battery due to internal resistance.
The Formula: \(\epsilon = I(R + r)\) or \(\epsilon = V + Ir\)
Analogy: Imagine EMF is your "Gross Pay" on a job. Internal resistance is like "Income Tax." The Terminal PD is your "Take-home Pay"—the money you actually get to spend in the shops (the circuit)!
Key Takeaway: As you draw more current from a battery, the "lost volts" increase, and the terminal PD drops. This is why car headlights might dim slightly when you start the engine!
Final Summary Review
• Current is charge per second (\(I = Q/t\)).
• Ohmic conductors have constant resistance; filament lamps don't.
• Resistivity is a material property (\(\rho = RA/L\)).
• Series: Add resistances. Parallel: Add reciprocals.
• EMF is total energy; Terminal PD is what the circuit actually gets.
You've got this! Keep practicing the formulas, and soon these "invisible" rules will feel like second nature.