DC circuits are everywhere around you - from your phone...
Physics443 views·Updated 5 Jul 2026·4 pagesUnderstanding A-Level Physics: Direct Current Circuits Explained
1 / 4
1
of 4
Electric Circuits and Conservation Laws
Current is simply the rate at which electric charge flows through a circuit - think of it like water flowing through pipes. When you measure current with an ammeter, you must wire it in series because it needs to count all the charge flowing through that point.
In parallel circuits, something brilliant happens with charge conservation. The current splits at junctions, but the total current going in always equals the total coming out: I₁ + I₂ = I_T. It's like a river splitting into tributaries - the total water flow remains constant.
Voltage represents potential energy per unit charge, whilst potential difference is the energy difference between two points. In series circuits, energy conservation means all the voltage drops across components add up to the source voltage: V₁ + V₂ = V_T.
Key insight: Current splits in parallel, voltage splits in series - remember this and you'll master circuit analysis!
2
of 4
Resistance and Potential Dividers
Resistance behaves differently depending on how components are connected. In series circuits, resistances simply add up: R_total = R₁ + R₂ + R₃. However, in parallel circuits, you use reciprocals: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃.
Potential dividers are incredibly useful circuits that split voltage based on resistance values. The voltage across each resistor depends on its proportion of the total resistance. Using the formula V = V_s × R/, you can calculate exactly how voltage divides.
The beauty of potential dividers lies in their stability - changing the supply voltage doesn't affect the voltage ratios between components. This makes them perfect for creating reference voltages in electronic devices.
Pro tip: Potential dividers are like sharing pizza - the bigger resistor gets the bigger slice of voltage!
3
of 4
LDRs, Thermistors, and EMF
Light Dependent Resistors (LDRs) and thermistors are brilliant components that change resistance with environmental conditions. As temperature increases, a thermistor's resistance decreases, whilst an LDR's resistance drops when more light hits it.
These components work perfectly in potential divider circuits for sensing applications. When a thermistor heats up, its resistance falls, so less voltage drops across it and more across the fixed resistor. This creates automatic temperature sensing systems.
Electromotive force (EMF) represents the energy a cell gives to each coulomb of charge. However, real batteries have internal resistance that reduces the voltage available to your circuit. The terminal voltage you actually get is always less than the EMF.
Real-world connection: Your phone battery voltage drops when you're gaming heavily - that's internal resistance in action!
4
of 4
Internal Resistance Calculations
The relationship between EMF, terminal voltage, and internal resistance follows a simple equation: E = V + Ir. This means your usable voltage (V) equals the EMF minus the voltage lost to internal resistance.
Rearranging gives you V = E - Ir, which looks like the straight-line equation y = mx + c. When you plot terminal voltage against current, the y-intercept gives you the EMF, whilst the gradient tells you the internal resistance (as a negative value).
This graphical method is incredibly powerful for finding a battery's internal resistance experimentally. As current increases, terminal voltage decreases linearly - a clear sign that internal resistance is stealing some of your battery's energy.
Exam tip: Remember the gradient is negative - higher current means lower terminal voltage due to internal resistance losses!
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Physics443 views·Updated 5 Jul 2026·4 pagesUnderstanding A-Level Physics: Direct Current Circuits Explained
DC circuits are everywhere around you - from your phone charger to car batteries. Understanding how current flows and voltage behaves in these circuits is crucial for your physics studies and will help you make sense of the electronics you...
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of 4
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Electric Circuits and Conservation Laws
Current is simply the rate at which electric charge flows through a circuit - think of it like water flowing through pipes. When you measure current with an ammeter, you must wire it in series because it needs to count all the charge flowing through that point.
In parallel circuits, something brilliant happens with charge conservation. The current splits at junctions, but the total current going in always equals the total coming out: I₁ + I₂ = I_T. It's like a river splitting into tributaries - the total water flow remains constant.
Voltage represents potential energy per unit charge, whilst potential difference is the energy difference between two points. In series circuits, energy conservation means all the voltage drops across components add up to the source voltage: V₁ + V₂ = V_T.
Key insight: Current splits in parallel, voltage splits in series - remember this and you'll master circuit analysis!
2
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Resistance and Potential Dividers
Resistance behaves differently depending on how components are connected. In series circuits, resistances simply add up: R_total = R₁ + R₂ + R₃. However, in parallel circuits, you use reciprocals: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃.
Potential dividers are incredibly useful circuits that split voltage based on resistance values. The voltage across each resistor depends on its proportion of the total resistance. Using the formula V = V_s × R/, you can calculate exactly how voltage divides.
The beauty of potential dividers lies in their stability - changing the supply voltage doesn't affect the voltage ratios between components. This makes them perfect for creating reference voltages in electronic devices.
Pro tip: Potential dividers are like sharing pizza - the bigger resistor gets the bigger slice of voltage!
3
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LDRs, Thermistors, and EMF
Light Dependent Resistors (LDRs) and thermistors are brilliant components that change resistance with environmental conditions. As temperature increases, a thermistor's resistance decreases, whilst an LDR's resistance drops when more light hits it.
These components work perfectly in potential divider circuits for sensing applications. When a thermistor heats up, its resistance falls, so less voltage drops across it and more across the fixed resistor. This creates automatic temperature sensing systems.
Electromotive force (EMF) represents the energy a cell gives to each coulomb of charge. However, real batteries have internal resistance that reduces the voltage available to your circuit. The terminal voltage you actually get is always less than the EMF.
Real-world connection: Your phone battery voltage drops when you're gaming heavily - that's internal resistance in action!
4
of 4Sign up to see the content. It's free!
- Access to all documents
- Improve your grades
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By signing up you accept Terms of Service and Privacy Policy
Internal Resistance Calculations
The relationship between EMF, terminal voltage, and internal resistance follows a simple equation: E = V + Ir. This means your usable voltage (V) equals the EMF minus the voltage lost to internal resistance.
Rearranging gives you V = E - Ir, which looks like the straight-line equation y = mx + c. When you plot terminal voltage against current, the y-intercept gives you the EMF, whilst the gradient tells you the internal resistance (as a negative value).
This graphical method is incredibly powerful for finding a battery's internal resistance experimentally. As current increases, terminal voltage decreases linearly - a clear sign that internal resistance is stealing some of your battery's energy.
Exam tip: Remember the gradient is negative - higher current means lower terminal voltage due to internal resistance losses!
We thought you’d never ask...
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
You can download the app from Google Play Store and Apple App Store.
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
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