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30 Dec 2025
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Understanding A-Level Physics: Direct Current Circuits Explained
1 / 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!
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!
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!
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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The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
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This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
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Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
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Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
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The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
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In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
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very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
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I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
Understanding 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... Show more
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!
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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!
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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!
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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...
What is the Knowunity AI companion?
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.
Where can I download the Knowunity app?
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The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user