Electricity: Direct current and circuits
Electricity is perhaps the most disliked and least well understood topic in A-level Physics. While it can be difficult, by learning the following it should be approachable and nowhere near as bad as you may expect. Please see the videos accompanying this article on my profile to get examples of these principles in practice, in order to see how easy they make circuit questions.
Voltage, current, resistance
Understanding how to think of these three properties of circuits is key an excellent analogy to use is the idea of water flowing through pipes.
Current is the flow of electrons around a circuit current is the amount of water flowing through the pipes. Since it is measuring the amount of water flowing, it can be measured at any single point.
An ammeter is used to measure the amount of current flowing at a point we don t want the addition of an ammeter to a circuit to change the circuit we are measuring, so they are designed to effectively have zero resistance and are always just put in series with components that are already there.
We do not connect ammeters in parallel with a circuit because doing so not only would not measure the flow of current at a particular point in the circuit, but also would provide a new path for water to flow down and so change the circuit - essentially it would be adding a new very large pipe.
Because current is like the flow of water in pipes, any junction where the circuit splits is like a pipe splitting into two separate pipes - the total water into a junction is equal to the total water out of the junction - this is why current splits in parallel circuit setups. How the water splits down both pipes depends on how thick each pipe is (e.g. the resistance) - if one path has double the resistance of the other it is like a pipe that is half as thick. Water flows down both paths, but double the water flows down the larger (lower resistance) pipe compared to the thinner pipe.
Water does not build up at any point in a circuit, so the total water flowing out of the battery is the same as the amount of water flowing back in at the end - hence current is the same at any point in a circuit, as long as it has not split down multiple paths.
Resistance is the thickness of the pipes a high resistance is like a very thin pipe - it is very difficult for water to get through and so not much of it can flow at once - high resistance means lower current.
Components in series means there is only one pipe the more components that are added the harder it is for the water to get through and hence the thinner the pipe and the higher the resistance. However, adding a component in parallel is equivalent to adding a new pipe it does not affect the old pipe s thickness and provides a new path for the water to travel down. That is why adding resistors in parallel always decreases the total resistance of the circuit it is effectively just adding more pipes.
V = IR, so R = V/I, meaning resistance can be defined as the number of volts (amount of energy/pressure) required for one amp (a certain amount of electrons flowing per second) to be able to flow through an object. More electrons flowing means more energy/pressure (voltage) is required to push them through the object.
Voltage is perhaps the least well understood property. A useful way to think of it is the pressure of the water flowing in the pipes. Higher pressure means more water can flow through pipes of the same thickness, i.e. higher voltage means higher current at the same resistance (V = IR).
A different analogy, which is an extremely helpful (though should not be used in definitions, but is safe to use otherwise) way to think of and find voltages is to view voltage as the difference in the amount of energy each electron has at two points. This method is key to making circuit questions trivial.
Each electron starts by being charged up with energy by the battery, so a 12V battery in a circuit means each electron starts with 12 units of energy.
Each electron then takes one path around the circuit, using up its energy as it goes through each component it meets, and finishes the lap of the circuit back at the other side of the battery, with 0 energy.
It then goes through the battery to be charged up with energy equal to the voltage of the battery to complete the lap again.
Since voltage (also known as potential difference) is defined as the difference in energy of an electron at two points, asking the question what is the voltage at one point does not make sense as a question it s like asking what is the distance between point A?
The question that would make sense is what is the distance between point A and point B? Hence, when measuring voltage, you are always measuring the energy at a point A and comparing it to the energy at point B. This is why voltmeters are never connected in series in a circuit, only in parallel.
Because voltmeters are designed to measure the voltage between two points on a circuit, it is important that they do not affect the circuit when they are added to one. Therefore, voltmeters are designed so that electrons cannot flow through them, so that attaching one to a circuit does not offer a new additional path for electrons to flow through. In other words, current cannot flow through a voltmeter, which is accomplished by essentially creating voltmeters that have effectively infinite resistance. Since current = voltage/resistance, an infinite resistance means no current flows, no matter what the voltage of the circuit is.
Each electron takes one path around the circuit and has, in our example, 12 units of energy to spend as it goes around. Components in series means the electron has to go through each one of them, and shares its energy between them.
The amount of energy each component gets is determined by the resistance of the component. For example, an electron going around a circuit with 2 resistors in series, one of 1 ohm resistance and the other of 2 ohms, will share its energy between the two according to the ratio of their resistances. Because the 2 ohm resistor has twice the resistance of the 1 ohm resistor, it gets twice as much energy from the electron. Hence, in this case, the electron will give 8 units to the 2 ohm resistor and 4 units to the 1 ohm resistor. E.g. the voltage across the 2 ohm resistor is 8 and across the 1 ohm resistor is 4.
For components connected in parallel, each electron just chooses a path to go down. For this reason, since each electron only needs to go through one of the components to complete its lap, it can use up all of its energy on the one component it goes through. This is why, for two resistors in parallel, voltage across each of them is not shared. In our example each resistor would receive all 12 units of energy from any electrons choosing to flow through them, so the voltage across each resistor is 12.
A final useful method of approaching questions is to consider each loop around a circuit as a separate circuit. This is an extremely powerful and useful tool that makes many questions trivial and can be seen demonstrated in the accompanying videos.
