Current, Voltage & Circuits

A background note on circuits, current, voltage, and useful analogies for teaching these concepts.

Current

“Current” refers to the flow of electric charges in a material, for example, a wire. The charges that flow are negative, and in the form of electrons. However, the direction of current is defined as the direction of positive flow - the electrons are actually flowing opposite to the current. This can be confusing, but for most applications, you don't need to worry about it. Just remember that current is the flow of charge through the wire.

In equations, current is represented as 'I'.

Voltage

Voltage is what makes the charges flow. When there is more than one charge in an area, they exert a force on each other: same-sign charges repel one another, and different-sign charges attract one another. This is simple when you only have two charges. In electrical systems however, there are far too many charges to be able to think of all the forces individually. Instead, we combine all these forces into something called an 'electric field'. At any point, the electric field represents the force on a charge due to all the other charges in the relevant area. The voltage between two points is the difference in the electric field of those points - basically the difference between the forces that would act on a particle at those two points. Voltage is often measured between a point in an electric field and “ground” - the point where there are no net forces on the charge.

Charges will go in the direction that minimizes the forces on them - that is, from a strong electric field to a weak electric field. This can happen without any extra force, just like a ball rolling down a hill. In a circuit, there is a 'voltage' (i.e. a difference in electric field) between the positive and negative terminals of a battery, which causes charges to move between them when the ends are connected to each other in a circuit.

To get charges to move from a point of low voltage to high voltage, extra energy needs to be used to push them there (like carrying a ball up a hill). This is what happens inside a battery to keep current flowing - the battery takes the current that has collected at its negative end and uses chemical energy to “push” it through the battery to the high-voltage positive end, where it then travels through the circuit to the negative end again (like rolling the ball down the hill again…).

The 'voltage across a component' is the difference in electric field across that component - it can also be thought of the energy a charge loses to go through the component.

In equations, voltage is represented as 'V'.

Circuits

Charges, in general, flow only through conductors (metals). Therefore, to go from a point of high voltage to low voltage, the two points must be connected by something conductive. A circuit is such a path that forms a complete loop, so that the charges are brought from a point of low voltage to a point of high voltage, and flow back to the point of low voltage.

As we put energy into the charges to get them from low voltage to high voltage, they lose energy as they flow from high voltage to low voltage. This energy is changed from electric potential energy (energy that the charge has by virtue of being at a higher voltage) to some other form of energy, such as heat, light, or motion (kinetic). For example, as current is flowing through a light bulb, the electric potential energy is converted to light and heat energy. If we put a resistor in our circuit, the resistor would dissipate power into heat. By the time the current gets back to the battery, it has dissipated all its energy. This means that the voltages across each of the components in a circuit must add up to the voltage of the battery.

One important thing to remember is that in order for current to flow in a circuit, the circuit must be a complete loop - otherwise there will be an “end” where all the charges get stuck, and there wont be any free charges to move through the circuit.

Resistance and Ohm's Law

Resistance is how much a component in a circuit 'resists' charge going through it. High resistance components only let a small amount of current through them, while low resistance components let lots of current through.

In a circuit powered by a battery, the voltage in the circuit will always be the same - it is determined by the voltage across the battery terminals (this is why batteries are labelled with specific voltages). However, the amount of current in a circuit is determined by resistance - if the components in the circuit are very low resistance, they will pull a lot of current through the circuit, and if they are high resistance, they will not.

In equations, resistance is represented as 'R'.

The relationship between voltage, current and resistance is characterized by Ohm's Law: V = IR : Voltage = Current x Resistance.

You can see that if the voltage is kept constant and the resistance increases, the current has to decrease, and vice versa.

Power

The rate of energy conversion in some piece of a circuit is referred to as the “power dissipated” in that component. Power is a measure of energy/time, and dissipated just means “gone away” - technically it's been transformed into something else (like, for instance, heat - which you can feel if you touch a resistor in a circuit), but it is no longer in the circuit. The power dissipated in some part of a circuit is equal to the voltage difference across that part of the circuit times the current flowing through that part of the circuit: P = IV.

Useful Analogies

A lot of the above information is somewhat abstract and can be hard to visualize. There are, however, some useful analogies that can be used to make this easier.

• The charges are like water in a stream or pipe.
• The current is how much water is flowing through the stream or
• pipe at some place. Voltage is like the height of the stream or pipe. Naturally, the water will flow from a high place to a low place.
• To get it from a low place to a high place, you must put energy into it, either using a pump, or a lot of buckets.
• Once there, the water has a lot of energy. As it flows back to the low place, the energy will be changed into sound (think of a waterfall), heat (friction of the water on the stream bed) or motion of leaves or other things the water might hit.
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