The Shedding Light on Electricity series exposes the shocking truth about electricity. Yes, positively a bad pun to begin with, but we promise to conduct ourselves really well from now on. This series teaches students watts of stuff (sorry, couldn’t help ourselves) about electricity including how it’s produced, how it’s used in our homes, how it’s controlled, and how we keep ourselves safe from nasty shocks. This is high-voltage education that is hard to resist!
In Episode 3, Electric Current, we take a look at what electric current is (it’s basically the flow of electrons through a wire) and at how electric current is measured. We then examine the different ways that electric current flows in series and parallel circuits.
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The Transcript (which can be used as a textbook)
Part A: Introduction
Part B: Amps and Ammeters
Part C: Current Affairs
Part A: Introduction
Hi everyone and welcome back to the Shedding Light on Electricity series. In this episode, episode 3, we’re going to take a deeper look at electric current.
I’ve already mentioned that electric current is basically the flow of electrons through a wire of some sort when you have a complete circuit, but electric current is a measurable quantity and we can calculate how much will flow in different circumstances. So, how do we measure electric current? Well, let me explain.
Atoms are made of protons, which have a positive charge, neutrons which have no charge, and electrons, which have a negative charge. The protons and neutrons make up the nucleus of the atom, and the electrons kind of move around the nucleus. Now, there is a force of attraction between protons and electrons. The force is called “electromagnetism”. This force usually stops the electrons from flying away from the atom.
However, in metals, the outermost electrons can move easily and randomly from atom to atom. Since electrons are all negatively charged, electromagnetism actually causes them to repel each other.
If one electron moves towards another electron, there are forces of repulsion between the two electrons so the second one is pushed away even though there is no actual contact between them. This electron can then push on another one. If another electron comes along, the process repeats.
The really complicated chemical reactions that occur inside batteries push electrons around a circuit because of all the forces of attraction and repulsion between the protons and electrons in the chemicals.
Generators push electrons around a circuit because of the electromagnetic force on the electrons in the coils of wire as they spin between the magnets.
Things that allow electrons to flow through them are called conductors, all metals, for example, are conductors, while things that don’t allow the flow of electrons are called insulators, plastic and rubber for example. Electric current can increase or decrease depending on the voltage of the power source and the type of and number of components in the circuit.
When the voltage is low for example, not much current flows and so the light globe is fairly dim. This power pack allows me to increase the voltage, and when I do, more current flows and so the light globe gets brighter.
To picture electric current you can think of water in a set of pipes. The water pump is kind of like the battery, the pipes are like the wires, the water is like the electrons, and the water wheel is kind of like a motor. A bigger pump can pump more water so a bigger current will flow. Thinner pipes might restrict the current.
In the case of a hose, if water is already in a hose and you turn the tap on, water will immediately start coming out of the open end because the water near the tap pushes on the water here which pushes on the water here and so on.
It’s the same with electricity. All the atoms in a wire already have electrons. As soon as some are pushed out of the battery they push on the ones next to them which then push on the ones next to them and so on. So, you could have wires 100 km long, but as soon as you press the switch the current will flow in the whole circuit almost instantaneously. There might be like a millionth of a second delay as the electrons all get pushed around, but you’ll never notice it.
Electric current is actually a measure of how many electrons are passing a given point in a circuit per second. It’s measured in amperes or amps for short. The symbol for amps is capital A. This unit was named after French scientist André-Marie Ampère, whose experiments in the 1820s led to important discoveries about how electricity works.
The current flowing through this light globe is about 0.02 amps (0.02 A). The current flowing through this electric heater is 8 amps (8 A). So how do I know that?
Well, thanks to Andre-Marie Ampere, and other scientists, we can measure current using what’s called an ammeter. Since it measures amps, I don’t know why they called it an ammeter and not an ampmeter. Maybe ammeter is just easier to say. Anyway, this ammeter is used for school experiments. This device is called a multimeter, but it has a setting that allows you to measure amps. This one allows you to safely measure the amount of current in appliances by plugging it into a socket and then plugging the appliance into it. Let’s take a closer look at how they operate.
Part B: Amps and Ammeters
Ammeters are placed in series with whatever you want to measure the current in.
In a circuit diagram it looks like this. This ammeter has 2 scales, a 1 Amp scale and a 10 amp scale. The black terminal is the negative terminal and it’s connected to whatever wire is closest to the negative terminal of the battery. Since I plugged into the terminal labelled 10 A, then I have to read off the 10 Amp scale, which is on the top. Here the ammeter is showing a current of 3.2 amps. Each little division (on the scale at the top) is 0.2 amps. That’s 2.0, 2.2, 2.4, 2.6, 2.8, 3.0 and the needle is showing 3.2, 3.2 amps. But, what exactly is an amp?
Well, if an electric current of 1 amp is flowing through a light globe or whatever, it means 6.25 billion billion electrons are flowing through the light globe per second. It’s an unbelievable number of electrons. A current of 2 amps means that 2 times this value, 12.5 billion billion electrons are passing through the light globe per second. 3.2 Amps in the light globe is 3.2 x 6.25 billion billion electrons per second which is 20 billion billion electrons per second.
Obviously giving the current in amps is much easier.
So what’s the current in this light globe? It’s 20 billion billion electrons per second. What? 3.2 Amps. Oh, why didn’t you say so?
If I keep exactly the same arrangement but use one of these little light globes with a 6 V battery, the current is less than 1 Amp. Now, to get a little more accuracy, I can now plug into the 1 Amp terminal and, reading off the 1 Amp scale now, we can see that the current is about 0.3 amps.
Different light globes are designed to allow different amounts of current to flow through them depending on the brightness required and on the voltage that they’ll be using.
Now a quick word about words. This little light globe has about 0.3 amps running through it. This is often expressed by saying that the light globe draws 0.3 amps as in it draws it out of the battery. So, just as a syringe draws a liquid out of a container, the light globe draws current, in this case 0.3 amps, out of the power supply. Of course the light globes aren’t actively doing anything, the current is being pushed through them by the batteries but we still say that they draw current. This light globe draws 0.3 amps, and this one draws about 3.2 amps.
These kinds of ammeters are only used for school experiments. Different ammeters have different terminals. This one measures amps and milliamps.
One milliamp, written 1 (lowercase)m (capital) A, is 1/1000 of an amp, so 1 A equals 1000 mA. So for example, 500 mA = 0.5 A, 150 mA = 0.15 A, and 3.2 Amps = 3,200 mA.
Here I’ve plugged the wire into the terminal labelled 500, so I have to read off the 500 mA scale, which is at the top. The reading is about 290 mA which is 0.29 A. That’s 200 mA, 250, 60, 70, 80, 290 mA.
Now sometimes, you might connect the ammeter and the needle will move backwards. To fix this, just swap the wires around. The needle moves because of a small electromagnet inside the ammeter and for the force to move the needle the correct way, the wire connecting the red terminal which is the positive terminal, to the battery has to trace back to the positive terminal of the battery. The wire connecting the black terminal (the negative terminal) has to trace back to the negative terminal of the battery.
Multimeters typically have a digital display. Multimeters have different settings depending on what you want to measure: voltage, resistance (both of which we’ll talk about more in our next episode) and current.
To measure current, I turn the knob to the 10 Amp setting, plug one of the wires into the 10 Amp terminal and the other wire into the COM terminal. COM stands for common, which in this case means that you always use this terminal for one of the wires but you only ever use this terminal or this terminal for the other wire, depending on what you want to measure. As we can see, the current is 0.29 Amps.
This device allows me to measure the current in appliances. You plug it in and then plug in the appliance. This toaster is drawing just under 3 and a half amps. This kettle is drawing just over 10 amps.
Different household items draw different amounts of current. The more power needed, the more current is drawn. Things that are used to generate heat draw a lot of current, while electronic equipment and lighting generally don’t draw all that much. By the way, in Australia and most of the world, electricity is supplied at a voltage of 240 Volts so all household items are designed to run off 240 V. However, as I said different items draw different amounts of current depending on what power is required.
In North America and a few other places around the world, the electricity is supplied at a voltage of about 120 Volts. A smaller voltage means that electrical devices in the USA and Canada need to draw more current, about double what you see in this table, to produce the same amount of power.
So is the amount of current coming out of the battery the same as the current going through the light globe and is it the same as the current going back into the battery? And what happens in circuits where there are many components connected in series and parallel? Let’s take a look.
Part C: Current Affairs
So here we have a simple circuit with a car headlight connected to a car battery and I’ve placed a multimeter set to read amps in series with the headlight.
The current is 3.2 amps. Even though this is a multimeter, I’ve used the symbol for an ammeter in the circuit diagram because when it’s on the 10 A setting, it’s acting as an ammeter.
Now what if I put another multimeter set to read amps on the other side of the light globe? Well, this ammeter also reads 3.2 amps.
This shouldn’t be surprising. Electric current involves the movement of electrons. If you’ve got 3.2 amps, 20 billion billion electrons per second going through this point, then you’re going to get exactly the same number at this point. Electrons can’t just disappear or appear out of nowhere. So in a simple circuit with a light globe and a battery, the current at every point along the circuit is the same, in this case 3.2 amps. 3.2 amps are coming out of the battery, 3.2 amps are going back into the battery, and 3.2 amps are going through the head light.
Now a quick word of warning. Sometimes the reading on a multimeter jumps around a bit, so if you have to record the data just write down either one because it won’t make much difference to any graphs you draw or calculations you make. In this case a difference of 0.01 out of 3.2 is a difference of just 0.3%.
In this circuit, the reading on the ammeter is about 290 millliamps, so we know that a current of 290 millliamps is coming out of the battery, is going back into the battery and is flowing through the light globe.
Now what if I put more than one component into a circuit?
Well, as you can see, the current in every part of the circuit is the same: 0.2 amps. A current of 0.2 amps is coming out of the battery, the same current is passing through both light globes, and the same current is flowing back into the battery.
So, you don’t need a separate ammeter for every single part of your circuit. If you just use one ammeter and the components are connected in series then whatever the ammeter is measuring is the current in all the components, in this case the two light globes.
So all components that are connected in series will have the same current passing through them.
In this circuit comprising a milliammeter, a resistor, and an LED, the reading on the milliammeter is about 9 mA. I’ve connected the wire to the terminal labelled 50, so I have to read of the 50 mA scale: 10, 20, 30 40, 50, so that’s 9 mA. We therefore know that there are also 9 mA passing through the resistor and through the LED.
Now what happens when parallel connections are involved?
Using three multimeters again set to read amps, we can see that a current of 0.54 amps is flowing out of the battery but at this point here, the current splits up. A current of 0.27 amps flows through light globe 1 and the same current flows through light globe two. The current splits equally because the two light globes are identical.
But what happens if two devices connected in parallel are not identical?
This circuit is the same as the previous one, but one of the light globes has been replaced with an electric motor. Now, the current flowing out of the battery is 0.31 amps, the current in the light globe is 0.28 amps (which is pretty much what it was when the two globes were connected) and the current in the motor is 0.03 amps. The current splits at the junction but more current flows through the light globe than the motor.
However, the mathematics is pretty clear: 0.28 amps + 0.03 amps = 0.31 amps. The reason that less current flows into the motor is that the motor has what’s called a greater “resistance” to electricity. The wires inside the motor are different to the wires and the filament inside the light globe. We’ll be looking more at resistance in our next episode.
Now quantities in science are usually given a symbol. Symbols make it easier to express data and to write formulas. The symbol used for Amps is the letter A, capital A, but the symbol used by scientists for current isn’t C, it’s I (capital I).
Andre-Marie Ampere himself used “I” in the 1820s to stand for the French “Intensite de Courant” which means current intensity (although he didn’t name amperes after himself; scientists officially started using amperes as the unit for electric current in his honour in the 1880s, around 50 years after he died). So for the circuit, I can write I = 3.2 A which means the current is 3.2 Amps.
Now on this circuit diagram, I’ve labelled different parts of the circuit. So how do I write, for example, the current in the milliammeter or the current at Point B? Well, scientists use subscripts. I can write “I” (normal size) (subscript) milliameter, Imilliammeter = 9 mA, which means that the current passing through the milliammeter is 9 mA. The subscript is like a label which tells everyone which particular current you’re talking about. Since everything here is connected in series, I also know that the current at Point A is also 9 mA (IPoint A = 9 mA) as it is in the resistor, in the LED, and at Point B.
So the laws of Physics are pretty clear when it comes to electric current in components connected in series and in parallel.
In a circuit where the components are connected in series, the current in each component is the same, in this case 9 mA. IBattery = IPoint A = Iresistor = ILED = IPoint B
In a circuit where the components are connected in parallel, the currents in each parallel branch add up to the current flowing into the branches. Ibattery = Ilight globe + Imotor
The current will split evenly if the components are identical but in this case more current flows into the light globe than into the motor.
So let’s take a look at this circuit, a pretty impractical circuit, but a circuit none-the-less. If I told you that the milliammeter was registering 300 mA of current flowing out of the battery, and that all the light globes are identical, then you should easily be able to work out the current in every light globe and at the two labelled points. So how much current is flowing in light globe 1? Well it must be 300 mA. Whatever current is coming out of the battery is going through light globe 1. At this point the current splits, evenly, so light globes 2 and 3 are getting 150 mA each. At this point the current rejoins again, kind of like two rivers joining up, so the current in light globe 4 is 300 mA. At this point the current splits again, but this time it’s a 3-way split, so each light globe gets 100 mA. So, what’s the current at Point A? Well at this point the current from Light globes 6 and 7 join up so the current at Point A must be 200 mA. And what’s the current at point B? Well, that must be 300 mA, which is 200 + 100. This is the current flowing back into the battery, which is equal to the current flowing out of the battery.
This is a far more realistic circuit showing some of the lighting in a home. Depending on the power rating of the light globes that are installed, when they’re switched on, a different amount of current will flow in each branch. Different light globes draw different currents. However, mathematically, 0.02 A + 0.04 A + 0.06 A = 0.12 A and it’s 0.12 Amps that are flowing into the home (from the power stations) and the same amount of current flows back out of the home. At this point the current splits evenly because the light globes are identical.
If the current into a house and the current out of a house are not the same, it may mean that someone’s getting electrocuted, and in Episode 5 of our series we’ll look at how that can occur and what we’ve done about it.
But, why do some light globes draw more current than others? Well, that’s what we’re going to look at in our next episode. See you then.