Everyone knows what electricity is. Or do they?
In many or most parts of the world, you can have light, wind, television, smart phone and all these nice things at the flick of a switch. But of course, have all these at our finger tips does not mean we know or even need to know how they work. We just need the money to buy them.
In this and the next few topics, we shall look at the basics of how electricity works. We are most familiar with using "current electricity". This is the form of electricity in action when we press a switch on the wall and the room lights up and air conditioner starts to cool the room.
The switch is like a tap that allows electric current flow through light bulbs, cookers and smart phones. But what is "electric current"? What is actually flowing through the wires?
What we know from the work of scientists over the past few centuries is that everything we see around us are made up of atoms. And atoms are in turn made up of 3 main types of even smaller particles - protons, neutrons and electrons.
The proton and the electron have a physical property called electric charge. This is a property in a similar sense to mass. While masses attract each other with a gravitational force, proton and electron attract each other with an electrical force.
An electron is nearly 2000 times lighter than a proton or neutron. When we press the switch, it is actually just electrons that flow through the wire and light up the bulb.
And for this first topic on electricity, just to start simple, we are more interested examples where the electrons do not move much.
When learning about electricity in physics, we usually start with this topic called "static electricity". The word "static" means at rest, not moving.
When I studied and worked in England, I often felt a sharp needle prick on my finger when I touch a desk or a shirt. But I could not find any needle or sharp point. From what I learnt in physics, I quickly realised that it was a tiny electric spark.
Students in Singapore who have not travelled to cold countries may be less familiar with the effects of static , as it is simply called. What happens is that through contact or friction, some electrons can be "rubbed" off from one body on to another.
The amount of electrons is tiny, but because it stays around a tiny spot - like a few strands of my hair or a small spot on the bookshelf, the voltage can be very high, like hundreds of volts.
If I happen to touch this, a tiny spark could jump between my body and the object. It may feel like a needle prick and can be quite painful, though often harmless. Despite the high voltage, the current only last for a very short time.
In Singapore, we do not normally experience this because mosture in the humid air would quickly help to conduct away this electric charge on objects.
Using the rubbing idea above, it is possible for example to put some electric charge on say two light plastic balls, each hanging on a thread. And when we bring the two threads close to each other, we would find that the two balls seem to push each other away.
In other words, the electric charges on the balls repel each other. This can happen when the charges are both positive or both negative. Another way to look at this is that an object with an electric charge can exert a force on another electric charge at a distance.
Physicists like to describe this by call it "action at a distance".
But of course, if it is too far away, the force would be practically zero. There is a name for the region around the electrically charged object where an electric force can act. It is called the electric field.
If you do physics exercises on this topic, you would often see questions on electric field lines. They show roughly the direction of the force on a small positively charged object nearby.
In everyday life, we don't normally think that there is anything similar between weight of a ball, and the electricity in our light switch. But actually, there are quite a bit of similarities.
Gravitational force can attract a body with mass from a distance. Electric force can attract a body with electric charge from a distance.
If you double the distance between two bodies with mass, the gravitational force drops to one quarter. If you double the distance between two bodies with electric charge, the electric force drops to one quarter.
Another similarity.
The first property is called "action at a distance". The second property is called "inverse square law".
These are the formulae for these two forces.
Both have 1/r² in the formula. That's why they are called "inverse square laws".
In an earlier blog, I have talked about g, the acceleration due to gravity. It is normally assumed to be the acceleration when you drop say a stone from a small height, where it is 9.8 m/s2. So it can be related to the weight of the stone by this formula
where m is the mass of the stone.
But there is another way to describe or define g. We can rearrage the equation by dividing both sides of the equation by m:
So the acceleration due to gravity is the same number as the weight per unit mass, or weight per kg if you like.
Weight is a force. So we can also say that g is the is the gravitational force per unit mass. There is another name for this - gravitational field strength.
From this familiar example of object in a "gravitational field", we have the idea of an "electric field strength" in an electric field.
electric field strength = electric force / charge
or
From Coulomb's law above, we can get the formula for electric field strength around a point charge
If you find it hard to understand these formulae, do not worry. This article is just to give you some idea of what kind of thinking is involved in this topic. It usually takes some exercises (not the physical type!) to get used to these ideas.
In the above few paragraphs, we have seen some similarity between electric force and gravitational force. This similarity naturally carries over to potential energy, since potential energy is directly related to force.
From my earlier blog on gravitational field, I have talked about the idea of potential energy. In the simplest case, this can be just the work we have to do on a stone - if we want pick it up from the ground and lift it up to a certain height.
In the more abstract case, the "ground" reference can be changed to "infinity". This means that instead of picking up from the ground, we "pick" the stone from a place so far from Earth that it cannot feel any of gravity, and bring it back to Earth's surface.
A very abstract idea that seems completely detached from everyday life! In a way, we can think of it as a mathematical technique, like a step to more "useful" calculations.
With the help of some mathematics, the amount of work needed to bring a unit electric charge from infinitely far away - to a nearby metal sphere with an electric charge Q, is given by this formula:
Again, this looks similar to the one for gravitational potential.
I have talked about electric field around a metal ball. That is kind of a simple shape in physics, in the sense that there is a simple formula for the electric field around it.
There is another shape that has a pattern for the electric field, and a simple formula for the strength - a pair of parallel metal plates, connected to a battery.
While the ball shaped conductor is useful to help us learn about electric charge and electric field, the the parallel plates are actually very useful in electrical and electronic devices.
Force on a a charged particle between the plates can be calculated if we know the electric field strength. Recall that this means the force per unit charge. In the case of the parallel plates, this is equal to voltage between plates divided by distance between them.
When a battery is connected to two metal parallel plates, one plate gains a positive charge, the other plate a negative charge. This has the effect of creating an electric field in between the plates. What is special about the electric is that it is uniform and parallel. What this mean is that if we place a small charged particle in between the plates, it feels a force. And if we move it anywhere within the volume between the plates, it feels almost the same force in the same direction.
This type of electric field has been used by scientists to study the motion of particles like electrons and protons, and has allowed them to make important discoveries and measurements.
Finally, let me just mention a direct connection between electric field strength and potential energy.
Both are rather abstract terms, but we can make them easier to understand by talking about their basic meanings directly:
The "reference point" can be anywhere convenient. In the case of the sphere, this point is infinitely far away. (What does that mean?!) For the parallel plate, it can be either plate or some fixed distance away. Yet more statements that seems to makes physics sound very abstract. But they really mean exactly what they say. Just takes some getting used to.
You can learn these concepts and more at Dr Hock's maths and physics tuition.