GCSE Science/Current, voltage resistance and Ohm's law

From Wikibooks, open books for an open world
Jump to: navigation, search

Electronics GCSE Science/Electricity


What is electricity anyway?[edit]

So far in this module we have been using words like electricity, current, voltage, and resistance without actually explaining in depth what these words mean. On this page you will learn what these words mean, and how they relate to one another.

What is electricity on a microscopic level[edit]

Let's think about a wire, made of copper. You should already know that it is made of particles called copper atoms, which have a positive nucleus surrounded by negative electrons. We need not concern ourself with the nucleus, because it does not move. They are stuck in a rigid crystal structure. Instead we need to look at the electrons. Copper, like all metals, has some loose electrons. These electrons are not held rigidly by the atoms but are free to roam about anywhere as long as they stay somewhere in the metal. We call them conduction electrons because they are the ones that conduct electricity. To see how electric current flows imagine a small petri dish, with an even smaller one glued inside. The channel between the two dishes is then filled with peas. The channel represents the wires in a circuit, the peas the electrons.


Peas in a bowl.png

Charge is the quantity of free charged particles (in this case electrons). The SI unit of charge is the coulomb.

You could count the electrons, but there are an awful lot of electrons! Each electron carries a tiny charge, of −1.602×10-19 coulomb. We can turn this figure on its head and say that if we grouped together 6.2415×1018 electrons we would have −1 coulomb of charge.

Electric field:-[edit]

An electric field is a region where an object experiences a force due to its charge.

The strength of the electric field (electric field strength E ) is described by considering how much force is experienced by a unit charge (a charge of 1 coulomb) when it's placed in the field.

The electric field at any point is therefore expressed in newtons per coulomb.

Some fields (such as the field between two parallel charged plates) have the same field strength throughout the field - uniform fields. Others (particularly radial fields due to isolated point charges) do not have constant field strength.


In a battery there is a negative and positive terminal. The conventional current flows from positive to negative (the large line to the small line). Because opposites attract, these charges in the battery will be attracting each other, but they can not move directly to each other through the battery because of the chemical processes. If there is a complete external circuit, this attraction from the battery will give the free electrons in the metal (i.e. the wire) a force which will make them move. If you think about it these electrons are being forced to the other side of the battery because of this attraction. We call this driving force the Electro-motive Force (e.m.f). Emfs are measured in volts, and are sometimes referred to simply as voltage. The larger the emf (the voltage) the more quickly electrons flow round the circuit. What is the rate of flow of the electrons?....the current. so, a larger voltage means a larger current.

Current[edit]

Now imagine that you were to put your finger on one pea and push it in a clockwise direction. All the peas would move because they are all touching one another. This is just what happens when an electric current flows.


Current is the flow of charged particles (the particles are usually electrons). The SI unit of current is the ampere (symbol A).

To understand how current is defined think of standing at a given point in the wire. Electrons are flowing past you.


One Ampere is a flow of one coulomb going past every second.

1 A = 1 C·s-1 (1 A = 1 C/s)

Definition of the ampere[edit]

BUT this flow of charge idea is NOT the definition of the ampere. The ampere is defined in terms of the force produced between two wires each carrying identical currents:-


The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 x 10-7 newton per meter of length.

Voltage[edit]

Having looked at charge and current, we now need to look at what voltage means.

As you know, electrons repel each other. If you hold one electron near another electron, you have to push against it to hold it in place. If you try to bring it even closer, you have to do work (force times distance) to get it to that new position.

The voltage between one point and another is simply how much work per coulomb is required to move any small test charge from point A to point B.

In most electronic components, it doesn't matter much which path the test charge takes in-between point A and point B.

Voltage is related to the energy of the charges. Let's go back to the peas in the petri dish. They can be pushed slowly or they can be pushed quickly. The faster they go, the more energy they have. It's a similar situation for the electrons, except the push isn't provided by a finger! It's provided by the battery.

The battery gives the charges energy. This energy is given up to the various components in the circuit, e.g. bulbs, resistors etc. The energy per unit charge is called the voltage (or the potential difference).

Definition of the volt[edit]

One volt means one joule of energy per coulomb of charge.

More accurately it has 2 definitions:

Electromotive Force is the amount of energy converted from non-electrical to electrical form when driving 1 coulomb of charge around a completed and closed circuit.

Potential Difference is the amount of energy converted from electrical to non-electrical form when driving 1 coulomb of charge around a completed and closed circuit.

The potential difference between 2 points in a conductor is defined as 1 volt, if 1 Joule of energy is converted from electrical form to non-electrical form, when 1 coulomb of charge per second (1 amp) flows through it. Note: This will only occur between 2 points in a conductor, that has a resistance, defined as 1 ohm. See resistance below.

1 V = 1 J·C-1 (1 V = 1 J/C)

Resistance and Ohm's law[edit]

Resistance is easy to understand. It's just how difficult it is for the charges to flow through an electrical component or from one point to another in an electrical circuit. Imagine a group of walkers travelling down a road. They approach a fork in the road. To the left is a flat straight road leading to a nearby town. To the right is a huge mountain, over which a steep and winding road traverses. This road also leads to the nearby town.

Naturally all the walkers chose the left route. Let's now suppose that there are millions of walkers. They are jam packed on the road, and they are all in a hurry to get to the town as quickly as possible. Now when they come to the fork in the road which way should they go? Most will still go to the left, but a few might chose to go to the right, the road is more difficult but there is no traffic jam, so they might get there quicker.

It's a similar thing with moving charges. Like charges repel, so they would rather not pack in very closely together. Some routes, like wires have very low resistance, while other routes like bulbs have much higher resistance. More charges will go down a low resistance route than a high resistance one. The unit of resistance is the ohm, represented by the Greek letter Omega (Ω).

Ohm's Law[edit]

This law relates resistance, current, and voltage. It's very easy to remember because it's obvious when you think about it.

Let's think of a wire carrying current from a battery, to a bulb then back to the battery. The voltage of the battery provides the energy of the flowing electrons. Let's assume we want to increase the rate of flow of charge. Remember that current is the rate of flow of charge so we want to increase the current.

  • Increase the voltage If we increase the voltage, we increase the energy of the charges. Each charge will then flow faster, so the current will increase. (A fast moving river carries more water per hour than a slow one of the same size). So current must be proportional to voltage.
  • Increase the number of charges flowing The more charges that flow, the bigger the current will be. (A wide river carries more water per hour than an equally fast flowing but narrower one.) The way to increase the number of charges is to decrease the resistance. So the current must be proportional to 1÷Resistance. We say that it is inversely proportional to the resistance.
  • So, the current is proportional to the voltage, and is inversely proportional to the resistance.

So Current = Voltage \times \frac{1}{Resistance}

Current = \frac{Voltage}{Resistance}

Rearranging gives :

Voltage = Current \times Resistance

In symbols:

V=IR

This is Ohm's law. Learn it by heart.

Q2) A wire has a resistance of two Ω and a voltage of 3 V is applied. What is the current?

Q3)A wire has a 5 V potential difference applied. the current is 10 A, what is the resistance?

Q4) A current of 2 A is flowing through a wire with a voltage of 20 V. What is the resistance of the wire?

(Remember to give the units for all your answers).


Summary
  • The unit of current is the ampere (A), and means one coulomb per second.
  • The unit of potential difference is the volt (V), and means one joule per coulomb.
  • The unit of resistance is the ohm (Ω), and is equal to voltage over current.
  • Ohm's law, V=IR, determines the relationship between these three units.

Answers | <<Circuits | Circuits Part 2>>