Topic 6

What does the current through an electric circuit depend on?

Current Electricity

In the last topic we looked at static electricity.  In this topic we will look at current electricity, the electricity that runs through wires made of conducting materials.  For current electricity to flow, we need:

• a complete circuit;

• conducting materials;

• a source of voltage (e.g. a battery).

All metals are conducting materials, as is the non-metal carbon.  Silicon and germanium care called semi-conductors which means that they can conduct electricity under certain circumstances.  Some compounds can be made to conduct electricity under certain circumstances as well.

Insulating materials keep parts of the circuit separate, for example the positive and negative terminals of a battery.  If the two are not kept separate, there will be a short circuit.  This can cause an electrical explosion or a fire.  The picture shows damage due to a short circuit.

Circuit Diagrams

We don't have to be good artists to draw good circuit diagrams.  They are useful because:

• they show how the components are laid out;

• they show how the components are connected to each other;

• they show what is NOT connected together, which is just as important as what is;

• anyone can understand how the circuit works.

To make sense of circuit diagrams you must learn the following symbols:

It is also important that you know not just how parts of the circuit are connected, but also how parts are NOT connected:

The connection between wires is shown by the black circle, which can be thought of as representing a blob of solder.

Quantities in Electricity

Electricity has two important measurements from which other measurements can be worked out:

• Potential Difference or voltage

• Current

Current

Current is a flow of charge.  We measure charge not in the total number of electrons, but in packets called coulombs (C).  (You buy a kilo of sugar, not 1 000 000 crystals of sugar.)

1 C = 6 × 10¹⁸ electrons

You can see why we don't count the number of electrons flowing.

Current is measured in ampères or amps (A).  If 1 coulomb of charge flows every second, a current is 1 amp.

1 A = 1 C/s

Voltage

Potential difference is the "electrical pressure" in a circuit.  (The correct definition is energy per unit charge, but you may find that a difficult concept at this stage.)  Look at this picture of a water circuit:

The pump pumps water at a certain pressure.  The work that can be got out of the load depends on two things:

• The number of litres of water passing every second;

• The pressure the water is under.

This model can apply to an electric circuit:

• The pump is the battery;

• The load is the load (e.g. bulb, motor, etc);

• The water flow is the current;

• The pressure is the voltage.

There are all sorts of other models to explain voltage.  Ask your teacher.

Now answer Question 1.

Sources of Voltage

Three obvious sources of voltage are:

• Cells (batteries);

• Generator;

• Power pack, which you use in your physics practicals.

Strictly speaking a battery consists of two or more cells, although to describe a single cell as a battery is perfectly OK.

The positive terminal of a cell is represented by the long line on the symbol; the negative is shown by the short line.

Conventional current flows from positive to negative.  Although we know that positives don't move and electrons (negatives) do, the early physicists got it wrong.  To get round it physicists have brought out the idea of conventional current.  In these notes and all text books, we will regard all currents as conventional.

Each cell gives out a voltage of 1.5 volts.  So two batteries wired in series as below will give out a voltage of 3.0 V.

Question 2.  What is the voltage from these combinations of cells?

(a) 

(b)

ANSWER

Relating Voltage and Current

The circuit below shows how to measure voltage and current:

Notice that:

• The voltmeter measures voltage and is in parallel across the component;

• The ammeter measures current and is wired in series with the component.

We find that as the voltage doubles, the current doubles as well.  We say that the voltage is proportional to the current.

The resistance of the resistor is the ratio of the voltage and the current.  Resistance is the degree to which a component opposes the flow of electricity.

In triangle form:

The physics codes for resistance and voltage are simple enough to understand.  The code I for current comes from the French, Intensité du Courant (intensity of the current).

The strange unit symbol for resistance, W, is a Greek capital letter "Omega", a long "O" (Ō).  (It looks like a seal seen from the back.)

Worked Example

A German physicist, Georg Simon Ohm gave us an important rule.  This is Ohm’s Law, which states:

The current in a metallic conductor is directly proportional to the voltage between its ends provided that the temperature and other physical conditions are the same.

A conductor that obeys Ohm’s Law is called an ohmic conductor.

Question 3 What are the key points to Ohm’s Law?   ANSWER 

Question 4  A 15 ohm resistor is connected across a 3 V supply.  What is the current?  ANSWER

If we plot a voltage current graph we see the following:

This graph is a straight line.  Note that we put the voltage on the vertical axis and the current on the horizontal axis.  If we measure the gradient, we get the resistance.  Notice that the graph goes through the origin. 

Question 5  Why does the line go through the origin?  ANSWER

Voltage-Current Graph for a Lamp

A light bulb is not an ohmic conductor.  Its graph looks like this:

Question 6  Why is the graph of a light bulb shaped like this?  ANSWER

Question 7  A lamp takes a current of 0.3 A at a voltage of 6 V.  What is its resistance?  Some students measure the resistance of the lamp with an ohmmeter and find that the resistance is only 4 ohms.  Explain why this happens.  ANSWER

Some Electronic Components

Electronic components do interesting things with electricity as we will see.  If you want to see more about electronic components, see Electronics AS Module 1.

Diodes

A diode is a one way "electrical valve".  The picture below shows the idea.  (This would lend itself to a splendid animation, but I don't have the program to do web animations.  Sorry.)

If the water is pumped forward, the valve is opened as above.  If the water is pumped backwards, the valves slam shut as below:

If the pump is worked hard enough, eventually the valves will burst open.

Diodes look like this.  You can see that there are different types.

The diode is made of semi-conductor material.  It behaves in the following way:

• If the diode is connected so that it can conduct electricity, we say that the diode is forward biased.  It allows electricity to flow.

• If the diode is the other way round, it is reverse biased.  It does not allow electricity to flow.

• If you increase the voltage across a reverse-biased diode, eventually a breakdown voltage is reached, and the diode conducts.  This can wreck the diode.

This can be shown as a voltage current graph.

The forward biased diode starts to conduct at +0.6 V.  The reverse biased diode breaks down at about -30 V.  The breakdown voltage varies according to the type of diode.

Question 8  Describe what is happening on this graph at Points A to D.  ANSWER

The light emitting diode (LED) is a special kind of diode that when forward biased gives out light.  It is possible to get LEDs that have different colours, or emit infra red light, as in a remote controller for your TV set.

LDR

The light dependent resistor (LDR) has a resistance that decreases as the light level goes up. 

The most obvious use for the LDR is as a light detector.  Light detectors can be found in:

• counting systems;

• burglar alarms;

• automatic switches for street lights;

• some kinds of smoke detectors;

• flame-out detectors.

Thermistor

The thermistor is a component whose resistance goes down as its temperature rises.  Here is a picture of a thermistor protecting a power supply.

Thermistors have uses wherever temperature is detected electronically.  Examples include:

• electronic thermometers;

• heat detectors.

Basic Electrical Circuits

Components in electrical circuits can be wired:

• in series, where the current passes through on component after another.

• in parallel where each component has its own branch.

Series Circuit

In this circuit the current has to flow through resistor R1 then R2, which are in series.  There are three points to learn about a series circuit:

1. The current is the same all the way round;

2. The voltages across the resistors add up to the battery voltage.  In other words,         V1 + V2 = VB, where VB is the battery voltage;

3. The resistances add up to a total resistance.  R total = R1 + R2.

Look at this circuit:

Question 9  What are the readings on voltmeters V1 and V2?  How do they compare to the battery voltage?  What is the total resistance of the circuit?  ANSWER

Parallel Circuit

The diagram shows a circuit of two resistors in parallel.

You can see that an electron can pass through either R1 or R2, but not both.  There are two main points to learn from a parallel circuit:

1. The voltage is the same across each resistor.  So the voltage across R1 and the voltage across R2 and the battery voltage are all the same.

2. The currents in each branch add up to the total current.  If ammeter A1 reads current I1, A2 reads I2, and AT reads total current I tot, I tot = I1 + I2

Now look at this circuit:

Question 10.  What are the currents through the 2 ohm and 4 ohm resistors?  What is the total current in this circuit?  ANSWER

Now answer Question 11

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