Electricity Tutorial 1 - Basic Electrical Measurement

The instruments that are of most use to the physicist and the electrical engineer are the voltmeter and ammeter.  With these, we can directly measure:

We can then use the data to get:

The multimeter is often used, as it can measure voltage and current (but not at the same time).

Notice that there is an analogue meter (a meter with a scale) and a digital meter (a meter where the read-out is a number).  It is important that you learn how to use these instruments correctly in your practical work.  If in doubt, ask your teacher.

Current is measured with an ammeter, which is wired in series with the component.  The voltmeter is wired in parallel with the component.

 

Current and Charge

The base electrical quantity is current, the flow of charge.  All other electrical quantities are derived from it.  Current is measured in ampères, or amps (A). 

 

Charge is measured in coulombs (C), which is defined as:

1 coulomb is the quantity of charge carried past a given point if a steady current of 1 amp flows for 1 second.

A single electron carries a charge of 1.6 × 10-19 C

1 coulomb is equivalent to 6.2 ´1018 electrons.  It is much more convenient to use this rather than counting individual electrons.  You would buy a 1 kg bag of sugar rather than counting all the crystals in it.

Question 1

What do you think an electron is?  

Answer

Charge and current are linked by a simple formula:

            Charge (C) = current (A) ´ time (s)

 Q = It

  In the syllabus the formula is written in physics code as:

  The symbol D is Delta, a Greek capital letter 'D', meaning “change in”.

  There are some important multipliers for current:

 

These are useful when we are dealing with small currents.  However we must remember to convert to SI units for doing calculations.  Watch out for this bear trap!

 
Question 2 Can you show that 1 coulomb is 6.2 ´1018 electrons? Answer
Question 3 A charge of 1.24 C flows in a period of 0.63 s.  What is the current? Answer

 

 

Potential Difference

Potential Difference is defined as energy per unit charge.

The unit of potential difference is the volt (V).  Using the definition, we can define the volt as Joules per Coulomb

1 V = 1 JC-1.

Potential difference (V) = energy converted (J)

                                                Charge (C)

In physics code we write:

Potential difference is often referred to as voltage. 

Conventional current goes from positive to negativeElectrons carry energy around the circuit; they go from negative to positive.  In the early days, physicists didn't know about the electron, which is why they got it all wrong.  Correction would require a complex re-write of the Laws of Physics, a task which no-one is likely to be bothered to tackle.  So all conventional currents are from positive to negative.  All currents are treated as conventional.

 

Sources of Voltage

A single cell normally provides about 1.5 volts.  A lab-pack might provide 15 V, and a generator can provide up to 25 000 V.  Without a voltage source, current cannot flow. 

 

If we connect the cells in series, we have a battery of cells.  The battery below has four cells in series.  Its voltage is 4 × 1.5 = 6 V.

We can wire cells in parallel as well.  This enables us to get a higher current.  A minibus might have two 12 V batteries in parallel to provide the massive current (1000 A) taken by the starter motor for its diesel engine.

This combination of cells will give us an output voltage of 3 V, not 6 V.  We would be able to get twice the current from this set of cells.

 

The combination of the cells shown below gives out a potential difference of 4.5 V:

This is because the voltage of the parallel combination is 1.5 V

 

The two cells in the parallel combination will last twice as long, since the current will be half that taken from the two series cells.

Batteries are often rated in amp-hours.  A 1 amp hour battery can give out a current of 1 amp for 1 hour.  The charge contained is:

Q = 1 A × 3600 s = 3600 C

 

Question 4

A cordless drill operates using a 14.4 V battery pack.  The battery is rated at 2 amp hours which means that it can deliver a current of two amps for a period of 1 hour.  How much energy is held by the battery? 

Answer

Resistance in a wire is the opposition of a wire to the flow of electricity.  It is caused by collisions between the electrons and the atoms in the wire.  The hotter the wire, the more chance there is of a collision.  Therefore hot wires have more resistance.  The formula for resistance is:

Resistance (ohms) = potential difference (volts)

                                 current (amps)

In physics code we write this as R = V/I 

Or more commonly:

V = IR

The unit for resistance is ohm (W).  (The curious symbol ‘W’ is Omega, a Greek capital letter long Ō.)

Question 5 What do you understand by the term resistance?  Answer
Question 6

Use the circuit below to answer the questions:

 

V
I
R

 

(a)

 

0.30 A

 

18 W

 

(b)

 

12 V

 

88 W

 

(c)

 

14.4 V

 

0.52 A

Answer

  Watch out for these bear traps in electrical calculations:

  Time must be in seconds

  Make sure you convert milliamps to amps