Circuit Electricity
Potential difference and energy, current and charge, resistance - the big concepts underlying the operation of electric circuits - will be the focus of this segment of practical work. It is crucial that you develop a sound understanding of these ideas in order that you may design circuits for a variety of applications. The understanding required is much more than facility in manipulating Ohm’s Law and the series and parallel resistance relationships. These experiments will be valuable in helping you improve your skills in the construction and analysis of circuits - careful measurement techniques will be emphasised. In addition, the experiments should help deepen your understanding of how circuits function.
Electric Circuits – A Review?
We commence with some background information on electrical circuits, and a simple introductory experiment, Electric Circuits- A Review?, to enhance your understanding of the basics of electrical circuit operation.
You may be familiar with the initial ideas of measurement in electric circuits - ammeters are connected in series to measure the rate at which charge passes through an electrical component, while voltmeters are placed in parallel to track the difference in potential energy per charge between one side of the component and the other. There is a lot more to it though ...
The Correct Use of Voltmeters and Ammeters
How do the meters affect the quantities being measured? After all, they provide paths for the charge and so are themselves part of the electric circuit. You will explore this area in the experiment The Correct Use of Voltmeters and Ammeters.
Constructing a DC Power Supply
The final experiment begins by introducing the operation of two components crucial to the design of AC circuits - the Capacitor and Diode. Both of these devices are nonlinear - that is the current through them is not directly proportional to the voltage across them. Clearly Ohm’s Law will be of limited use in analysing circuits including capacitors and diodes. You will be introduced to the characteristics of electronic devices, which is a graph of current versus voltage for that device. The capacitor and diode are then used to produce a D.C. power supply from an AC. source.
Throughout these experiments you will develop confidence in your ability to wire up and make measurements in electrical circuits. At the same time, this practice in constructing and analysing circuits will improve your understanding of the underlying relationships between current, voltage and resistance.
Background Information
Sources of E.M.F.
In general, a D.C. (direct current) circuit consists of a source of E.M.F. (often a battery or a single cell) and a closed circuit around which a current can flow. E.M.F. stands for the archaic and misleading term "electromotive force". The source of E.M.F. is a device which maintains a potential difference between its terminals, and so provides the energy to maintain a current around the circuit. The E.M.F. is a measure of the electrical energy supplied to charge which travels from one terminal to the other. It is measured in joule per coulomb, ie. volt (V).
Sources of E.M.F. may be combined in series to give a voltage output equal to the algebraic sum of their individual voltages; eg. consider a string of dry cells:
cell 1 maintains A at a potential VAB above B
cell 2 maintains B at a potential VBC above C, etc.
Therefore, the potential difference (P.D.) between A and E is:
VA - VE = VAE = VAB + VBC + VCD + VDE.
This is sensible - each cell supplies energy to charge which passes through it, and the energy provided by the whole string of cells is equal to the sum of these energy contributions.
A battery consists of a number of cells connected in series. The voltage quoted on a battery is its nominal voltage. The actual E.M.F. could be significantly less, depending on the internal resistance of the battery which is affected by the battery's age and history. A portion of the nominal voltage is then “lost” due to the energy dissipated as the charge passes through the battery itself.
Ohm's Law
When a potential difference, V, is applied across a conductor (eg. by connecting the ends of the conductor to the terminals of a battery as shown in the diagram), charge, flows through the conductor. The rate at which charge flows past each point in this circuit is the current, I, measure in coulomb per second, ie. ampere (A).
This cause/effect relationship between potential difference (P.D.) and current for many types of conductor is described by Ohm's law, I V. It therefore follows that V/I is a constant, which is called the resistance R (ohms). Thus,
R = V / IorV = I RorI = V / R.
I (in amps) is the current in a conductor of resistance R (in ohms) when a P.D. of V (in volts) is maintained between its ends. It is an experimental fact that the resistance R of many conductors remains essentially constant as the current is varied, and this will be assumed throughout the following experiments unless stated otherwise. There are some common exceptions to this sort of behaviour however - the resistance of the fine wire in light globes increases with temperature, and semiconductor materials experience a decrease of resistance with temperature and often when exposed to light.
Notes on Wiring Up Electrical Circuits
- Before starting to wire up, draw the circuit diagram in your practical book, showing all the components you are actually going to use. You should always label (i) all components with suitable symbols or values, and (ii) the positive terminal of every meter with a + sign. Remember to show the direction of current flow.
- It is helpful to arrange the major components of your circuit in the same relative positions as those they occupy in your circuit diagram.
- In connecting up:
(i)If the circuit contains more than one loop, wire up one loop at a time. ie. first wire one loop completely from one terminal of the input voltage to the other, then wire up the second loop between the two points where it meets the first, and so on.
(ii)When including meters, place ammeters in the circuit when you wire up the loop containing the component of interest, but place voltmeters in the circuit after all components have been wired into the circuit.
(iii)It is often useful to include a switch in order that the battery not be drained unnecessarily.
(iv)Connect the battery last.
(v)When you do finally connect the battery, momentarily touch the terminal with the lead, check that all meters turn the right way and that they are not overloaded.
Make sure every member of your group gains experience in wiring these circuits. This is a skill which will be useful throughout the semester.
Colour Codes for Resistors
Four Coloured Bands:If the resistor has 4 coloured bands, then the resistance value can be decoded as follows:
bands A,B and C / band DBlack / 0 / Silver / ± 10%
Brown / 1 / Gold / ± 5%
Red / 2
Orange / 3
Yellow / 4
Green / 5
Blue / 6
Violet / 7
Grey / 8
White / 9
Example:(370,000 ± 5%)
= (370 ± 18) k
Five Coloured Bands:If the resistor has 5 coloured bands, then use:
bands A, B, C and D / band EUse colour code above / Red / ± 2%
Brown / ± 1%
Example(33,200 ± 2%)
= (33.2 ± 0.7) k
Physics 121/2 and 141/2 Laboratory ManualC-1