When two lamp bulbs are connected in series all the current that flows through the first bulb in one second must also flow through the second bulb. A series circuit is used in order to have the same value of current.
In a series circuit the total resistance equals the sum of all the separate resistances. Series circuits are usually automatically controlled so that the current is constant.
There are several advantages in using lamps in series. Namely:
1. A number of low-voltage lamps can be connected to a high-voltage circuit.
2. One wire is required from lamp to lamp.
3. Most important is the saving in wire material.
The chief disadvantages are that all the lamps must burn at the same time.
A parallel circuit has the main line and parallel branches. A parallel circuit is used in order to have the same value of voltage. In such a case, if we turn on the light in one room, we may see that both bulbs are bright.
In parallel circuits the total current is equal to the sum of all the currents that are passing through the branches of that given circuit.
(880)
Series circuit (a) and parallel circuit (b)
Answer the questions
1. What is series circuit?
2. What is parallel circuit?
3. What is the difference between series and parallel circuit?
Resistance and Resistivity
Every material offers some resistance to the flow of electric current through it. The resistance of a conductor depends on its material. It also depends on its temperature. Materials change the value of resistance with change in their temperature. Different materials also have different melting points. Good conductors, like the metals copper, silver, and aluminium, offer very little resistance, while non-conductors, like glass, wood and paper, offer a very high resistance. The resistance of copper, silver, and aluminium, is rather high.
Resistance of conductors and their resistivity have different units. The unit by which resistance is measured is called the ohm. The unit of resistance is the ohm while the init of resistivity is the ohm.m.
There are several factors that determine the electrical resistance of any wire: a) the material which is composed of; b) the size of the wire; c) its temperature.
The resistance of a wire is proportional to its length and inversely proportional to its cross-sectional area (the temperature of a conductor remains constant). This is Ohm’s law.
(920)
4.1 Translate the following word – combinations: metal conductor, steel conductors, aluminium and copper conductors, high conductivity, low resistivity, conducting materials, resistant material, different units, independent units.
Answer the questions
1. What does the resistance of a conductor depend on?
2. What materials have low value of resistance?
3. What materials have high value of resistance?
4. What is the unit of resistance?
5. What is the unit of resistivity?
6. What is Ohm’s law?
Measuring units and instruments
Any instrument which measures electrical values is called a meter. An ammeter measures the current in amperes. The abbreviation for the ampere is amp. A voltmeter measures the voltage and the potential difference in volts.
The current in a conductor is determined by two things, the voltage across the conductor and the resistance of the conductor. Every material object offers some resistance to the flow of an electron current through it. Good conductors like the metals, copper, silver and aluminium offer very little resistance. Nonconductors such as glass, wood and paper offer very high resistance. The unit by which resistance is measured is called the ohm. The resistance in practice is measured with the ohmmeter.
A wattmeter measures electrical power in watts. Very delicate ammeters are often used for measuring very small currents. A meter whose scale is calibrated to read a thousandth of an ampere is called a milliammeter. One whose scale is calibrated in millionth of an ampere is called a microammeter or galvanometer. If an ammeter or voltmeter is connected to a circuit to measure electric current or potential difference, the ammeter must be connected in series and the voltmeter in parallel.
(1020)