ELECTRICAL ENERGY (CONTINUED) ENOTE

WEEK THREE

SUBJECT: BASIC SCIENCE

CLASS: GRADE 9

TERM: SECOND TERM

TOPIC: ELECTRICAL ENERGY (CONTINUED)

AMMETER AND VOLTMETER AND GALVANOMETER: Ammeters and voltmeters are changed forms of galvanometer, a device that is used to detect small currents. Galvanometers are modified to be either ammeters or voltmeters. Galvanometers that are calibrated to measure currents of different amounts are called ammeters. When they are modified to measure voltages, they are called voltmeters. A galvanometer is constructed from coils on springs, a needle and a stationary magnet. When a current is passed through the coils, the needle is deflected. This deflection is proportional to the amount of current. Galvanometers as ammeters can directly measure DC current but to measure AC, a diode is placed inside the device. The purpose of this diode is to limit the current flow to one direction. Without it, the needle would fluctuate back and forth or not move at all, and it would not be possible to take a reading. Ammeters and voltmeters are the opposite of each other not only in terms of construction, but of circuit placement.

Ammeter:  Electric current is measured using an ammeter. Current is the measure of the rate of electron “flow” in a circuit. It is measured in the unit of the Ampere, simply called “Amp”, (A). The most common way to measure current in a circuit is to break the circuit open and insert an “ammeter” in series (in-line) with the circuit so that all electrons flowing through the circuit also have to go through the meter. Ammeters must have current passing through them in order to measure it, and so must be placed in series inside a circuit. Polarity must be observed, and their positive and negative leads must be connected to the positive and negative sides of the circuit, respectively. Also their internal resistance must not be large or it will not block too much current, and this will affect the way the circuit functions and hence the reading.

Voltmeter: Voltmeters measure the potential difference or voltage in a circuit, and are galvanometers that are placed in series with a resistor. Ideal galvanometers must have an infinite resistance, and this is approximately having the value of the resistor to be very large. A diode must be placed inside in order to measure AC currents. Voltmeters should not have any current passing through them, and so are placed in parallel with the circuit they are to measure. Their leads are placed on each side of the component of interest, and as with ammeters, polarity must be observed. If their internal resistance is too small it will not block enough current, and this will affect the measurement.

FUSES AND CIRCUIT BREAKER

(i) A fuse is an electrical device, a type of low resistance resistor that acts as a sacrificial device to provide over current protection. A fuse interrupts excessive current (blows) so that further damage like overheating or fire is prevented.

(ii) The circuit breaker is an absolutely essential device in the modern world, and one of the most important safety mechanisms in your home. Whenever electrical wiring in a building has too much current flowing through it, these simple machines cut the power until somebody can fix or resolve the problem. Without circuit breakers (or the alternative, fuses), household electricity would be impractical because of the potential for fires and other disasters resulting from simple wiring problems and equipment failures.

ELECTRIC METER READING AND BILLING

Electric Meter: Your electric bill is calculated in kilowatt-hours (kWh) used in a monthly billing cycle. An electric meter measures kWh. A kWh is 1000 watts of electricity used for one hour. One 100 watt bulb glowing for 10 hours equals one kWh. As the meter is read from month to month, the difference in these readings accounts for your consumption and you are billed accordingly.

  There are two types of electric meters used to measure residential consumption. One type o9f electric meter has numerals similar to the odometer in your car. As energy is consumed these columns of numerals revolve, tallying the amount of kWh used. The other type of electric meter has four or five dials that look similar to clock faces. In this meter, the right dial measures kWh. This means that the appliances have to use 1000 watt-hours to move the dial’s point one number. When this dial completes one revolution, the dial to the left advances one number. The first, third and fifth dials move clockwise; the second and fourth dials move counter clockwise.

How to Read Your Electric Meter: Read all the dials from right to left, and write the numbers down from right to left. When the hand on any dial is between two numbers, read the smaller number.  If the pointer appears to be exactly on a number, record the next lowest number unless the pointer on the dial to its right has passed zero.

Calculating Your Energy Consumption:  Meters are highly accurate instruments. One way to monitor your consumption is to read your meter and check it daily or weekly. If you read your meter at the same time each day, subtract the previous day’s reading from the present reading to determine the number of kilowatt-hours used in one day. By reading your meter at the same time each day, you will get an exact total of the electricity you have used. By noting high consumption activities, such as air conditioning in the summer, you will know where you are spending your electric energy. Reading your meter often, along with careful observation of the weather and appliances being used, can help you manage energy consumption and evaluate the effectiveness of appliances.

ASSIGNMENT

OBJECTIVE QUESTIONS:

1. A television set of 40 watt is used for 2 hours per day for 30 days. Calculate the quantity of energy consumed by the television in kw. A. 0.42 B. 2.4 C. 24.0 D. 72.0 E. 2400

2. Two 4 ohms resistors connected in series have a total resistance of – A. 2Ω B. 4Ω C. 8Ω D. 16Ω E. 32Ω

3. The electrical symbol below represents a/ an.                   A. ammeter B. cell C. fuse D. resistor E. switch

4. A dry cell stores – energy. A. chemical B. Electrical C. Heat D. Nuclear E. Solar

5. Calculate the current passing through a conductor of resistance 4Ω if a potential difference of 15V is connected across its ends. A. 60A B. 19A C. 11A D. 6.25A E. 3.75A

THEORY QUESTIONS:

1. Differentiate between ammeter and voltmeter.

2. What is a galvanometer?

3. What is a fuse?

4. Explain the working of an electric meter.