Answers to Question EE101-Experiment 1

Answers to Question and Problems 1. Describe the type of meter movement used in analog meters. The type of meter movement is coil. As current flows in the movable coil which were supported by jeweled bearings between the poles of a permanent magnet, then the core will be magnetized. The coil experiences torque and rotates to align its plane perpendicular to the line joining the poles of the permanent magnet. The pointer deflects according to the change in the direction of current flow. 2. What device is used to extend the range of a DC ammeter and how is it connected to the basic meter? A shunt resistor is used to extend the range of a DC ammeter. It allows the measurement of too large current values since it is placed in series with the load so that all of the current to be measured will flow through it. It is connected across the meter terminals and may be located inside (internal shunts) or outside (external shunts) the meter case. 3. What device is used to extend the range of a DC voltmeter and how is it connected to the basic meter? A device called multiplier resistor is also added on the circuit to extend the range of dc voltmeter. It will then affect the voltage in the voltmeter but not the current. Adding a resistor with the same resistance with the voltmeter, the range will be doubled.

4. Why is it necessary that an ammeter be a low resistance instrument? Why must a voltmeter be a high resistance instrument? Ammeters are used in series while voltmeters are used in parallel between two points, where the measurement is to be made. Ammeter is necessary to be a low resistance instrument and the voltmeter must be a high resistance instrument so that these meters would not alter the behavior of the circuit. 5. What type of basic meter is configured as a wattmeter? Electrodynamometer meter is configured as a wattmeter. An electrodynamometer is an instrument used for measuring the electric power. 6. Two 150V voltmeters are being compared; meter A has 5kΩ/V while meter B has a total meter resistance of 750kΩ. Which is the more sensitive meter? Why? Voltmeter sensitivity is expressed in ohms per volt. It is the resistance of the voltmeter at the full-scale reading in volts. Using the equation for computing voltmeter sensitivity which is total resistance over the input resistance, meter B is more sensitive meter who has 5KΩ/V than meter A which has 33.33 Ω/V. 7. What

current

is

required

for

full

scale

deflection

of

a

galvanometer having a current sensitivity of 50μA per scale division? The meter has exactly 50 divisions on either side of the mid-scale index. I =50

μA × 2∗50 divisions division I =5 mA

8. What potential difference appears across the galvanometer described in problem 6 when the pointer is fully deflected? The meter resistance is 10Ω. voltmeter sensitivity=5 k Ω/ V

(

I m= 5

kΩ V

−1

)

=2× 10−4 A

V =I m R m=( 2×10−4 A ) (10 Ω) V =2 mV

9. Determine the series resistance needed to convert a galvanometer to a voltmeter reading 15V on full scale deflection if a current of 2.5mA causes full scale deflection and the resistance of the coil is 10Ω. V T =I m ( R s+ R m ) Rs =

VT 15 V −R m= −10 Ω Im 0.0025 A

Rs =5990 Ω

10.

Determine the shunt resistance needed to convert a

galvanometer to an ammeter with a full scale reading of 10A if the scale requires 2.5mA to ca resistance of 10Ω.

use full deflection with a coil

Rsh=

I m Rm ( 0.0025 A ) (10 Ω) = I T −I m 10 A−0.0025 A

Rsh=2.501 m Ω