Technical Card in Physics
DC Voltage
Grade: 9
Duration: 3 periods.
Objectives:
1. Know how to use of digital multi-meter.
2. Measure DC voltage and an electric current using digital multi-meter.
3. Know the laws of voltages and currents in an electric circuit.
4. Measure a DC voltage using an oscilloscope.
Procedure and observation:
Activity 1: Know use of digital multi-meter.
- Materials: 1- A multi-meter.
Take a multi-meter and ask a question:
What is a multi-meter? After we receive students’ answers we select the correct one : “multi-meter is an instrument of multiple uses, used in electric measurement”.
• Can be used as a voltmeter which measures DC or AC voltage.
• Can be used as an ammeter which measures DC or AC currents.
• Can be used as an ohmmeter which measures resistance of conductors,
and this depends on the mode and the scale used.
Activity 2: Characteristics of a DC voltage.
Materials used:
- Low frequency generator (DC – Power supply).
- Digital multi-meter.
- Connecting wires and a switch.
Procedure and Observation: 1-Adjust the DC – Power supply at 20V. The terminals of the (LFG) are connected to the multi-meter such that the (-) to com.
The multi-meter adjusted to DC V mode and scale at 20V.
We read the reading of multi-meter during successive intervals of time. What do we observe? Reading remains the same voltage.
Repeat the same procedure, but instead of (LFG) replace it by a car battery?
What do you observe? If we make a table V(t) = constant=12.5v.
Its graph is a straight line V(t)= 12.5v.
We conclude that DC Voltage supplied remains constant with time,and its graph is a horizontal straight line.
| 4 | 3 | 2 | 1 | t(s) |
| 12.5 | 12.5 | 12.5 | 12.5 | V (v) |
Activity 3: The use of digital ammeter:
Materials:
- Multi-meter.
- DC – Power supply or (LFG) DC mode adjusted at 12V DC.
- Lamp (12v; 20w).
- Switch + connecting wires.
Procedure and observation:
- Adjust ammeter and select the highest range (10A).
- Connect the com of ammeter in series with opened switch.
- Connect the 12v lamp in series with ammeter.
- Observe and read current during different intervals of time.
- Write the readings in a table:
| 4 | 3 | 2 | 1 | t(s) |
| 0.63 | 0.63 | 0.63 | 0.63 | I(A) |
The current remains constant in DC V
Activity 4-1: Law of current and voltage in DCV case of series connection.
Materials:
- DC – Power supply 6v.
- 2 lamps L1 (6v; 60mA) and L2 (6v; 60mA).
- 2 digital ammeters.
Procedure and observations:
- Adjust ammeters and select highest range 10A.
-We increase the voltage of (LFG) until lamps L1 and L2 light, and then observe the readings of A1 and A2. What do you conclude? Current is the same in lamps L1 and L2 so in series connection current remains the same.
Law of voltage addition
If ammeters are removed, their mode are changed to voltmeter mode, connected in parallel across L1 and L2 and a third one across both L1 and L2, we get from observation and reading, V= V1 + V2 = 5.3 + 5.7 = 11V
Activity 4-2: Laws of currents and voltages in case of parallel connection.
Materials DC – Power supply
- 3 digital multi-meters
- 2 lamps
- Switch
- Connecting wires
Procedures:
Adjust the three multi-meters as ammeter mode.
Adjust ammeter on the highest range(10 A).
Connect an ammeter (A) on the main line, then connect
(A1) and (A2) with lamp L1and L2 respectively, in parallel branches.
When switch closed and generator adjusted until two
lamps light, write the readings of A, A1, and A2.
We conclude from these readings that the main current
I= I1+I2.
Next step:
- Adjust the three multi-meters as volt meter mode of scale 20v, and then connect them as in fig (4.2).
- Start adjusting (L.F.G( D.C mode slowly until lamps light.
- Write reading of V, V1, V2.
- What do you conclude?
Activity 5: Using of an Oscilloscope in the measurement of DC voltage.
Materials used:
- (LFG) DC mode.
- Oscilloscope.
- Digital voltmeter.
Procedure and observation:
1. Turn on (power on).
2. Adjust the sharpness of the horizontal line (by focus button).
3. Let the horizontal passes in O.
4. Select the 2ms/div for horizontal sensitivity.
5. Adjust the LFG to DC mode (6vmode).
6. Adjust the vertical sensitivity (2v/div)
7. Adjust the digital voltmeter at 20v.
Connect the voltmeter to LFG and to oscilloscope.
What do you read on the voltmeter and on the screen of oscilloscope?
The reading of voltmeter is 6v, and on the screen the line displaced up word by 3 div. So,V= 3x2= 6volts=Sv xy (where Sv vertical sensitivity and y number of div on vertical.