Chapter 5: DC voltage.

Transcription

1 Chapter 5: DC voltage. 1. Introduction to electric circuits A. Electric generator (dry cell) - An electric generator transforms any form of energy into electrical energy e.g. Battery or dry cell converts chemical energy into electrical energy. - A dry cell has two distinct poles one positive and the other negative. B. Electric circuit. An electric circuit consist of: - An electric generator (dry cell). - An electric appliance (lam or motor). - Connecting wires. - Switch. C. Representing an electric circuit by a diagram Component lamp Dry cell Connecting wire Symbol Open switch Closed switch motor 2. Voltage and current A. Voltage. - Voltage is the energy carried by each electron. - The symbol of voltage is U. - The SI unit of voltage is volt symbol V. other units 1 V = 1000 mv, 1 KV= 1000 volt. - The instrument used to measure the voltage is called voltmeter, symbol - The com should be close to the negative pole so that to have a positive reading on the voltmeter. - A voltmeter must be connected in parallel across the dry cell or electric appliance. ( figure) - Note: U AB =-U BA B. Electric current - The electric current is the flow of electric charges. - The symbol of the electric current is I and represented on the circuit by an arrow indicating the direction of the flow. - The electric current has a definite direction, by convention the electric current comes out of the positive pole of the dry cell and enters through its negative pole - The SI unit of the electric current is the ampere {A}.

2 - Common units: - Milli-ampere (ma) - Micro- ampere (µa) - The electric current is measured by an instrument called the ammeter. - The ammeter has two terminals the common (com) and the positive terminal (+) - The com terminal is connected to the side of the negative pole of the generator. - The ammeter is connected along one path with the component (series connection) (make figure). 3. Direct voltage and direct current. - The voltage between two points A and B will be called direct if its value remains constant with time. Batteries are sources of direct voltage. - A direct current is the current that flows in one direction it is characterized by a unique sense and value. - When the voltage is direct voltage then the corresponding current is direct current. 4. Measurement of voltage. - In an open or closed switch the voltage across the terminals of a connecting wire can be considered as zero. - In an electric circuit the voltage across a close switch is zero, if the switch is open the voltage is not zero. - In an electric circuit the voltage across the terminals of a load (lamp, motor) is zero if the switch is open and different from zero if the switch is closed - In an electric circuit the voltage across the terminal s of a battery is always different than zero. 5. Grouping of lamps. A. Grouping in series. In grouping in series all the lamps are connected one next to the other. B. Grouping in parallel. In grouping of lamps in parallel all the lamps are connected across each other and have two points in common. 6. Laws of voltage. A. Law of addition of voltage. In grouping of lamps in series the voltage across them is added up. B. Law of uniqueness of voltage. In parallel grouping of lamps the voltage across each lamp is the same or equal 7. Laws of current

3 A. Law of uniqueness of voltage. In grouping of lamps in series the current is the same in all the lamps. B. Law of addition of current. In grouping of lamps in parallel the current in the main branch is the sum of the current in the branches 8. Multimeter. a. Definition. The multimeter is a multipurpose measuring instrument that can be used for measuring, voltage, currents and resistances. b. Structure of multimeter The multimeter is composed of a screen, a knob and terminals. i. Screen: to display the measured value. ii. iii. Knob: it is used to select the role (functioning as an ammeter, a voltmeter or an ohmmeter) of the multimeter and the convenient scale. Terminals: a multimeter usually has three terminals a negative terminal (com) and the others are positive (A, V, Ω). c. How to use a multimeter: 1. Select the convenient zone (ammeter, voltmeter of ohmmeter). 2. Insert the convenient terminals. 3. Select the highest scale and move down to the convenient scale. 4. The convenient scale is greater but closest to the measured value. 5. If the multimeter is used to function as a voltmeter it is then connected in parallel across the load; and if it is used to function as an ammeter it is then connected in series with the load. 6. If the measured voltage is higher than the convenient scale a warning sign is given by the multimeter which is a digit one to the left of the screen. 7. If the positive pole of the battery is connected to the com of the multimeter we will observe a negative sign on the screen.

4 8. The voltage measured by the multimeter functioning as voltmeter is always from the positive terminal to the com. 9. Oscilloscope A. Definition: - An oscilloscope is an instrument used to measure and visualize the voltage. - The function of an oscilloscope is simple: it draws a graph of voltage against time, voltage on the vertical or y axis, and time on the horizontal or X-axis. B. Description of an oscilloscope. The oscilloscope consists of: i. Screen: the screen of this oscilloscope has 8 squares or divisions on the vertical axis, and 10 squares or divisions on the horizontal axis. Usually, theses squares are 1cm in each direction. ii. Sensitivity or controls: sensitivity controls allow you to change the vertical or horizontal scales of the voltage time graph. We have the vertical sensitivity Sv (Scale on the y-axis) gives the number of volts per division (Volt/div or volt/cm) and the horizontal sensitivity (scale on x-axis) gives the number of seconds per division (s/div or ms/div) iii. Entry channels: an oscilloscope has a positive termoinal which is called phase or y- channel. And negative terminal which is called com or mass.

5 C. How to use an oscilloscope. - Press the power button to start the oscilloscope. A spot appears on the screen. Bring the spot to the point of intersection of the two axes of the screen - The oscilloscope must be connected in parallel across the appliance. - To measure the voltage between the terminals of a battery connect the positive pole of the battery to the phase of the oscilloscope and the negative pole of the battery to the ground of the same channel. - The spot moves upward. - To find the voltage we use the following rule: U=Sv y Where Sv is the vertical sensitivity in (V/div) Y is the number of divisions in div U is the measured voltage in V - If the connections of the oscilloscope are reversed the spot will be displaced downwards. - The oscilloscope measures the voltage from the phase to the ground. - In the presence of the sweeping (horizontal motion of the spot) the spot becomes a line that is displaced either upwards or downwards when connected to a battery. - Symbol of phase and ground.

6 Exercise 1: The voltage of the dry cell is 12 V. What is the indication of the voltmeter in each of the following figures? Exercise 2: In order to measure the voltage U PN between the terminals P(+) and N(-) of a car battery (G), some students performed the following experiment They use a multimeter functionning as a voltmeter. the students connect the terminals of the multimeter to the terminals of (G). the multimeter reads (-12V) a. Draw the sketch showing the connections of the terminals P and N of (G) to the terminals (V) and com of the multimeter. b. Does the multimeter display the voltage U PN or U NP? c. Deduce the value of the voltage U PN. Exercise 3: The circuit in figure 1 contains a dry cell G maintaining between its terminals A and F a constant voltage U AF = 6 V, an ammeter (A), a voltmeter (V), lamps L 1, L 2, L 3, L 4, a closed switch k, the phase E and the ground D of an oscilloscope.

7 The figure (2) represents the oscillogram observed on the screen of the oscilloscope regulated with the vertical sensitivity S V = 0.5 V/div. The voltmeter indicates a voltage U BC = 2.5 V. 1. The switch K is closed. Give the value of the voltage U EF across its terminals. (0.5 pt.) 2. What is the nature of the voltage measured by the oscilloscope? Justify. (0.5 pt.) 3. What voltage do you visualize U ED or U DE? Why? (1 pt.) 4. Determine the value of the voltage U ED; then deduce the value U DE. (1 pt.) 5. Determine the voltage U CD then deduce the voltage across lamps 1 and 2. (1 pt.) 6. The ammeter A indicates 400mA. a. Determine the current that passes through L 3. (1 pt.) b. If the current that passes through L 1 is 150mA, determine the current that passes through L 2. (1 pt.) 7. We join points C and D with a connecting wire, a. The lamps L 1 and L 2 will turn off. Justify Why. (1 pt.) b. The brightness of lamps L 3 and L 4 increases. Justify why. (1 pt.) Exercise 4: The figure (a) represents a battery branched across an oscilloscope and the figure (b) represents the figure obtained by the oscilloscope 1. What is the nature of the voltage measured by the oscilloscope? Justify. 2. What voltage do you visualize U PN or U NP? Why? 3. Why does the luminous line move upwards? 4. What is the value of the measured voltage? Given : vertical sensitivity: Sv=5V/div Exercise 5: The waveform I represent the electric voltage U AB. The vertical sensitivity is SV=5V/div 1. Give the type of the voltage U AB represented by waveform I. justify. 2. The value of this voltage is negative. Why? 3. Determine the value of U AB.

8 4. Is the phase of the oscilloscope connected to the point A or to B?why? 5. Give the name of a source of tension may deliver such voltage.