What electrical property of the DC voltage source discussed at the beginning of

Question

What electrical property of the DC voltage source discussed at the beginning of lab determines which direction the galvanometer needle swings??

Voltage Supplies There are two types of voltages: 1. AC Voltage. Voltages that vary in time by reversing their polarity are commonly referred to as AC (alternating current) voltages. For example, the standard wall socket supplies an AC voltage that changes its polarity 60 times per second with a peak voltage difference between the two slots of about 150 volts. 2. DC Voltage. Voltages that are constant in time are referred to as DC (direct current) voltages. DC voltages are most commonly available from batteries, ranging from the small batteries in toys, cell phones, and flashlights, to the larger variety used in automobiles. These supply DC voltages ranging from 1.5 to 12 volts and are created by chemical processes. However, DC voltages are also available from “power supplies,” devices that convert the standard wall socket AC voltage to DC voltage. In the case of a DC voltage we use the following symbol to represent it. The DC voltage source has two terminals, a positive terminal (anode) and a negative terminal (cathode). The anode is characterized as having a voltage that is V volts higher than the cathode. Positive charges will be pushed from the anode to the cathode if a path or “circuit” is available to them. The anode or positive terminal is visualized as being “uphill” from the cathode or negative terminal. The labs we do will often use a DC source that converts AC wall voltage into a variety of DC voltages, including a 5 volt and a 12 volt source. The voltage is measured with respect to the ground or common plug.

Voltage differences are measured with a voltmeter. This course will use two types of voltmeters. The most commonly used device is called a digital multimeter. It is capable of measuring voltages, currents, and other properties of electrical circuits such as resistance and capacitance, which we will study later. The second device is a galvanometer. It is an analog device that can measure only voltages. However, it has the capability of displaying voltages that change as long as it does not occur too quickly. We simply attach the two probes from the voltmeter to the two points of interest, and the voltmeter reads the voltage difference between the positive (red) probe and the negative (black) probe. The latter is sometimes called the “ground” or “common” point for reasons we will discuss later in the semester. Note that voltage differences can be positive (the red probe point is at a higher voltage than the black probe point) or negative (the red probe point is at a lower voltage than the black probe point). In electricity, the sign of the voltage difference is called the polarity of the voltage. This terminology stems from visualizing electric charge as a “pole” (in the same sense as the north and south pole of the Earth) of electric flux. In the case of the digital multimeter it must be configured to measure DC voltages by placing the voltage switch on “DC” and placing the dial on an appropriate range of voltages. It cannot measure a voltage difference greater than the top of the range and will read 0L if such a voltage is measured. It will indicate the polarity of the voltage by reading out either a positive or a negative voltage. In the case of the galvanometer, the range of voltages that are represented is chosen by holding one of its three buttons down. We will investigate its behavior in today’s lab. The galvanometer indicates a positive polarity if the needle swings to the right and a negative polarity if it swings to the left.

Explanation / Answer

The property of maintaining the constant current accounts for the deflection in galvanometre. If the positive of battery is connected to the positive of galvanometre then the deflection of neddle is towards the positive reading (right) but if the negative of battery is connected to the positive of galvanometre then the deflection of neddle is towards the negative reading (left).

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