Physics 2: Five Pre Lab questions "Galvanometer​"

timer Asked: Mar 20th, 2019

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Experiment: Electrical meters - Galvanometer

I just need answers to the 5 pre - lab questions attached.

Experiment 6: Electrical meters The subject: electricity means “of amber" or "like amber", from ἤλεκτρον, elektron, the Greek word for "amber". Dramatis Personae (by entrance) • • • • • Luigi Galvani (1737 – 1798) In 1780, he discovered that the muscles of dead frogs' legs twitched when struck by an electrical spark. Alessandro Volta ( 1745 – 1827) inventor of the electrical battery in 1799. André-Marie Ampère (1775 – 1836) French physicist. Hans Christian Ørsted (1777 – 1851) discovered that electric currents create magnetic fields, which was the first connection found between electricity and magnetism. Georg Simon Ohm (1789 – 1854) He found that (in most cases) there is a direct proportionality between the potential difference (voltage) applied across a conductor and the resultant electric current. Purpose (1) To use a galvanometer as a milliammeter. (2) To convert a galvanometer into a reading voltmeter. (3) To determine the characteristics of a galvanometer. Apparatus (a) a galvanometer, (b) a resistance decade box, (c) a rheostat (for part 2) (d) batteries Theory a) basics A galvanometer is an instrument used to detect and measure very small electric currents. Its main working part is a tiny coil suspended between the poles of a permanent magnet. The coil will experience a torque when a current passes through the wire of the coil. The coil is attached to a pointer and a spring so that the amount of deflection of the pointer is proportional to the current in the wire of the coil. The maximum deflection of the galvanometer needle is limited by the current which the galvanometer resistance can support. Too much current will damage the galvanometer. Note that the galvanometer (as any other current carrying element of an electrical circuit) has an internal resistance, which we shall indicate as RG. b) a galvanometer as a milliammeter To measure a current the meter has to be placed in series in the circuit so that the current flows through it. As mentioned above, the total resistance of the circuit has to be such that the current does not exceed the maximum current the galvanometer can support. To be a reliable meter, the internal resistance RG of the meter has to be low, so that it does not significantly alter the current flow in the circuit itself. Figure1 shows the basic circuit for a galvanometer used as a milliammeter Figure 1: a galvanometer as a milliammeter c) a galvanometer as a voltmeter The voltage range of the galvanometer can be extended by connecting a resistor, called a multiplier, in series with it. (See Fig. 2). Then the voltage VAB between points A and B may be made larger while keeping iG for the full meter deflection the same. Figure 2: a galvanometer as a voltmeter The potential difference between A and B is VAB = iG (RM + RG). Since the galvanometer deflection is proportional to iG , it will also be proportional to VAB . By a convenient choice of RM , this deflection can be calibrated to read the voltage VAB directly. Important: The resistance RM must be chosen large enough so that for the range of VAB used, the current for maximum galvanometer deflection will not be exceeded. In addition, the maximum iG of your galvanometer should be small enough to not significantly disturb the original voltage, VAB , existing between points A and B before the galvanometer and RM are connected. Without the galvanometer branch, we have VAB = IRAB. The resistance (RM + RG) must be large enough so that the current I through RAB is not significantly changed by connecting the galvanometer branch. After connection, we will have that VAB = (I – iG) RAB = (RM + RG) iG. If (RM + RG) >> RAB, then iG <
Prelab Name Class Section 1) if a 9.0V battery is connected in series to a resistor with resistance R and a galvanometer whose full scale range is 1 mA, what is the minimum resistance R needed to avoid damaging the galvanometer? Show your calculations to get credit. 2) should a voltmeter be connected in series or in parallel? Explain why. 3) should its internal resistance be high or low? Explain why. 4) should an ammeter be connected in series or in parallel? Explain why. 5) should its internal resistance be high or low? Explain why.

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