Physics TAS

2009-04-11 9:33 am

Physics TAS

1.) Exp: To determine the internal resistance of a voltage source.
1.Record the voltage by connecting the voltage source across the voltmeter
2.Then connect the voltage source with a variable resistor, the voltmeter is connected across the voltage supply. Varying the variable resistance and record the p.d. and I. Plot a graph of p.d. vs I.
I want to ask why the emf found from the graph(y-intercept) is used but not the one recorded in procedure 1?
Also, what are the precaution of the exp?

2.) Exp: Studuing a simple harmonic oscillator
Use ticker-timer to obtain a tape of a simple pendulum. use the tape to plot a x-t graph, the v-t graph and the a-x graph.
My Q is, What is the physical significance of the slope of the v-t graph?

Thank you very much.

回答 (1)

天同

2009-04-11 9:48 pm

✔ 最佳答案

1. Let r be the internal resistance of the voltage source, R be the resistance of the variable resistor, and E be the emf of the source.
By Ohm's Law, E = I(R+r)
i.e. E = IR + Ir
but IR = V, the voltmeter reading
hence, E = V + Ir
or V = E - Ir
A plot of V against I is therefore a straight line, with slope equals to r (internal resistance of source) and y-intercept to E (emf of source).
The voltmeter reading in recorded in step(1) only gives you the "terminal voltage" of the source, i.e. the potential difference between the +ve and -ve poles, but NOT the emf, since the potential difference across the source internal resistance cannot be measured by the voltmeter.
Precaution: the internal resistance of the voltmeter must be high compared with the max resistance of the variable resistor used.
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2 The slope of the velcity-time graph gives the acceleration of the pendulum bob.

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