drop in p.d. supplied (TAS)

Sze Wai

2009-12-18 4:25 am

1) How to show that the lost p.d., the current and the internal resistance of the power supply can be related by E-V =Ir?
2) Why EHT supply has high internal resistance?

回答 (1)

天同

2009-12-18 5:13 am

✔ 最佳答案

1. This just follow directly from Ohms' Law.
Let E be the emf of the power supply
r be the internal resistance
R be an external load resistance
I be the current through the load

Hence, E = I(r+R)
i.e. E = Ir + I.R
But what you could measure is I.R, which equals to the terminal voltage V of the power supply

Therefore, E = Ir + V
or E - V = Ir
The p.d. [I.r] cannot be measured directly

2. The high resistance is to limit the power output of the EHT supply. Since a EHT supply provides a high voltage, a small current (in order of mA) could provide quite a large power already.

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