ALL CIE PHYSICS DOUBTS HERE !!!

[ashwinkandel]

Current is not given in the question. Only Voltmeter reading and resistance of voltmeter and other two resistors are given.

[astarmathsandphysics]

i.The voltmeter has a resistance of 1200 ohm. calculate the emf of the battery.

Where is the voltmenter reading to answer this question>?

[ashwinkandel]

I think you forgot to read : "A student builds the circuit in the diagram, using a battery of negligible internal resistance. The reading on the voltmeter is 9.0V."

[astarmathsandphysics]

here

[astarmathsandphysics]

Notice that as the resistance of the voltmenter increases, the voltage across the other tow resiststors is more nearly equal to the ratio of their resistances, so it is desirable for resistance of voltmenter to be as high as possible.

[ashwinkandel]

I am still confused with one thing.. the reading shown on that voltmeter... What does that indicate ? I thought it was the p.d across 400 ohm resistor but in calculation you have used it as p.d of the resistor itself..?

[astarmathsandphysics]

You don't understand what a voltmeter is.

A voltmeter is just a very high resistance ammeter, scaled to read voltages. Both ammeter and voltmenter actually detect the current passing through them. The voltmeter is scaled for voltage using V=IR, and since it is always in parallel, it actually gives the voltage across that paralleled component.

[ashwinkandel]

Thanks i got what you meant.Now my mind is clicking the concept. I really appreciate the help that you have been providing . 

[ashwinkandel]

The diagram shows a piece of silicon of width 32mm and length 36 mm. The resistance of the silicon between the points P and Q is 1.1 Mohm. Silicon has a resistivity of 2.3*10^3 Ohm*m.
a. Calculate the thickness of the piece of silicon.[3]
b. Calculate the current which would pass through the silicon if a potential difference of 12V were applied across P and Q.[2]
c. Discuss how the current would change if it were large enough to cause the silicon to become significantly warmer.[3]

[ashwinkandel]

A student is investigating the properties of a semiconducting diode. the diagram shows the circuit she builds.
a.
i. Sketch the graph to show how the current through the diode would vary as the voltage across it is increased from 0 to 1.0 V [1]
ii.The supply is now connected in the reverse direction and once more the potential difference across the diode is increased from 0V to 1.0V. Complete the I-V graph . [1]
b.Suggest why the safety resistor is required.[2]
c.When the potential difference across the safety resistor is 1.4V the current through it is 20mA. Calculate the resistance of the safety resistor. [2]

[astarmathsandphysics]

Tonight

[astarmathsandphysics]

here

[ashwinkandel]

Q1.
In order to switch on a heater when the temperature falls below a set value a potential divider is connected to a switching circuit. When the input voltage to the switching circuit falls below 0.5 V it switches the heater on.
a. Copy the Fig 13.16 and add a suitable potential divider circuit to trigger the switching circuit.
b. Explain how the operator could lower the temperature at which the heater is switched on.

[ashwinkandel]

Q2.
A student is asked to compare the emf.s of a standard cell and test cell. He sets up the circuit as shown in Fig. 13.18 using the test cell.
i. He achieves balance when the distance AB is 22.5 cm. He repeats the experiment with the standard cell of emf of 1.434 V. The balance point using this cell is at 34.6 cm. Calculate the emf of the cell

[ashwinkandel]

Q3. A car battery needs to supply a current of 200A to turn over the started motor. Explain why a battery made of a series of dry cells would not be suitable for a car battery.