Electricity and magnetism test questions

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When petrol is poured into the tank of a car it has to travel through a metal pipe. The metal pipe can become _positively_ charged during the filling process. The petrol moving against the metal of the pipe can cause _electrons_ to be rubbed off the metal into the petrol. If electrons are removed from the metal, the metal is left with a _positive_ charge.
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this charging process would not take place if petrol was a _conducting_ liquid because although _electron_ _transfer_ would still take place so the pipe would still become _positively_ charged and then the positively charged pipe would immediately create an _attractive_ force on the _negative_ electrons in the liquid. Because the liquid is a _conductor_ electrons would be _free_ to _move_ back into the pipe and _neutralise_ the charge.
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If a positively charged metal pipe is allowed to touch the neutral metal rim of a car's petrol tank, just before contact is made, a spark could jump between the two. The charge flow is sufficient to create a spark because the _positively_ charged pipe exerts an attractive force on the _electrons_ in the neutral rim.This force causes electrons in the rim to gather at the _surface_ making it _negatively_ charged. The _attractive_ force on the electrons gets stronger as the two surfaces get _closer_ together. When they are very close the force is strong enough to pull _electrons_ out of the rim into the pipe. This flow of electrons is a _spark._
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To prevent a spark from being produced the metal pipe is connected to earth. This prevents a spark from being produced because when the rubbing between the moving petrol and the metal pipe removes an _electron_ from the pipe, the pipe gets a _positive_ charge.The _positively_ charged pipe immediately attracts _electrons_ from earth and they move _into_ the pipe and _neutralise_ the charge.
Switch 1 and switch 4 are closed; switch 2 and switch 3 are open. (i) Calculate the current through the resistor.
0.90 A
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Switch 2 is now closed. Closing switch 2 will _not_ have an effect on the current calculated above. This is because the batteries are connected in _parallel._ When batteries are connected in _parallel_ the source _voltage_ is the same as the voltage of each individual _battery._ The current in the resistor is V/R and as neither _V_ nor R have changed the _current_ hasn't changed. because
Switch 1 and switch 3 are closed; switch 2 and switch 4 are open. (i) Calculate the current through switch 3.
0.33 A
Switches 1, 3 and 4 are still closed and the current through switch 4 is 0.90 A. Calculate the total resistance of the circuit.
7.3 