GCSE Questions: Static Electricity

Q9. When two different materials listed below are rubbed together they become electrostatically charged. The polarity of the material that is higher in the list would become positive.

 

(a) The diagram below shows a nylon rod being rubbed with a silk cloth.

(i) Explain why both materials become electrostatically charged. Include in your answer the polarity of each material.

The nylon of the rod is higher up in the list than the silk of the cloth so the nylon will become positive and the silk negative.

When the silk cloth rubs the nylon rod electrons are transferred from the rod to the cloth. The rod loses electrons, which are negative, and therefore becomes positively charged and the silk cloth gains electrons and becomes negatively charged.

The first mark was for saying electrons were transferred - if you said that protons were transferred you did not get the mark.

Saying they moved 'from the nylon rod' or 'to the silk cloth' - in otherwords a correct direction of travel - got you the second mark.

To get the last mark you had to state the final charge of both the rod and the cloth.

[3 marks]

(ii) What difference would it have made if a PVC rod had been rubbed with the silk cloth rather than a nylon rod?

PVC is below silk in the table so the silk cloth would have lost electrons and become positive while the PVC rod would gain electrons and become negative.

any one point got you the mark - but it is best to be thorough!

[1 mark only]

(b) The charged nylon rod is put onto an electric balance.

A second identically charged nylon rod is moved towards the rod on the balance, as shown in the diagram above. Explain what will happen to the reading on the balance as the two rods get closer to each other. (The rods do not touch or discharge).

Like charges repel - so as both of the rods are positively charged they repel each other. The rod on the balance pan will experience a force pushing it away from the rod in the person's hand - downwards. The readout on the balance will therefore increase.

Note that the weight of the rod does NOT increase - saying so will lose you a mark. The pull of gravity of the rod is due to its mass (which does not change) - so its weight doesn't change!

[3 marks]

 

(c) A driver often becomes electrostatically charged when in a car. This happens because the driver's clothing rubs against the car seat.

(i) The charge on a driver who has just got out of his car is 3.0 mC.

When the driver starts to close the car door a spark jumps across the air gap between the driver and the door. The spark transfers 27 J of energy.

Calculate the potential difference between the driver and the car as the driver starts to close the door.

Q = 3.0 mC

Q = 3.0 x 10^-3 C

E = QV

V = ^E/Q

V = ²⁷/(3.0 x 10^-3)

V = 9000 V

[4 marks]

(ii) A scientist investigated how the charge on a driver changed when the driver wore clothes made from different materials. The results from the investigation are given in the table below:

 

A student looked at the data and concluded:

'The charge on a driver will always be less if the driver wears clothes made from cotton rather than any other type of material.'

Suggest two reasons why this may not be a valid conclusion.

Not all types of material were tested - only three were tested.

Other factors may affect the charge (on the driver) eg material of the seats, temperature, humidity

[2 marks]