Practice questions on Static electricity and circuits

Target Language or Knowledge:

QUESTION ONE: Dusting cloth On a packet of dusting cloths there is a claim that the cloths “act like a magnet” to dust. When the cloth is used it is rubbed against the surface that is to be cleaned and becomes charged. (a) Explain how the cloth becomes charged.
• Electrons are rubbed either from the cloth to the surface or from the surface to the cloth.
The diagram below shows part of a charged cloth that is being brought close to a piece of dust. In this situation the cloth has a positive charge. Explain what happens to the charges inside the piece of dust.
• The positively charge cloth attracts the negatively charged electrons inside the piece of dust and they move upwards, giving the top of the piece of dust a negative charge. • Because electrons have moved upwards, the bottom of the piece of dust is left positively charged.
Explain why the cloth “acts like a magnet” to the piece of dust.
• The top surface of the piece of dust has a negative charge and so it is attracted towards the positively charged cloth. • The bottom surface of the piece of dust has a positive charge and so it is repelled from the positively charged cloth. • Because the bottom of the dust is further from the cloth than the top of the dust the attractive force is stronger than the repulsive force so, overall the dust is attracted to the cloth – the cloth “acts like a magnet”.
The positively charged cloth is put down on top of another, similar, cloth that is uncharged. A short time later the similar cloth is picked up and it, also, is now slightly charged. Explain how the similar cloth became charged.
• The positively charged surface of the cloth attracts the electrons in the similar cloth. • In the places where the two cloths are touching each other surface electrons are able to pass from one cloth to the other. • Because electrons have been removed from the similar cloth it gains a positive charge.
Both charged cloths are left untouched overnight. During this time both cloths lose all the charge they had. Explain why the cloths lose their charge.
• Particles of water vapour in the air, if they are in contact with a cloth, allow electrons to be attracted into the cloth. • A charged cloth attracts water vapour particles in the same way that it attracted dust particles. • Over a long period of time the quantity of water vapour particles that have come in contact with a cloth is such that the number of electrons that have been transferred across are able to neutralise the positive charge.
In the circuit diagrams below, both power sources have a voltage of 12 V and all resistors have a resistance of 20 Ω. Calculate the current that is flowing through resistor A in the series circuit.
0.30 A
Explain why the current through resistor A in the parallel circuit is twice the current calculated above
In a parallel circuit the voltage of each branch is the voltage of the source branch and so the voltage across resistor A is 12 V. In a series circuit the voltage of the source is shared between the resistors and so the voltage across resistor A is half the source voltage and so is 6 V. As I = V/R and R is the same, if the voltage across resistor A in the parallel circuit is twice the voltage in the series circuit, the current through resistor A in the parallel circuit must be twice the current in the ser
Calculate the current that is being drawn from the power source in the parallel circuit
1.2 A
Calculate the power that is being drawn from the power source in the series circuit.
3.6 W
Calculate the resistance of the parallel circuit.
10 Ω