Electricity - 2.5.1 Static Charge (GCSE Physics AQA)

Static Charge

Charging Materials

• Certain insulating materials can become charged. You can charge certain insulating materials by rubbing them together. For example, if you rub together an uncharged plastic rod with an uncharged cloth, both will get charged.
• Electrons can move from rod to cloth. Electrons are negatively charged particles, which move from the rod to the cloth during rubbing. Protons, which are positively charged particles, never move. The rod becomes positively charged as it loses electrons. The cloth becomes negatively charged as it gains electrons.

• The positive charge equals the negative charge. The positive charge of the rod should be equal to the negative charge of the cloth. We need to consider the number of electrons moving from the rod to the cloth. The number of electrons gained by the cloth will be equal to the number of electrons lost from the rod.

Forces Exerted by Charged Objects

• Forces are exerted by charged objects. Two electrically charged objects can exert a force on each other without directly touching. This force is an example of a non-contact force.
• Forces can attract and repel. Two objects with the same type of charge will repel, whilst two oppositely charged objects attract. The force between unlike charges is called electrostatic attraction. The force between like charges is called electrostatic repulsion.
• Electrostatic forces can cause movement. Similar to other forces, electrostatic forces can cause movement. Electrostatic attraction causes objects to move closer together, whilst electrostatic repulsion causes objects to move further away from each other. If we think back to our example with the rod and the cloth, the positive rod will attract the negative cloth, so they will move closer together.

Static Electricity

• Rubbing surfaces leads to static electricity. When we rub two surfaces together, we produce static electricity. This occurs when electrons move from one surface to the other. We get a build up of static positive charge on one surface, and an equal static negative charge on the other surface.
• A charge difference is created. As charge builds up on a surface, we start to create a difference in charge between the charged surface and the earth. Usually, the earth is at zero volts. As we charge the surface, the difference in charge between the surface and the earth will get bigger and bigger.
• Static electricity leads to sparking. Eventually, the charge difference between the earth and charged surface gets so big that electrons will jump from the charged surface to the earth. As the electrons jump, we will be able to see a spark.

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An Example of Static Electricity

Question: Laura takes an uncharged balloon and a jumper. She rubs the balloon against the jumper for several seconds. Laura then touches the jumper. She experiences a small electrical shock. Why has this happened?

1. When the balloon is rubbed against the jumper, we get electron transfer. As the balloon rubs against the jumper, electrons will be transferred from the balloon to the jumper. This results in a negatively charged jumper and a positively charged balloon.
2. When Laura touches the jumper, the electrons are transferred to her. Compared to the jumper, Laura is positively charged. Therefore the electrons jump across the gap between the jumper and Laura’s hand, creating a spark (and therefore an electrical shock).
3. The electrons are trying to get to the earth. Electrons are always trying to get to the earth. This means that they will go through Laura’s body and travel straight down into the earth. In this situation, we would refer to Laura as an ‘earthed’ object; she is in contact with the earth, therefore providing a pathway for the electrons.

General Uses of Static Electricity

The AQA specification does not ask you to memorise uses of static electricity. But often, exam questions are based on real life scenarios. If you’ve read about the scenario before, it can help you answer the question very easily. So read these 3 uses and 3 dangers of static electricity, but there is no need to memorise.

Question:

1. The only way electrostatic charges can be produced is by the movement of electrons.
2. Static electricity causes hairs to attract each other and therefore stand up on end
3. If you rub a plastic rod with a cloth, the rod becomes positively charged, and cloth negatively charged.
4. You can charge two insulators by using friction.

Which of the above statements are true?

1. True – this is true, electrons can move but protons cannot.
2. False – this is incorrect, as electrons are transferred from the object rubbed to the hair, causing an accumulation of charge. As the hair strands are all the same charge, negatively charged, the hair strands repel each other and stands on end.
3. True – During rubbing, electrons move from the rod to the cloth. Protons never move. The rod becomes positively charged as it loses electrons. The cloth becomes negatively charged as it gains electrons.
4. True – Friction can cause the movement of electrons, so this is true.

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