The Physics Question Bank

Spec links: 4.2.5.1 Static charge (Physics only); 4.2.5.2 Electric fields (Physics only); 4.2.4.2 Energy transfers in everyday appliances (equation used in example) · Reading time: about 4 minutes

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A warning at the pump

Look closely at the signs at a petrol station. Some warn drivers not to get back into the car while refuelling. Why? Sliding across a car seat can build up a static charge on your body. When you reach back to the nozzle, a tiny spark could jump. Petrol vapour can catch fire, so even a small spark is a danger.

Charging by friction

All materials are made of atoms. Each atom has a positive nucleus with negative electrons around it. When two insulating materials are rubbed together, electrons can be rubbed off one material and onto the other. This is charging by friction.

The material that gains electrons becomes negatively charged. The material that loses electrons becomes positively charged by the same amount. Only electrons move. The positive charges stay where they are.

Rub a balloon on your jumper and electrons move from the jumper onto the balloon. The balloon becomes negative and the jumper becomes positive. Insulators like these hold on to the charge, because it cannot flow away.

Pushing and pulling

Charged objects exert a force on each other without touching. This is a non-contact force. Two objects with the same type of charge repel. Two objects with opposite charges attract. That is why your charged balloon sticks to the jumper.

Every charged object is surrounded by an electric field. Any other charge placed in the field feels a force. The field is strongest close to the object. The further away you go, the weaker the field and the smaller the force.

We draw electric fields with lines and arrows. Around an isolated positive sphere, the lines point straight outwards, like the spokes of a wheel. Around a negative sphere, they point straight inwards.

Why sparks jump

As charge builds up, the potential difference between the charged object and the earth gets bigger. Eventually the electric field becomes so strong that it pulls electrons off the air particles. The air is ionised and becomes a conductor. Charge suddenly rushes across the gap. We see a spark.

Lightning is the same thing on a huge scale. Inside a storm cloud, ice and water particles collide and become charged. Charge builds up until the potential difference between the cloud and the ground is enormous. Then a giant spark leaps down through the air.

Making it safe

Fuel tankers are connected to the ground with a metal wire before fuel is unloaded. Aircraft are connected too. The wire lets charge flow safely away, a process called earthing.

A lightning strike transfers a lot of energy in a very short time. We can use the equation E = P t to compare. A 2000 W kettle switched on for 300 s transfers E = 2000 × 300 = 600 000 J. A single lightning flash can transfer about 1 000 000 000 J. That is more than a thousand times as much, released in a fraction of a second. That is why lightning conductors are fitted to tall buildings. They give the charge a safe path to the ground.

Key words

• Static charge: electric charge that builds up on an insulating material and does not flow away.
• Electron: a negatively charged particle that orbits the nucleus of an atom; it is the only particle that moves when objects are charged by friction.
• Charging by friction: rubbing two insulators together so that electrons are transferred from one to the other.
• Electric field: the region around a charged object in which another charged object experiences a force.
• Non-contact force: a force that acts between objects that are not touching, such as the electrostatic force.
• Earthing: connecting an object to the ground with a conductor so that charge can flow safely away.

Questions

1. Name the particles that move when two insulators are rubbed together. [1 mark]
2. Why are fuel tankers connected to the ground before fuel is unloaded? [1 mark]
3. Where is the electric field around a charged object strongest? [1 mark]
4. A plastic rod is rubbed with a cloth. The rod becomes negatively charged. Explain how the rod became negatively charged and what charge the cloth now has. [3 marks]
5. Two balloons are both rubbed on the same jumper and then hung side by side. Explain what happens to the balloons. [2 marks]
6. A small hand dryer has a power of 1200 W. It is switched on for 25 s. Calculate the energy it transfers. Give the unit. [3 marks]
7. The energy transferred by one lightning flash is about 1 000 000 000 J. An electric heater has a power of 2500 W. Calculate how long the heater would take to transfer the same amount of energy. Give your answer in seconds. [4 marks]
8. Explain how a spark is produced when charge builds up on an object, and why sparks are dangerous at a petrol station. Use ideas about charge, potential difference and electric fields in your answer. [6 marks]

Show answers

1. Electrons (1)
2. So that charge can flow safely away / to stop static charge building up and causing a spark (1)
3. Close to the object (1)
4. Electrons transferred (1); from the cloth to the rod (1); the cloth becomes positively charged (1)
5. Both balloons gain the same type of charge (negative) (1); like charges repel so the balloons push apart / move away from each other (1)
6. E = P t (1); E = 1200 × 25 (1); E = 30 000 J (1)
7. t = E ÷ P (1); t = 1 000 000 000 ÷ 2500 (1); t = 400 000 (1); s (1)
8. Indicative content:
   • rubbing / friction transfers electrons so charge builds up on an insulator
   • as charge increases, the potential difference between the object and the earth increases
   • the electric field near the object gets stronger
   • a strong field can ionise the air / remove electrons from air particles
   • ionised air conducts, so charge flows across the gap as a spark
   • a spark transfers energy and is hot
   • petrol vapour is flammable / can ignite
   • a spark could cause a fire or explosion, so earthing or avoiding charge build-up is important
   • Level 3 (5–6 marks): a detailed, coherent explanation linking charge build-up to potential difference and field strength, ionisation of air and the danger with flammable vapour; Level 2 (3–4 marks): an explanation with some links, e.g. charge builds up and causes a spark that could ignite petrol; Level 1 (1–2 marks): simple relevant statements, e.g. sparks can start fires

More reading: See all 30 GCSE Physics comprehension articles