Electric Charge
5 marks · NormalA science class uses plastic drinking straws to investigate static forces. First, they identify which straw materials are conductors and which are insulators. Then, they compare how a charged straw interacts with both an uncharged straw and a suspended, previously-rubbed straw.
Describe how you could use a simple circuit with a bulb to determine whether a straw is an electrical conductor or an insulator.
The students suspend an uncharged plastic straw and bring a second, charged plastic straw near it. What will happen? Explain your answer.
Next, both straws are rubbed with the same cloth so they gain the same charge. Predict and explain the observation when one is brought close to the other.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Set up a simple electrical circuit with a bulb and two wires with exposed ends. Place the straw between the ends to complete the circuit. If the bulb lights up, the straw is a conductor. If the bulb does not light, it is an insulator. · must show use of bulb and wires, place straw in circuit, light/no light indicates conductor/insulator
(b) The uncharged straw will be attracted to the charged straw due to induced charge/opposite charges attracting. · must show attraction, because of induced charge
(c) The straws will repel each other because both have the same type of charge (like charges repel). · must show repel each other, same charge, like charges repel
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Describes correct use of a bulb circuit with the straw to test for conduction
A1 (a) · 1 mark · Links observation (bulb on/off) to conductor/insulator result
B1 (b) · 1 mark · States straws attract and correctly refers to induced charge/opposite charges attracting
M2 (c) · 1 mark · Predicts repulsion for both straws having same charge
A2 (c) · 1 mark · Explains with 'like charges repel'
Worked solution
a) Connect the ends of a complete circuit with a bulb, leaving a gap for the straw. Insert the straw. If the bulb lights, the straw conducts electricity (conductor). If not, it's an insulator.
b) The uncharged straw will move toward the charged straw. The charged straw induces an opposite charge in the nearest part of the uncharged straw, so they attract.
c) Both straws will repel each other because after rubbing with the same cloth, they have the same charge. Like charges repel.