2023 A/L Physics Past Paper · Question 12 (English Medium)
The statement about field lines that is not true
Which of following is not true about the electric field lines?
Answer and explanation
Answer: (4) Electrostaic field lines form closed loops.
What the question asks
We want: the statement about electric field lines that is not true.
What the words mean:
A field line is a line drawn to show the direction of the force on a small positive charge in an electric field.
A closed loop is a line that goes round and comes back to where it started.
Infinity means very far away, as far as we can think.
Relevant theory
For this purpose, a type of hypothetical lines known as "electric lines of force" are used.
A large number of such lines of force would pass through a very small region in an electric field and no two lines of force would ever cross or even touch each other.
Electric flux lines are continuous and are supposed to emerge from positive charges and
end at negative charges.
Reading the options
The three quotes give three facts.
First, field lines are imagined lines that help us study a field.
Second, no two lines cross or touch.
Third, the lines are continuous. They come out of positive charges and end at negative charges.
For example, think of a thread on a spool. It has a start on the spool and an end at the needle. A thread tied into a ring has no start and no end.
A field line is like the thread with a start and an end. We test each option against the three facts.
Reasoning and calculation
Test each option against the three facts.
(1) Lines start on positive charges and end on negative charges. This is the third fact, so it is true.
(2) A single positive charge has no negative charge near it. Its lines go on outwards, to infinity. This is true.
(3) No two lines cross. This is the second fact, so it is true.
(5) Field lines are imagined lines. This is the first fact, so it is true.
(4) A closed loop has no start and no end. But a field line has a start on a positive charge and an end on a negative charge. So field lines cannot form closed loops. This is not true.
Why the other options fail
This is true. The lines come out of positive charges and end at negative charges.
This is true. A single positive charge has no negative charge near it. Its lines go on outwards, to infinity.
This is true. No two lines of force cross or touch.
This is true. Field lines are hypothetical lines used to study the field.
Conclusion
Field lines start on positive charges and end on negative charges, so they cannot form closed loops.
The statement that is not true is (4).
When a question asks for the statement that is not true, test each option and mark each one true or not true.
For field lines, remember the start and the end: they start on positive charges and end on negative charges.
Exam tip