A student investigates how the number of turns in a coil changes the output voltage. Fig. 1 shows the apparatus. The student moves the same magnet through t...
Question 1 Report
A student investigates how the number of turns in a coil changes the output voltage. Fig. 1 shows the apparatus. The student moves the same magnet through the centre of each coil in the same time and records the largest voltage shown by a data logger. Table 1 shows the results. The resistance of the 400-turn coil is 10 ohms.
| number of turns on coil | maximum output voltage / V |
|---|---|
| 100 | 0.30 |
| 200 | 0.60 |
| 300 | 0.90 |
| 400 | 1.20 |
(a) State two variables that the student keeps constant in this investigation. [2]
(b) Describe the relationship shown by Table 1. [2]
(c) Calculate the maximum output voltage expected from a coil with 550 turns. [2]
(d) Explain why moving the magnet through the coil more quickly produces a larger output voltage. [3]
(e) Calculate the largest current in the 400-turn coil. [2]
(f) When the student pulls the magnet out of the coil, state how the direction of the induced current compares with when the magnet is pushed in. [2]
(g) State the energy transfer that occurs while the magnet is moved by the student. [1]
(h) Describe one change to the apparatus, other than changing turns, that would increase the output voltage. [2]
Answer Details
(a) Two control variables are needed. Valid examples include keeping the magnet movement time or speed constant, using the same magnet strength, moving it through the same distance and direction, keeping the coil position fixed, and using the same data logger and circuit. These controls ensure that turns are the factor being tested. [2]
(b) Output voltage increases as the number of turns increases. It is directly proportional: doubling turns from 100 to 200 doubles voltage from \(0.30\text{ V}\) to \(0.60\text{ V}\). [2]
(c)
\[\frac{0.30\text{ V}}{100\text{ turns}}=0.0030\text{ V per turn}\]
\[550\times0.0030=1.65\text{ V}\]
Expected voltage = \(1.65\text{ V}\). [2]
(d) Moving the magnet faster changes the magnetic field, or magnetic flux, through the coil in a shorter time. The rate of change of magnetic flux is greater, so a larger emf and output voltage are induced. [3]
(e)
\[I=\frac{V}{R}=\frac{1.20}{10}=0.12\text{ A}\]
Largest current = \(0.12\text{ A}\). [2]
(f) Pulling the magnet out induces a current in the opposite direction to pushing it in. [2]
(g) The student's kinetic energy, ultimately from chemical energy in their body, is transferred to electrical energy. [1]
(h) One valid change is to use a stronger magnet, move the magnet faster, use an iron core, or use a coil with larger area. Each increases the change in magnetic flux, so it induces a larger output voltage. [2]
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