What field strength depends on
Definition
Magnetic field strength
A measure of how large a force a magnet or magnetic material would experience at a particular point in a magnetic field.
- The magnetic field around a current-carrying wire is not equally strong everywhere.
- The strength of the field at any point depends on two factors, the size of the current in the wire and the distance of the point from the wire.
- A larger current produces a stronger magnetic field.
- The field is strongest close to the wire and gets weaker further away from it.
- On a field diagram, these changes are shown by the spacing of the circular field lines, because closely spaced lines show a strong field.
Effect of current
- Current is the rate of flow of charge, so a larger current means that more charge passes along the wire every second.
- Moving charge is what creates the magnetic field, so more charge flowing each second produces a stronger field at every point around the wire.
- Increasing the current therefore increases the field strength, and decreasing the current decreases it.
- On a diagram, a larger current is shown by drawing more circles, packed closer together, around the same wire.
- With no current at all, there is no magnetic field around the wire.
- To compare the effect of two different currents fairly, the field must be measured at the same distance from the wire each time.
Effect of distance
- For a fixed current, the field becomes weaker as the distance from the wire increases.
- A diagram shows this with circles that are close together near the wire and get further and further apart moving outwards.
- A plotting compass very close to the wire swings round sharply to follow the wire's field, while one placed far away barely moves, because there the wire's field is much weaker than the Earth's field.
- The field does not suddenly stop at some distance, but it becomes too weak to detect a long way from the wire.
- To compare the field at different distances fairly, the current must be kept the same each time.
Practical
Investigating field strength around a wire
- Aim: to find how the strength of the magnetic field around a straight wire changes with the current and with the distance from the wire.
- Apparatus: a low-voltage d.c. power supply, a variable resistor, an ammeter, a switch, connecting leads, a long straight copper wire, a horizontal card with a hole for the wire, a clamp stand, a plotting compass, a protractor and a ruler, or a magnetic field sensor connected to a data logger in place of the compass.
- Method, changing the current:
- Push the wire vertically through the hole in the card and connect it in series with the switch, variable resistor, ammeter and power supply.
- With the switch open, place the plotting compass 3 cm3\ \text{cm}3 cm due north of the wire, so that the wire's field there will act at right angles to the Earth's field, and note the starting direction of the needle.
- Close the switch, set the current to 1.0 A1.0\ \text{A}1.0 A, and use the protractor to measure the angle the needle turns through.
- Increase the current in steps of 1.0 A1.0\ \text{A}1.0 A up to 5.0 A5.0\ \text{A}5.0 A, recording the angle each time and opening the switch between readings.
- Method, changing the distance:
- Fix the current at 4.0 A4.0\ \text{A}4.0 A using the variable resistor.
- Measure the angle of turn with the compass at 2 cm2\ \text{cm}2 cm, 4 cm4\ \text{cm}4 cm, 6 cm6\ \text{cm}6 cm, 8 cm8\ \text{cm}8 cm and 10 cm10\ \text{cm}10 cm due north of the wire, measuring each distance from the centre of the wire to the centre of the compass.
- Repeat every reading three times and calculate a mean angle for each current and each distance.
- Variables: in the first test the current is the independent variable and the distance is controlled, in the second test the distance is the independent variable and the current is controlled, and in both tests the angle of turn of the needle is the dependent variable.
- Results: the needle turns through a larger angle as the current increases, and through a smaller angle as the compass is moved further from the wire.
- Conclusion: the strength of the magnetic field increases with the current and decreases with the distance from the wire.
- Watch out: the Earth's field and steel bench frames also act on the needle, so keep the apparatus in the same place for every reading, and check the ammeter before each reading because the current can drift as the wire warms up.
- Safety: a current of several amperes makes the wire hot, so switch off between readings and do not touch the wire.
Example
Ranking the field at different points
- A wire carries a current of 2 A2\ \text{A}2 A, and the field is compared at point A, 1 cm1\ \text{cm}1 cm from the wire, and at point B, 4 cm4\ \text{cm}4 cm from the wire.
- The current is the same for both points, so only the distance differs.
- Point A is closer to the wire, so the field at A is stronger than the field at B.
- The current is then increased to 6 A6\ \text{A}6 A, and the field at A is checked again.
- The distance has not changed but the current is larger, so the field at A is now stronger than it was before.
Example
Spotting an unfair comparison
- A student measures the field 6 cm6\ \text{cm}6 cm from a wire carrying 3 A3\ \text{A}3 A, and then 2 cm2\ \text{cm}2 cm from a wire carrying 1 A1\ \text{A}1 A.
- The larger current tends to make the first field stronger, but the larger distance tends to make it weaker.
- Both factors have changed at once, so the student cannot tell which one caused any difference in the readings.
- A fair test changes only one factor, so the student should measure both fields at the same distance to find the effect of the current.
Exam technique
Stating how field strength changes
- Use comparative words, writing that a larger current gives a stronger field, rather than only saying that the current affects the field.
- Give both factors when asked what the field strength depends on, because naming only the current or only the distance earns part of the marks.
- In an explanation of an investigation, name the factor you are changing and say which factor you are keeping the same.
- When a question gives you a diagram, refer to the spacing of the field lines, saying that the field is stronger where the circles are closer together.
Common Mistake
- Do not say that the field disappears a short distance from the wire, because it only gets weaker with distance.
- Do not give potential difference as the factor, because the field depends on the current, and a higher potential difference only strengthens the field if it makes the current larger.
- Do not compare readings taken at different distances and different currents, because the effect of each factor cannot then be separated.
Self review
- Name the two factors that affect the strength of the magnetic field around a wire.
- What happens to the field strength when the current increases?
- Where is the field around a wire strongest?
- How does a field line diagram show that the field gets weaker further from the wire?
- What must be kept the same when investigating the effect of distance on field strength?
