🧰 You need: pen, pencil, ruler, calculator. Time: 45 minutes. Course: Combined and Separate. The F = BIl calculations are Higher tier only.
Information – read this first
- A magnetic field is the region around a magnet where a force acts on another magnet or on a magnetic material (iron, steel, nickel, cobalt).
- Field lines go from the north (N) pole to the south (S) pole. Draw an arrow on every line. Field lines never cross.
- The field is strongest at the poles, where the lines are closest together.
- Like poles repel. Unlike poles attract.
- A current in a wire makes a magnetic field of circles around the wire. The right-hand grip rule: point your right thumb along the current; your fingers curl in the direction of the field.
- A solenoid is a coil of wire. Outside, its field looks like a bar magnet’s. Inside, the field is strong and uniform (straight, parallel, evenly spaced lines).
- An electromagnet is a solenoid with an iron core. Make it stronger by: more current, more turns on the coil, or adding an iron core. It can be switched on and off.
- (HT) Force on a wire = magnetic flux density × current × length (F = BIl). B is in tesla (T).
What to do
- (2 min) Write the title Magnetic Field Patterns and today’s date. Underline both with a ruler.
- (8 min) Bar magnet: rule a rectangle about 6 cm × 2 cm in the middle of a space. Write N at one end and S at the other. Draw at least 8 curved field lines looping from N round to S, some close to the magnet and some further out. Put an arrow on each line pointing from N to S. Label where the field is strongest.
- (6 min) Two magnets attracting: draw two bar magnets end to end with a 3 cm gap, N facing S. Draw straight, parallel lines across the gap from N to S, with arrows.
- (6 min) Two magnets repelling: draw two bar magnets with N facing N. Draw the lines bending away from each other in the gap. Mark the point in the middle where there is no field with an X and label it neutral point.
- (5 min) Straight wire: draw a vertical line for a wire with an arrow labelled current pointing up. Draw three ellipses around it, getting further apart as they go out. Add arrows going anticlockwise when seen from above (right-hand grip rule).
- (5 min) Solenoid: draw a coil as a row of loops. Draw straight, parallel lines through the inside and curved lines around the outside, like a bar magnet. Label strong uniform field inside.
- (3 min) Write three ways to make an electromagnet stronger, and two uses (e.g. scrapyard crane, electric door lock, electric bell).
- (8 min) Write the heading Questions. Do Q1–Q3. Higher tier students also copy the worked example and do Q4–Q6.
- (2 min) Check your answers and correct in a different colour.
Questions
Q1 Which way do magnetic field lines point?
Show answer
From the north pole to the south pole.
Q2 How can you tell from a field diagram where the field is strongest?
Show answer
The field lines are closest together there.
Q3 Why is an electromagnet better than a permanent magnet for a scrapyard crane?
Show answer
It can be switched off to drop the scrap metal, and it can be made very strong.
Worked example (HT): A wire 0.5 m long carries a current of 3 A in a magnetic field of 0.2 T. Find the force.
F = BIl = 0.2 × 3 × 0.5 = 0.3 N
Q4 (HT) A 0.4 m wire carries 2 A in a 0.1 T field. Calculate the force on it.
Show answer
F = 0.1 × 2 × 0.4 = 0.08 N
Q5 (HT) A force of 1.2 N acts on a 0.8 m wire in a 0.3 T field. Calculate the current.
Show answer
I = F ÷ (B × l) = 1.2 ÷ (0.3 × 0.8) = 1.2 ÷ 0.24 = 5 A
Q6 (HT) A wire carrying 3 A in a 0.4 T field feels a force of 0.6 N. Calculate the length of wire in the field.
Show answer
l = F ÷ (B × I) = 0.6 ÷ (0.4 × 3) = 0.6 ÷ 1.2 = 0.5 m
⭐ Challenge
Describe how you could use a small plotting compass to draw the field pattern around a bar magnet.
Show answer
Put the magnet on paper and draw round it. Place the compass near the N pole and mark a dot where the needle points. Move the compass so its tail is at that dot and mark the new point. Keep going until you reach the S pole, then join the dots into a line with an arrow. Repeat from different starting points around the magnet.
