Electricity & Magnetism
Magnetism
1st Year · 2nd Year · 3rd Year (Junior Cert)
- ✓Define magnetism and identify magnetic materials.
- ✓Describe the properties of magnetic poles and their interactions.
- ✓Draw and interpret magnetic field lines around bar magnets.
- ✓Explain the principle of an electromagnet and list factors affecting its strength.
- ✓Describe the Earth's magnetic field and its significance.
Key concepts
Magnetism is a force of attraction or repulsion that acts between magnetic materials. Magnets have two poles: a North pole (N) and a South pole (S). Like poles repel each other (N-N, S-S), while unlike poles attract each other (N-S). Only certain materials, such as iron, nickel, and cobalt, are magnetic. These are called ferromagnetic materials.
A magnetic field is the region around a magnet where its magnetic force can be detected. Magnetic field lines are used to represent this field. They always emerge from the North pole and enter the South pole, forming continuous loops. The closer the lines are, the stronger the magnetic field. A compass can be used to show the direction of magnetic field lines, as its needle aligns with the field.
An electromagnet is a temporary magnet created by passing an electric current through a coil of wire, often wrapped around a soft iron core. The magnetic field exists only when the current is flowing. The strength of an electromagnet can be increased by: 1. Increasing the current flowing through the coil. 2. Increasing the number of turns in the coil. 3. Using a soft iron core (or a larger/better quality soft iron core). Electromagnets have many uses, such as in electric bells, relays, and lifting scrap metal.
The Earth itself acts like a giant bar magnet, generating a magnetic field. This field is believed to be caused by the movement of molten iron in the Earth's outer core. The Earth's magnetic North pole is actually located near its geographical South pole, and vice-versa. This magnetic field protects the Earth from harmful charged particles from the Sun (solar wind). Compasses work by aligning with the Earth's magnetic field.
Key facts to remember
- 1Magnets have two poles: North and South.
- 2Like poles repel, unlike poles attract.
- 3A magnetic field is the region around a magnet where its force is felt.
- 4Magnetic field lines always go from North to South.
- 5Electromagnets are temporary magnets created by electric current.
- 6The strength of an electromagnet depends on the current, the number of turns in the coil, and the core material.
- 7The Earth has a magnetic field that protects us from harmful solar radiation.
- 8A compass needle is a small magnet that aligns with the Earth's magnetic field.
Worked examples
Example 1
Draw the magnetic field lines around a single bar magnet, clearly indicating the direction of the field.
Answer
A diagram showing a bar magnet. Curved lines originate from the North pole and terminate at the South pole, with arrows pointing from N to S. The lines are denser near the poles and spread out further away.
Magnetic field lines never cross each other.
Example 2
A student wants to make an electromagnet stronger. List three ways they could achieve this.
Answer
1. Increase the current flowing through the coil. 2. Increase the number of turns in the coil. 3. Insert a soft iron core into the coil (or use a larger/better quality soft iron core).
Electromagnets are temporary magnets; their magnetism can be switched on and off.
Example 3
Explain how a compass works to indicate direction.
Answer
A compass contains a small, lightweight magnet that is free to rotate. The Earth has its own magnetic field. The North pole of the compass needle is attracted to the Earth's magnetic South pole (which is located near the Earth's geographical North pole). This causes the compass needle to align itself with the Earth's magnetic field lines, thus pointing towards the geographical North.
The compass needle points to the Earth's *magnetic* North, which is slightly different from the true geographical North.
Common mistakes
- ✗Confusing geographical North/South with magnetic North/South.
- ✗Drawing magnetic field lines crossing each other.
- ✗Forgetting to add arrows to magnetic field lines to show direction.
- ✗Believing that all metals are magnetic.
- ✗Thinking that electromagnets are permanent magnets.
Exam tips
- ★Always use arrows on magnetic field lines to show direction (N to S).
- ★Remember the rule for magnetic poles: 'Like repel, unlike attract.'
- ★When asked about electromagnets, mention their temporary nature and how to vary their strength.
- ★Be able to explain the basic function of a compass in relation to the Earth's magnetic field.
- ★Practise drawing magnetic field patterns accurately.
Ready to practise?
Try a problem on this topic
Snap a photo or type a question — get step-by-step working instantly.