Copy each passage into your book and fill in the numbered gaps using the word bank. Then check your answers. ← All topics
1. Magnets and magnetic fields (C+S)
The poles of a magnet are where the magnetic forces are (1)______. Like poles repel and unlike poles (2)______. A (3)______ magnet produces its own magnetic field. An induced magnet only becomes a magnet when placed in a magnetic field, and induced magnetism always causes a force of attraction. Magnetic field lines go from the north pole to the (4)______ pole. The Earth has a magnetic field, which is why a (5)______ points north.
Word bank: attract · compass · permanent · south · strongest
Check your answers
1 strongest · 2 attract · 3 permanent · 4 south · 5 compass
2. Electromagnetism (C+S)
When a current flows through a wire, a magnetic (1)______ is produced around it, in circles around the wire. Winding the wire into a coil called a (2)______ makes the field stronger; inside it the field is strong and (3)______. Adding an (4)______ core makes an electromagnet. An electromagnet is stronger if the current is increased or the number of (5)______ on the coil is increased, and it can be switched on and off.
Word bank: field · iron · solenoid · turns · uniform
Check your answers
1 field · 2 solenoid · 3 uniform · 4 iron · 5 turns
3. The motor effect (C+S, HT)
When a wire carrying a current is placed in a magnetic field, the wire experiences a (1)______. This is the motor effect. The direction of the force can be found using Fleming’s (2)______-hand rule. The size of the force is F = BIl, where B is the magnetic flux (3)______ in tesla. In an electric motor, a coil carrying a current (4)______ in a magnetic field. Reversing the current or the magnetic field (5)______ the direction of the force.
Word bank: density · force · left · reverses · rotates
Check your answers
1 force · 2 left · 3 density · 4 rotates · 5 reverses
3b. Loudspeakers and headphones (S only, HT)
Loudspeakers and headphones use the (1)______ effect. An alternating current flows through a coil that sits in the field of a permanent (2)______. The force on the coil keeps changing (3)______, so the coil and the cone attached to it (4)______. The cone makes the air vibrate, turning variations in current into variations in air (5)______ – sound waves. A microphone works the opposite way, using the (6)______ effect.
Word bank: direction · generator · magnet · motor · pressure · vibrate
Check your answers
1 motor · 2 magnet · 3 direction · 4 vibrate · 5 pressure · 6 generator
4. The generator effect (S only, HT)
If a wire moves relative to a magnetic field, or the field around it changes, a potential difference is (1)______ across it. If the wire is part of a complete circuit, a (2)______ flows. The induced current creates its own magnetic field that (3)______ the original change. An alternator produces alternating current using slip rings; a (4)______ produces direct current using a split-ring commutator. In a microphone, sound waves make a diaphragm and coil (5)______, which induces a varying current.
Word bank: current · dynamo · induced · opposes · vibrate
Check your answers
1 induced · 2 current · 3 opposes · 4 dynamo · 5 vibrate
5. Transformers (S only, HT)
A transformer has a primary coil and a secondary coil wound on an (1)______ core. An (2)______ current in the primary coil produces a changing magnetic field in the core, which induces an alternating potential difference across the (3)______ coil. A step-up transformer has (4)______ turns on the secondary coil than the primary. For a 100% efficient transformer, the power in the primary coil (5)______ the power in the secondary coil: VpIp = VsIs.
Word bank: alternating · equals · iron · more · secondary
Check your answers
1 iron · 2 alternating · 3 secondary · 4 more · 5 equals
