Magnetism and electromagnetism flashcards

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Which materials are attracted by a magnet?
Magnetic materials: iron, steel, cobalt and nickel. Most other metals, such as copper and aluminium, are not magnetic.
State the rules for the forces between magnetic poles.
Like poles repel and unlike poles attract. Both poles of a magnet attract an unmagnetised piece of iron.
What is the difference between a permanent and an induced magnet?
A permanent magnet keeps its magnetism. An induced magnet is magnetic only while in a magnetic field; iron loses it quickly, but steel keeps it longer.
What is the difference between a magnetically hard and a magnetically soft material?
Soft materials, such as iron, are easily magnetised and demagnetised, so are used in electromagnets. Hard materials, such as steel, stay magnetised and are used for permanent magnets.
What is a magnetic field and how is it shown?
A region where a magnetic material or a current-carrying wire feels a force. Field lines go from north to south, and are closer together where the field is stronger.
Describe the magnetic field pattern around a bar magnet.
Lines leave the north pole and curve round to enter the south pole. They are most crowded near the poles, where the field is strongest, and never cross.
How can you plot the magnetic field of a bar magnet?
Place a small plotting compass near the magnet, mark the direction of its needle, then move it so its tail is at the mark. Repeat to build up a line. Or use iron filings.
What does a compass show about the Earth?
The needle is a small magnet that lines up with the Earth's magnetic field and points to magnetic north. The Earth behaves as if it has a bar magnet inside it.
(Paper 2) Describe the field around a straight current-carrying wire.
Concentric circles centred on the wire, with spacing getting wider away from it. Reversing the current reverses the direction. A larger current gives a stronger field.
(Paper 2) What is the field of a solenoid like?
Similar to a bar magnet: strong and nearly uniform inside, with a pattern of lines outside from one end to the other. Reversing the current swaps the poles.
(Paper 2) How can you make an electromagnet stronger?
Increase the current, add more turns of wire, or put a soft iron core inside the coil.
Give two uses of electromagnets.
Lifting scrap metal in scrapyards, and electric bells and relays. They can be switched on and off by controlling the current.
What is the motor effect?
A current-carrying wire in a magnetic field experiences a force, because the wire's field interacts with the magnet's field. It is at its maximum when the wire is at right angles to the field.
How do you find the direction of the force on a wire in a magnetic field?
Use Fleming's left-hand rule: first finger = field (N to S), second finger = current, thumb = motion (force). Keep all three at right angles.
How can the force on a wire in a magnetic field be increased?
Increase the current, use a stronger magnet, or make a longer length of wire sit in the field. Reversing either the current or the field reverses the force direction.
Explain how a simple d.c. motor works.
A coil in a magnetic field carries current. The forces on its two sides are opposite, so it rotates. A split-ring commutator reverses the current every half turn so it keeps turning one way.
How can the speed of a d.c. motor be increased?
Increase the current, use stronger magnets or add more turns to the coil.
Explain how a loudspeaker works.
A varying current flows in a coil attached to a cone, inside a magnetic field. The motor effect makes the coil and cone vibrate, producing sound waves at the same frequency as the current.
What is electromagnetic induction?
A potential difference is induced across a conductor when it moves through a magnetic field, cutting field lines, or when the magnetic field through a coil changes. A complete circuit then carries a current.
How can you increase the induced p.d. in a coil?
Move the magnet or coil faster, use a stronger magnet, add more turns, or use a bigger coil area. Reversing the direction of motion reverses the induced p.d.
Describe how an a.c. generator works.
A coil rotates in a magnetic field, cutting field lines. A p.d. is induced; slip rings and brushes connect the coil to the circuit. The output reverses direction each half turn.
What happens to the output of a generator when it spins faster?
Both the size (peak p.d.) and the frequency of the alternating output increase.
(Paper 2) How does a transformer work?
An alternating current in the primary coil creates a changing magnetic field in the iron core. This induces an alternating p.d. in the secondary coil. It only works with a.c.
(Paper 2) What is the difference between a step-up and a step-down transformer?
A step-up transformer has more turns on the secondary than the primary, raising the voltage. A step-down transformer has fewer turns on the secondary, lowering the voltage.
(Paper 2) State the transformer equation relating voltages and turns.
Vp ÷ Vs = Np ÷ Ns. V is p.d. in V and N is the number of turns, with p for primary and s for secondary.
(Paper 2) A transformer has 1000 primary turns and 50 secondary turns. The primary is 230 V. Find the secondary p.d.
Vs = Vp × Ns ÷ Np = 230 × 50 ÷ 1000 = 11.5 V.
(Paper 2) State the power equation for a 100% efficient transformer.
Power in = power out, so Vp × Ip = Vs × Is. Stepping the voltage up means the current is stepped down in proportion.
(Paper 2) A 100% efficient transformer gives 11.5 V and 2.0 A in the secondary. Find the primary current at 230 V.
Vs Is = 11.5 × 2.0 = 23 W. Ip = 23 ÷ 230 = 0.10 A.
(Paper 2) Why are transformers used in power transmission?
Step-up transformers raise voltage and lower current in the cables, cutting energy lost as heat. Step-down transformers then reduce the voltage to a safe level for homes.

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