9702 · 20.4
Magnetic fields due to currents flashcards
Revision flashcards for Cambridge 9702 Magnetic fields due to currents (syllabus 20.4). Flip, recall, then mark a real past-paper question.
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How do electric currents create magnetic fields?
Electric currents, which are moving electric charges, inherently generate magnetic fields around them.
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What rule determines the direction of the magnetic field around a straight current-carrying wire?
The Right-Hand Grip Rule (or Right-Hand Rule).
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What is the formula for the force per unit length between two parallel wires carrying currents $I_1$ and $I_2$ separated by distance $r$?
$$\frac{F}{L} = \frac{\mu_0 I_1 I_2}{2\pi r}$$
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How does inserting an iron core affect a solenoid's magnetic field?
The magnetic field strength inside the solenoid significantly increases because iron is a ferromagnetic material with high magnetic permeability.
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Describe the magnetic field inside a long current-carrying solenoid.
It is strong, uniform (parallel field lines), and directed along the axis of the solenoid. Its pattern closely resembles that of a bar magnet.
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What does $\mu_0$ represent in magnetic field formulae?
It's the permeability of free space, a constant ($4\pi \times 10^{-7} \text{ T m A}^{-1}$) that quantifies how easily a magnetic field can pass through a vacuum.
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What is the magnetic field pattern around a straight current-carrying wire?
It consists of concentric circles centred on the wire, with the field strength decreasing with distance.
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Under what condition do two parallel current-carrying wires attract each other?
When they carry electric currents flowing in the same direction.
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How do you determine the North pole of a solenoid?
Using the Right-Hand Grip Rule: curl your fingers in the direction of the current around the coil; your thumb will point towards the North pole. Alternatively, if viewed from an end, an anti-clockwise current indicates a North pole.
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What is the formula for the magnetic field strength $B$ at distance $r$ from a long straight wire?
$$B = \frac{\mu_0 I}{2\pi r}$$
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What is the formula for the magnetic field strength B inside a long solenoid?
$$B = \mu_0 n I$$, where n is the number of turns per unit length and I is the current.
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In the solenoid formula $B = \mu_0 n I$, what does 'n' represent?
'n' is the number of turns per unit length of the solenoid, calculated as N/L where N is the total number of turns and L is the length.
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How does the magnetic field strength B vary with distance r from a long straight wire?
It is inversely proportional to the distance ($B \propto 1/r$).
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Under what condition do two parallel current-carrying wires repel each other?
When they carry electric currents flowing in opposite directions.
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Name one practical application of a solenoid.
Solenoids are used as electromagnets in devices like electric bells, relays, and circuit breakers.
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What is the relationship between the magnetic field strength B and the current I in a long straight wire?
The magnetic field strength is directly proportional to the current ($B \propto I$).