Home › Subjects › A-Level H2 Physics › Magnetic fields and flux density from currents, force on a conductor and force on a moving charge
A-Level Magnetic fields and flux density from currents, force on a conductor and force on a moving charge
What the A-Level syllabus expects for Magnetic fields and flux density from currents, force on a conductor and force on a moving charge, and how to practise it.
What the syllabus expects
- Understand a magnetic field as a force field that either current-bearing conductors or permanent magnets can create
- Draw the field-line shapes made by current flowing through a long solenoid, through a flat circular coil, and through a long straight wire
- Apply B = μ0I/(2πd), B = μ0NI/(2r) and B = μ0nI, one formula each for a long straight wire, for a flat circular coil, and for a long solenoid
- Recognise that slipping a ferrous core into a solenoid can change its magnetic field
- Understand that a wire carrying current can feel a force once it lies within a magnetic field
- Apply F = BIl sinθ in problems, settling directions with Fleming's left-hand rule
- Define magnetic flux density from the force experienced by a wire lying at right angles to the field, taken per unit current and per unit length
- Understand how the force on a current-carrying wire lets a current balance gauge a field's flux density
- Explain why parallel current-carrying wires attract or repel and work out which way the forces point
- Determine which way the force acts on a charge travelling through a uniform magnetic field
- Apply F = BQv sinθ in problems
- Describe and analyse how uniform electric and uniform magnetic fields bend beams of charged particles
- Explain how crossed electric and magnetic fields pick out charged particles of one particular speed
How it's examined
About 5% of the past-paper style questions in Rae's bank for this subject sit in this topic.
Worked examples
Example 1 (2 marks)
A coaxial cable carries high-frequency electrical signals, for instance television signals. The figure shows the usual layered structure of such a cable, with its conductor, insulator, metal braid and plastic jacket having radii r1, r2, r3 and r4. A current I passes one way along the central conductor while an equal current I passes the other way along the metal braid. (a) Write down Ampère's Law in its integral form, defining every symbol you use. [2]
Show the worked answer
Ampere's Law (integral form): the line integral of the magnetic flux density around any closed loop equals the permeability of free space times the net current enclosed by that loop. Closed-loop integral of B . dl = mu_0 * I_enclosed. Here B is the magnetic flux density, dl is a vector element of length along the closed loop, mu_0 is the permeability of free space, and I_enclosed is the net (algebraic) current passing through any surface bounded by the loop.
More A-Level H2 Physics topics
Physical quantities, units, measurement uncertainty and vector basics · Types of force, turning effects and conditions for equilibrium · Kinematics, uniformly accelerated motion, momentum and Newton's laws · Energy stores and transfers, work, kinetic and potential energy, fields, power and efficiency · Falling freely, the gravitational potential energy of a uniform field, and how air resistance changes the motion · Impulse and the conservation of momentum and energy · all of A-Level H2 Physics