Magnetic field strength and currents
Define B from the force on a moving charge, then on a wire, then between two wires.
Run a current through a wire near a compass and the needle swings, though nothing touches it. What is the wire making around itself?
The tesla, the unit of magnetic field strength, is equivalent to…
A 0.25 m length of wire carries 4.0 A at right angles to a uniform magnetic field of 0.20 T. The force on the wire is…
Two long parallel wires carry currents in opposite directions. The wires…
Two long parallel wires carry fixed currents. The separation of the wires is doubled. The force per metre becomes…
An electron (charge 1.6 × 10⁻¹⁹ C) moves at 3.0 × 10⁶ m s⁻¹ at right angles to a magnetic field of 0.020 T. The magnetic force on it is…
Show that the definition of field strength B = F / (q v) becomes B = F / (I L) for a straight wire carrying current I, and explain what q v turns into.
A moving charge creates which kind of field?
The distance between two point charges is doubled. The electric force becomes…
Key points
- • A moving charge, that is a current, creates a magnetic field. B points the way the N pole of a compass points.
- • B = F / (q v) = F / (I L). The unit is the tesla, T = N A⁻¹ m⁻¹.
- • The magnetic force is at right angles to both B and the motion.
- • Parallel wires: F / L = μ₀ I₁ I₂ / (2πr). Same direction attract, opposite repel.
Formulas
- B = F / (q v)
- B = F / (I L)
- F / L = μ0 I1 I2 / (2π r)
Key terms
- Tesla :
- the unit of B: one newton per ampere per metre
- Current :
- charge moving per unit time, I = q / t
Common errors
- • Using F = B I L when the wire is not at right angles to the field.
- • Forgetting that the force between wires is per metre.
- • Thinking the magnetic force acts along the field lines.