Electricity and Magnetism
Electricity and Magnetism on ORT Physics tests circuit analysis with real sources, the relationship between current, voltage and resistance, and the field ideas behind magnetic forces and electromagnetic induction.
Core rules
- Ohm's law V = IR applies to ohmic conductors at constant temperature; resistance R = ρL/A.
- Series: same current, voltages add, R_total = R₁ + R₂ + … Parallel: same voltage, currents add, 1/R_total = 1/R₁ + 1/R₂ + …
- Kirchhoff's laws: charge is conserved at a junction; the sum of potential differences around a closed loop is zero. For a real source, emf = I(R + r).
- Power: P = VI = I²R = V²/R.
- Capacitors: Q = CV; capacitors in parallel add, in series the reciprocals add; stored energy = ½CV².
- Magnetic force on a current-carrying wire: F = BIL sinθ; on a moving charge: F = qvB sinθ. Direction follows Fleming's left-hand rule.
- Faraday's law: induced emf is proportional to the rate of change of magnetic flux linkage. Lenz's law gives the direction that opposes the change.
Worked example
A battery of emf 12 V and internal resistance 0.50 Ω is connected to 6.0 Ω and 3.0 Ω resistors in parallel. The parallel combination satisfies 1/R = 1/6.0 + 1/3.0 = 1/2.0, so R = 2.0 Ω. Total resistance is 2.0 + 0.50 = 2.5 Ω, giving current I = 12/2.5 = 4.8 A. Terminal potential difference is 12 − (4.8 × 0.50) = 9.6 V. The branch currents are 9.6/6.0 = 1.6 A and 9.6/3.0 = 3.2 A, which sum to the 4.8 A total.
Traps
- Ignoring internal resistance when asked for terminal p.d.
- Adding parallel resistances directly instead of using reciprocals.
- Believing current is used up around a series circuit.
- Confusing the direction of the magnetic field with the direction of the force.
- Reversing Lenz's law so the induced emf aids the change.