Physics (university) · Part 08
Circuits and magnetism
Current, resistance and circuit rules, then magnetic fields and forces.
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- Electric currentThe rate that charge flows past a point, I = Q/t. The unit is the ampere, one coulomb per second.
- Ohm's lawFor many conductors, voltage equals current times resistance, V = IR. Resistance is in ohms.
- Series and parallel resistorsIn series, resistances add. In parallel, reciprocals add: 1/R = 1/R1 + 1/R2.
- Electric powerRate of energy use in a circuit, P = IV = I squared R = V squared / R. The unit is the watt.
- Kirchhoff's junction ruleAt a junction, the current going in equals the current going out, because charge is conserved.
- Kirchhoff's loop ruleAround any closed loop, the voltage rises equal the voltage drops, because energy is conserved.
- RC circuitA resistor and a capacitor together. Charging takes time given by the time constant tau = RC.
- Magnetic fieldA region where moving charges and magnets feel force. Measured in tesla. Field lines form closed loops.
- Field of a long straight wireA current makes circular field lines around the wire. Its size is B = mu0 I / (2 pi r).
- Magnetic force on a chargeA moving charge in a magnetic field feels a force F = q v B sin theta, at right angles to both. It changes direction, not speed.
- Circular motion in a fieldA charge moving at right angles to a uniform field follows a circle of radius r = m v / (q B).
- Force on a current-carrying wireA wire carrying current I in a field B feels force F = B I L sin theta.