Electricity, magnetism and simple circuits
Electricity, magnetism and simple circuits tests current, voltage, resistance, circuit rules and basic magnetic effects.
Circuits
- Current I is charge flow per second (ampere); conventional current flows positive to negative.
- Voltage V is energy transferred per coulomb (volt); measured across components in parallel.
- Resistance R = V/I (ohm). Ohm's law holds for ohmic conductors at constant temperature.
- Series: same current; voltages add; R total = R1+R2+... Parallel: same voltage; currents add; 1/R = 1/R1 + 1/R2.
- Power P = VI = I²R = V²/R; energy E = Pt.
Magnetism
- Magnets have north and south poles; like poles repel. Magnetic field lines go north to south outside a magnet.
- A current-carrying wire has a magnetic field; a solenoid is an electromagnet. The motor effect: a current in a magnetic field experiences a force.
- Increasing current, turns or adding an iron core strengthens an electromagnet.
Worked example
A 12 V battery is connected across a 4.0 Ω resistor. I = V/R = 12/4.0 = 3.0 A. Power P = VI = 12×3.0 = 36 W. In 60 s energy transferred E = Pt = 36×60 = 2160 J. If two such resistors are in series across 12 V, total R = 8.0 Ω, current = 1.5 A.
Traps
Putting an ammeter in parallel; using series voltage rule for parallel; adding resistances directly in parallel; confusing power and energy; forgetting conventional current direction.