MCAT Physics · Lesson 6
Circuits
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6.1 Current
Current is the flow of positive charge (opposite to electron flow). Conductivity, voltage sources, and current types are discussed, along with circuit laws and conservation principles.
- Conductivity:
- Reciprocal of resistance; unit = siemens (S), S/m for materials.
- Metallic conductivity:
- Metals lose outer electrons → free to move.
- Good electrical and thermal conductors.
- Metallic bond: sea of electrons moving past lattice.
- Electrolytic conductivity:
- Depends on ion concentration.
- Distilled water = low conductivity; seawater/orange juice = high conductivity.
- Measured using voltage across solution.
- Current:
- , SI unit = ampere (A = C/s)
- Electron flow: low → high potential, but conventional current opposite.
- Types:
- DC: single direction (batteries)
- AC: periodically reversing (household)
- Voltage:
- Potential difference from generators, galvanic cells, or batteries.
- Galvanic cells: spontaneous redox → emf (electromotive force, V = J/C)
- No motion → voltage = emf
- Circuit laws:
- Conservation of charge and energy.
- Kirchhoff’s laws:
- Junction:
- Loop:
6.2 Resistance
Resistance opposes current flow; depends on material properties, geometry, temperature, and configuration in circuits. Ohm’s law and power calculations included.
- Resistance:
- (ρ = resistivity, L = length, A = cross-sectional area)
- Resistivity:
- Superconductors: at T < 100 K
- Length: R ∝ L
- Cross-sectional area: Larger A → lower R
- Temperature: Higher T → higher R (mostly)
- Ohm's law:
- Power:
- Internal resistance:
- Discharging → current flows + → -
- Recharging → current flows - → +
- Series resistors:
- Current same; voltage additive:
- Parallel resistors:
- Voltage same; currents split.
- More conduction paths → less resistance.
6.3 Capacitance and Capacitors
Capacitors store charge at a specific voltage. Capacitance depends on geometry and dielectric materials. Series and parallel configurations affect voltage and charge storage.
- Capacitor properties:
- Two plates connected to voltage source: + on positive terminal, - on negative.
- Capacitance: , unit = farad (F = C/V)
- Parallel plate:
- E-field: , direction + → -
- Stored energy:
- Dielectric materials:
- Insulators placed between plates increase C:
- k ≥ 1
- Isolated capacitor: dielectric lowers voltage → increases capacitance
- Circuit capacitor: dielectric keeps voltage constant → increases stored charge
- Charge release:
- Discharge through wires or patient (defibrillator)
- Lightning: rapid natural discharge
- Capacitors in circuits:
- Series: share voltage, smaller effective C:
- Parallel: additive capacitance:
6.4 Meters
Devices to measure current, voltage, and resistance. Placement and resistance considerations affect accuracy.
- Ammeters:
- Measure current; inserted in series.
- Low resistance; ideal = 0 Ω
- Magnetic properties move needle
- Shunt used for high currents
- Voltmeters:
- Measure voltage across points; wired in parallel.
- Ideal = infinite resistance
- Ohmmeters:
- Measure resistance; own battery.
- Placed across 2 points; calculate resistance from voltage and current.
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