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Physics 20
| Question | Answer |
|---|---|
| What is a battery and what are its components? | A source of electric energy containing two dissimilar metal electrodes and an electrolyte solution in which the electrodes are partially immersed. |
| What role does the zinc electrode play in a battery? | It acts as the anode (−), where zinc atoms lose electrons: Zn → Zn²⁺ + 2e⁻ |
| What role does the copper electrode play? | It acts as the cathode (+), where copper ions gain electrons: Cu²⁺ + 2e⁻ → Cu |
| What is voltage? | The electric potential difference that pushes electrons through a circuit; the energy per unit charge supplied by a battery. Measured in volts (V). |
| What is electric current and how is it calculated? | The amount of charge per unit time passing through a surface perpendicular to charge motion. I = Δq/Δt. Measured in amperes (A). The conventional direction of the current is always opposite to the direction of motion of free electrons. |
| What is the formula for resistance of a wire? | R = ρ(L/A), where ρ is resistivity, L is length, and A is cross-sectional area. |
| Why does resistance increase with temperature? | Electrons tightly bound to the nucleus move into the space used by free electrons, creating additional obstacles that impede current flow. |
| How does resistance change with temperature mathematically? | R_T = R₀(1 + α(T − T₀)), where R₀ is resistance at reference temperature T₀ (20°C) and α is the temperature coefficient of resistance. |
| State Ohm's Law. | Current is directly proportional to voltage and inversely proportional to resistance: V = IR |
| What are the three electric power equations? | P = IV = I²R = V²/R |
| What is 1 kilowatt-hour equal to in joules? | 1 kWh = 3.6 × 10⁶ J |
| What is the key property of resistors in series? | They share the same current. R_eq = R₁ + R₂ + …, and V = V₁ + V₂. |
| What is the key property of resistors in parallel? | They share the same voltage. 1/R_eq = 1/R₁ + 1/R₂ + …, and I = I₁ + I₂. |
| What is terminal voltage and its formula? | The voltage across battery terminals, which is less than the EMF due to internal resistance: V = ξ − Ir. Also I = ξ/req |
| What is the difference between DC and AC current? | In DC, current flows in one direction only. In AC, current direction changes at a set frequency (60 Hz in the US), following a sinusoidal pattern. |
| What are the RMS formulas for AC circuits? | I_rms = I₀/√2 and V_rms = V₀/√2. Average power: P̄ = I_rms · V_rms |
| State Kirchhoff's Junction Rule. | The sum of all currents entering a junction equals the sum of all currents leaving it. |
| State Kirchhoff's Loop Rule. | The algebraic sum of all voltage gains and losses around any closed loop equals zero. |
| For capacitors in series, what is the equivalent capacitance formula? | 1/C_eq = 1/C₁ + 1/C₂ + … (same charge Q on each, voltages add) |
| For capacitors in parallel, what is the equivalent capacitance formula? | C_eq = C₁ + C₂ + … (same voltage across each, charges add) |
| What is an RC circuit and what is the time constant? | A resistor and capacitor in series with a DC source. The time constant τ = RC determines how quickly the capacitor charges or discharges. |
| What are the charging equations for an RC circuit? | Q = Q₀(1 − e^(−t/RC)) and V_C = V₀(1 − e^(−t/RC)) |
| What are the discharging equations for an RC circuit? | Q = Q₀e^(−t/RC), V_C = V₀e^(−t/RC), and I = I₀e^(−t/RC) |
| What does the direction of conventional current represent? | The direction in which positive charge flows — opposite to the actual direction of electron movement. |
| If you double the length of a wire, what happens to its resistance? | Resistance doubles, since R is directly proportional to length (R = ρL/A). |
| If you double the cross-sectional area of a wire, what happens to its resistance? | Resistance is halved, since R is inversely proportional to cross-sectional area. |
| Why is the terminal voltage of a battery always less than its EMF? | Because some voltage is "used up" overcoming the battery's own internal resistance, leaving less available for the external circuit. |
| What happens to equivalent resistance when you add more resistors in series vs. parallel? | In series, equivalent resistance always increases. In parallel, it always decreases — it will even be less than the smallest individual resistor. |
| Why do household appliances use parallel circuits rather than series? | In parallel, each appliance receives the full supply voltage and operates independently — if one fails, the others continue working. In series, they would share voltage and all fail together. |
| What does the time constant τ = RC physically represent? | The time it takes for the capacitor to charge to about 63% of its maximum charge, or discharge to about 37% of its initial charge. |
| What is the physical significance of resistivity (ρ)? | It is a material property that describes how strongly a substance opposes the flow of electric current, independent of the object's shape or size. |
| At the moment a switch is closed in an RC charging circuit, how does the capacitor behave? | It behaves like a short circuit (wire) — voltage across it is zero and current is at its maximum value I₀ = V₀/R. |
| When a capacitor is fully charged in an RC circuit, how does it behave? | It behaves like an open circuit — no current flows and the full source voltage appears across the capacitor. |
| Which is the positive bar and which is the negative? | The current (I) always flows out from the positive terminal (the longer bar) and the negative bar is the shorter bar. |
| Vo | Peak, maximum voltage |
| Io | Peak, maximum current |
| Po | Peak, maximum power. You can use all of the average power equations but without the 1/2. |
| Complete Lecture Problem 1 | -- |
| Complete Lecture Problem 2 | -- |
| Complete Lecture Problem 3 | -- |
| Complete Lecture Problem 4 | -- |
| Complete Lecture Problem 5 | -- |
| Complete Lecture Problem 6 | -- |
| Complete Lecture Problem 7 | -- |
| Complete Lecture Problem 8 | -- |
| Complete Recitation Problem 1 | -- |
| Complete Recitation Problem 2 | -- |
| Complete Recitation Problem 3 | -- |
| Complete Recitation Problem 4 | -- |
| How to solve for disapated power? | Simplify into a single Req, calculate total current, then find the voltage drop across individual resistors, then find the current in the individual resistor, then find the power disapated by the P = I^2 * R equation. |
| Complete Homework Problem 1 | -- |
| Complete Homework Problem 3 | -- |
| Complete Homework Problem 4 | -- |
| Complete Homework Problem 5 | -- |
| Complete Homework Problem 6 | -- |
| Complete Homework Problem 8 | -- |
| Complete Homework Problem 9 | -- |
| Complete Homework Problem 10 | -- |
Created by:
smurtab