Current Electricity Flashcards
(21 cards)
What is electric current and how is it defined?
Electric current is the rate of flow of electric charge through a conductor. It is defined as the amount of charge passing a point per unit time. The relationship between charge (Q), current (I), and time (t) is given by Q = I × t. One ampere (1 A) is equal to one coulomb of charge per second.
What is potential difference (voltage)?
Potential difference (voltage) is the energy transferred per unit charge as charge moves through a component. It is what ‘pushes’ the charge through the circuit. When a charge Q gains or loses energy E, the potential difference V (in volts) between two points is given by E = Q × V (or V = E/Q). In circuits, a higher voltage means a stronger driving force for current.
State Ohm’s Law and describe an ohmic conductor.
Ohm’s Law states that for an ohmic conductor at constant temperature, the current through the conductor is directly proportional to the potential difference across it. Mathematically, V = I × R, where V is voltage, I is current, and R is resistance. An ohmic conductor is one that obeys Ohm’s Law (for example, a metal wire or resistor at constant temperature): its V–I graph is a straight line through the origin, so its resistance R = V/I is constant.
How does resistance affect current for a given voltage?
The resistance of a component opposes the flow of current. For a given potential difference, a higher resistance means a smaller current flows (since I = V/R). Conversely, reducing resistance increases current if voltage stays the same.
How does the resistance of a filament lamp change as the current changes?
A filament lamp is non-ohmic: as the current (and voltage) increase, the filament’s temperature rises. As its temperature increases, its resistance increases. This causes the V–I graph of a filament lamp to be a curve that flattens out (it requires more voltage for each additional increase in current).
What is a diode and how does it behave in a circuit?
A diode is a component that allows current to flow only in one direction (forward) and blocks it in the reverse direction. In the forward direction (anode to cathode), it has a low resistance after its threshold voltage (~0.6–0.7 V for silicon) and conducts current. In reverse, it has a very high resistance and virtually no current flows. On a V–I graph, a diode passes current only in the forward quadrant (after the threshold) and shows almost zero current in the reverse.
How does a thermistor behave, and where is it used?
A thermistor is a temperature-dependent resistor whose resistance decreases as temperature increases. At higher temperatures, more charge carriers are available, so it conducts more easily. Thermistors are used in circuits for temperature sensing and control (e.g. thermostats, temperature sensors in refrigerators).
How does an LDR (light-dependent resistor) behave, and where is it used?
An LDR is a resistor whose resistance decreases as light intensity increases. In bright light, it has low resistance; in darkness, its resistance is high. LDRs are used in circuits that respond to light changes (e.g. automatic street lights or camera light sensors).
How do you measure the resistance of an unknown component?
To measure resistance, set up a circuit with a variable power supply, an ammeter in series, and a voltmeter in parallel across the component. Adjust the supply to different voltages, record the current I and potential difference V, and calculate R = V/I. Plotting V against I and finding the slope can also give R (for a linear device). This circuit diagram uses standard symbols (battery, resistor, ammeter, voltmeter).
What is a V–I characteristic and how do linear and non-linear elements differ?
A V–I characteristic is a graph of current (I) versus voltage (V) for a component. A linear element (ohmic) has a straight-line V–I graph (constant R). A non-linear element has a curve (varying R). For example, an ideal resistor is linear (straight line), a filament lamp has a curved V–I graph (due to rising R), and a diode’s V–I graph is non-linear (only conducts after a certain forward voltage and almost zero in reverse).
Describe a series circuit in terms of current, voltage, and resistance.
In a series circuit, components are connected end-to-end in a single loop: 1. Current: The same current flows through each component (I₁ = I₂ = … = I). 2. Voltage: The total supply voltage is shared (split) across the components. The sum of the individual voltages equals the supply: V_supply = V₁ + V₂ + … 3. Resistance: The total (equivalent) resistance is the sum of the resistances: R_total = R₁ + R₂ + …. Adding resistors in series increases the total resistance, because the charge must pass through each one sequentially.
Describe a parallel circuit in terms of current, voltage, and resistance.
In a parallel circuit, components are connected on separate branches across the same two points: 1. Voltage: The same potential difference (voltage) is across each component (V₁ = V₂ = … = V_supply). 2. Current: The total current from the supply splits into the branches. The sum of the branch currents equals the total current: I_total = I₁ + I₂ + …. Each branch current depends on its branch resistance (I = V/R). 3. Resistance: The equivalent resistance R_total of the parallel combination is less than the smallest individual resistance. This is because more branches provide extra paths for current. Quantitatively, 1/R_total = 1/R₁ + 1/R₂ + … (for two resistors in parallel, R_total = R₁R₂/(R₁+R₂)).
Why does adding resistors in series increase total resistance, whereas adding them in parallel decreases total resistance?
- Series: In a series connection, resistors are end-to-end. The current must flow through each resistor sequentially. Each resistor adds to the total opposition. Therefore, the total resistance is the sum: R_total = R₁ + R₂ + …. Adding more resistors in series makes it harder for current to flow, so R_total increases. 2. Parallel: In a parallel connection, resistors are on separate branches between the same nodes. Current can split and flow through all branches simultaneously. This provides extra pathways for charge to flow. The combined effect of multiple paths is a lower net opposition. Mathematically, 1/R_total = 1/R₁ + 1/R₂ + …, so adding an extra branch (resistor) makes 1/R_total larger, meaning R_total is smaller. 3. Comparison: Thus, series resistors simply add more opposition in the single path (increasing R_total), while parallel resistors create multiple paths (decreasing the overall opposition). This qualitative difference explains the opposite effects on total resistance.
How do you calculate equivalent resistance in series and parallel circuits?
For resistors in series, add their resistances: R_total = R₁ + R₂ + …. For resistors in parallel, use the reciprocal formula: 1/R_total = 1/R₁ + 1/R₂ + …. For two resistors, this gives R_total = (R₁ × R₂) / (R₁ + R₂). These formulas allow you to find the total resistance seen by the power supply.
What is the unit of electric current?
The ampere (A).
What is the unit of potential difference (voltage)?
The volt (V).
What is the unit of resistance?
The ohm (Ω).
What is the relationship between current, potential difference, and resistance?
V = I × R (Voltage = Current × Resistance).
What happens to current if resistance increases and voltage stays the same?
Current decreases.
What happens to current if potential difference increases and resistance stays the same?
Current increases.
What type of component has a constant resistance at constant temperature?
An ohmic conductor.
