Chapter 10: Current Electricity and DC Circuits Flashcards
Electric current, e.m.f, p.d., Resistance, Ohm's Law, Ohmic conductors, Resistance and Resistivity, Resistors in Series and Parallel, Potential Divider, Transducers, Thermisters, LDR
What is electric current (I)?
The rate of flow of charges (I = Q / T)
What instrument is used to measure current?
Ammeter
How should the Ammeter be connected to a circuit, and why?
It should be connected in series. As the ammeter has an extremely low resistance, if it is connected in parallel, then all the current in the circuit will flow through the lower resistance path, and no current will flow through the component, creating a short circuit
How does the conventional electric current flow?
From the positive terminal of the power source to the negative terminal
What is electromotive force in terms of work done?
The amount of work done by a power source to drive a unit charge around an electrical circuit. (AKA battery voltage)
What equation can be used to express electromotive force in terms of work done?
e.m.f. = Work Done (W) / Charge (Q)
What is electromotive force in terms of energy?
The amount of non-electrical energy (chemical potential energy) converted into electrical energy when a unit charge passes through the power source
What equation can be used to express electromotive force in terms of energy?
e.m.f. = Energy (E) / Charge (Q)
SI Unit: Volt (V)
What is Potential Difference?
The amount of electrical energy converted into non-electrical energy when a unit charge passes between two points of an electrical component
OR
The difference in voltage between two points of an electrical component
What equation can be used to express potential difference in terms of energy?
Potential Difference (V) = Energy (E) / Charge (Q)
SI Unit: Volt (V)
What instrument is used to measure potential difference / voltage?
Voltmeter
How should the Voltmeter be connected to a circuit, and why?
It should be conected in parallel. As the voltmeter has an extremely high resistance, if it is connected in series, the resistance of the circuit would be increased by an extremely large amount. Therefore, to measure the potential difference across two points in the circuit, the Voltmeter has to be connected in parallel
What is Resistance?
The ratio of voltage across a conductor to the current flowing through it at that point
What equation can be used to express Resistance in terms of Voltage and Current?
Resistance (R) = Voltage (V) / Current (I)
SI Unit: ohm
What does Ohm’s law state?
Current flowing through a conductor is directly proportional to the potential difference across it provided all physical conditions stay constant
What is an Ohmic conductor?
A conductor that follows Ohm’s law. That is to say that it’s resistance is constant
What is a non-Ohmic conductor?
A conductor that does not follow Ohm’s law. The current flowing through it is not directly proportional to the voltage flowing through it, as the resistance in non-Ohmic conductors are not constant, and vary with different variables
Why does the resistance of metals (good conductors) increase with higher current flow?
The metal heats up when a current flows through it, causing its temperature to rise. At higher temperatures, the metallic ions vibrate about their fixed positions more vigorously, which increases the number of collisions between the mobile electrons and fixed positive ions, which opposes and slows down the flow of electrons, causing resistance to increase
What are some factors that affect the resistance of a conductor?
Temperature: As temperature increases, resistance increases
Length: As length increases, resistance increases
Area of cross-section: As area of cross-section increases, resistance decreases
What is electrical resistivity?
A property of a material that quantifies how strongly it resist electrical current
What is the SI unit for resistivity?
ohm-meter
What equation describes the relationship between resistivity and resistance?
Resistivity (p) = Resistance (R) x Area (A) / Length (L)
OR
Resistance (R) = Resistivity (p) x Length (L) / Area (A)
SI unit: ohm-meter
What is the difference between resistance and resistivity?
The resistance of a conductor depends on its length and area, while the resistivity of a conductor is constant, independent of its dimensions
When are resistors in series?
When the same current physically flows through all the resistors
How is the potential difference (voltage) across a resistor related to its resistance?
Directly proportional
How can the potential difference across a power source in series be found?
Finding the sum of the potential differences across all the resistors
When are resistors in parallel?
When there is more than one path the electrons can flow through
How are the potential difference (voltage) across resistors in parallel related?
They are the same
How is the current flow through each resistor related to the resistance of each resistor?
Inversely proportional
How can total current be calculated in a parallel circuit?
The sum of all the currents at different branches
How can the total resistance be calculated in a parallel circuit?
1 / Rt = 1 / R1 + 1 / R2 + 1 / R3 …
How does a circuit short circuit?
When a path connected in parallel in the circuit has an extremely low resistance, such as when the path is only made of one wire, or consists of an ammeter. This path provides an easy path for the current, which draws an extremely large current to it
What are some consequences of a short circuit?
Overheats wires and may cause fire, causes battery to go flat (run out of energy) very quickly
What does a potential divider do?
It provides a voltage to an electrical component that is smaller than the supply voltage in a circuit. It can divide the supply voltage by any ratio
Assuming the potential divider consists of two resistors, how can the voltage across either of the resistors be calculated?
Voltage across Resistor 1 = Supply Voltage x (Resistance 1 / (Resistance 1 + Resistance 2))
Works for both Resistor 1 and 2
What are transducers?
Electrical or electronic devices that convert energy from one form to another
What is the difference between an input and output transducer?
Input: Converts non-electrical energy into electrical energy (Microphones, Photocells, LDR)
Output: Converts electrical energy into non-electrical energy (LED, Speaker)
How does a thermistor work?
It has a resistance that decreases when the temperature increases
Why and where are thermistors used
They are used to measure and regulate temperature, like in refridgerators and ovens
How does an LDR work?
It has a resistance that decreases when the light intensity increases
Why and where are LDRs used?
They are used to measure and detect changes in light intensity, like in smoke detectors
What steps should be taken to find the total resistance of a series-parallel circuit?
- Label all the resistance and current values
- Combine all the series branches into one equivalent resistance value (Rt = R1 + R2 + R3 …)
- Convert the parallel resistances into one equivalent resistance value in series (1 / Rt = 1 / R1 + 1 / R2 + 1 / R3 …)
- Repeat steps 2 and 3 until the circuit is simplified
