Physics - Voltage and Resistance Flashcards
Where does energy enter the circuit?
The power supply
Where does energy leave the circuit?
The components
What is voltage?
The electrical energy transferred [electrical working] to the component per unit charge passed
What is the relationship between voltage, energy and charge?
Voltage = Energy/Charge
What is a volt equal to?
1 J/C
How many Volts are in the mains vs in a cell?
230V - 1.5V (Though it can vary)
How is a Voltmeter connected to a component?
It is connected in parallel with the component
In a series circuit, how is the voltage divided if there are two components of differing resistance?
The voltage divides in proportion to the ratio of the other resistances. For example, in a circuit with 6V and with two components (one with 6ohms and another with 12 ohms) the voltage for the first component will be 2V and for the second it will be 4V.
In a series circuit, how is the voltage divided if there are two components of the same resistance?
The voltage divides by the number of them. For example, if there are 6V in a circuit with 3 components of the same resistance, the voltage for each will be 2V.
In a series circuit, will the voltage change if the resistance of all of the components doubles?
No - provided they are in the same ratio, the resistance will remain the same. However, the current would half as it is affected by the total resistance in a circuit.
In a parallel circuit, does the number of branches affect the voltage that each branch receives?
No - each branch will receive the same amount that the cell provides (e.g. 1.5V). This however draws more voltage from the cell so it will run out more quickly.
If you attach 4 1.5V cells correctly, what Voltage would they supply a series circuit with?
6V
If you attach 3 1.5V cells correctly, but attach 1 1.5V cell wrongly (wrong poles attaches), what voltage would be supplied to a series circuit?
3V - each incorrectly attached one makes another useless so effectively there are only two cells
What is the equation linking resistance, voltage and current?
Resistance (ohms) = voltage (volts)/current (amps)
R = V/I
What is charge measured in?
Coulombs - C
Give four ways that resistance is decreased in a wire
Make it out of a different material (e.g. copper), make it wider, make it shorter and make it cooler
What is Ohm’s law?
V = IR
Voltage = Current x resistance
What is Ohmic behaviour?
When something gives constant resistance regardless of its voltage or current (e.g. resistor or wire)
Describe a circuit used to investigate the I-V behaviour of a metal wire
Power bank connected to an ammeter, then a coil of wire, with a voltmeter connected in parallel
Describe a circuit used to investigate the I-V behaviour of a filament bulb
Power bank connected to an ammeter, then a filament bulb, with a voltmeter connected in parallel
Describe a circuit used to investigate the I-V behaviour of a diode
Power bank connected to an ammeter, then a diode, with a voltmeter connected in parallel
What does the I-V graph of a wire look like? What conclusion can you make?
A straight line - it obeys ohmic behaviour (it does have a linear relationship between voltage and current), and the current is directly proportional to the voltage. The resistance is constant
What does the I-V graph of a filament bulb look like? What conclusion can you make?
A straight line at the bottom, but then it curves to level off. It obeys ohmic behaviour at the start but then it curves and doesn’t - it does not have a linear relationship between voltage and current
The current isn’t proportional to the voltage because the temperature of the filament increases so the resistance increases
What does the I-V graph of a diode look like? What conclusion can you make?
It starts off as nothing (it’s just a horizontal line), but then suddenly (at about 0.7V) it rises rapidly to a very steep line.
It doesn’t obey Ohm’s Law, as it does not have a linear relationship between voltage and current. The resistance is extremely high until it reaches 0.7V and the resistance drops rapidly
Write down the relationship between charge, current and time in words and symbols
Charge = Current × Time
Q=It
Use a microscopic model to explain why metals resist the flow of current
Metals resist the flow of current because they are made of ionic lattices, and delocalised electrons. When the electrons flow through the lattices, they may collide with the metal ions, and transfer some of their kinetic energy in the process, thus slowing down and the current reduces.
Use a microscopic model to explain why the resistance of a metal increases as its temperature increases
As the temperature increases, the metal ions vibrate more, because temperature is linked to kinetic energy (temperature is kinetic energy amongst all of the particles). Therefore, the delocalised electrons collide with the ions more often and transfer their kinetic energy to the ions, slowing them down and the current decreases as a result.
Use a microscopic model to explain why metals heat up when a current flows
Current is the flow of a charged particle, and through wire it is often electrons that flow. When they flow they may collide with the fixed ions that make up metal lattices - and in the process transfer kinetic energy. Kinetic energy on a microscopic scale is often thermal energy on a macroscopic scale, and thus the metal heats up.
Write down the equation that give the combined resistance of three different resistors in series
R_T=R_1+R_2+R_3
Total resistance = Resistance 1 + Resistance 2 + Resistance 3
Write down the equation that give the combined resistance of three different resistors in parallel
1/R_T =1/R_1 +1/R_2 +1/R_3
1/Total Resistance = 1/Resistance 1 + 1/Resistance 2 + 1/Resistance 3
Describe the basic structure of the metallic state.
Fixed ions and delocalised electrons
Describe the breakdown of insulators and semiconductors so they can conduct
Enough voltage causes the electrons in an insulator to delocalise and conduct through the material
What is a semiconductor?
A material that conducts better than an insulator, but worse than a metal. Conduction is due to excited electrons in the material, therefore its resistance decreases as temperature increases
Describe and explain how resistance changes with length
Resistance increases in proportion to length due to the additional material the current must flow through. This is similar to putting resistors in series
Describe and explain how resistance changes with cross sectional area
Resistance decreases as cross sectional area increases. This is due to the additional paths that the current can flow through. This is similar to putting resistors in series.
