2.1 Current, potential difference and resistance Flashcards
Cell / battery
Provides the circuit with a source of potential difference. A battery is two or more cells.
Fixed resistor
A resistor limits the flow of current. A fixed resistor has a resistance it cannot change.
Variable resistor
A resistor with a slider that can be used to change its resistance. These are often used in dimmer switches and volume controls.
Thermistor
The resistance of a thermistor depends on its temperature. As its temperature increases, its resistance decreases and vice versa.
A thermistor is a non-ohmic conductor and a temperature-dependent resistor.
The resistance of a thermistor changes depending on its temperature.
As the temperature increases the resistance of a thermistor decreases and vice versa.
Light dependant resistor (LDR)
The resistance of an LDR depends on the light intensity. As the light intensity increases, its resistance decreases and vice versa.
A light-dependent resistor (LDR) is a non-ohmic conductor.
The resistance of an LDR changes depending on the light intensity on it.
As the light intensity increases the resistance of an LDR decreases and vice versa.
Diode
A diode allows current to flow in one direction only. Current flows through the diode when it is in forward bias position. They are used to convert AC to DC current.
A diode is a non-ohmic conductor that allows current to flow in one direction only.
This is called forward bias.
In the reverse direction, the diode has very high resistance, and therefore no current flows.
This is called reverse bias.
Light emitting diode (LED)
This is equivalent to a diode and emits light when a current passes through it. These are used for aviation lighting and displays (TVs, road signs).
Ammeter
Used to measure the current in a circuit. Connected in series with other components.
Voltmeter
Use to measure the potential difference of an electrical component. Connected in parallel with the relevant component.
Source of potential difference
For electrical charge to flow through a closed circuit, it must include a source of potential difference (p.d).
Sources of potential difference include:
A cell.
Batteries (multiple cells).
Electrical generator.
A cell makes one end of the circuit positive and the other negative.
This sets up a potential difference across the circuit.
This is sometimes known as the voltage.
The symbol for potential difference is V.
The potential difference across a component in a circuit is defined as The energy transferred per unit charge flowing from one point to another.
The energy transferred can also be called the work done.
V = W / Q
W = work done (J)
Q = charge (C)
Electric current
Electric current is the flow of electrical charge.
It is measured in units of amperes (A) or amps.
The symbol for current is I.
The size of the electric current is the rate of flow of electrical charge.
In other words, how much charge passes through a point each second.
In metals, such as a copper wire, the electrical charge that flows is electrons.
Therefore, the current in a circuit is a flow of electrons.
The unit of charge is the Coulomb (C).
This is defined as the quantity of charge that passes a fixed point per second when a current of 1 A is flowing.
Q = It
I = current (A)
t = Time (s)
Current in a loop
Electrons are negatively charged.
Therefore, they flow away from the negative terminal of a cell towards the positive terminal.
Conventional current is defined as the flow of positive charge from the positive terminal of a cell to the negative terminal.
This is opposite to the direction of electron flow, this is because conventional current was being put to use before the discovery of the electron.
In a circuit that is a closed-loop, such as a series circuit, the current is the same value at any point
This is because the number of electrons per second that passes through one part of the circuit is the same number that passes through any other part.
Resistance
Resistance is defined as the opposition to current:
The higher the resistance of a circuit, the lower the current.
This means that good conductors have a low resistance and insulators have a high resistance.
The symbol for resistance is R.
It is measured in Ohms (Ω).
Ω is the Greek capital letter ‘Omega’.
An Ohm is defined as one volt per ampere (1 V / A).
The resistance of a circuit can be increased by adding resistors (or variable resistors) to it.
Every electrical component has a resistance, even wires.
The current I through a component depends on both the resistance R of the component and the potential difference V across the component.
The greater the resistance R of the component, the lower the current I for a given potential difference V across the component.
The lower the resistance R of the component, the greater the current I for a given potential difference V across the component.
Calculating Current, Resistance & Potential Difference
V = IR
V = potential difference (V)
I = current (A)
R = resistance (Ω)
Ohm’s law
The current through a conductor is directly proportional to the potential difference across it.
Electrical conductors that obey Ohm’s Law are referred to as ohmic conductors.
Examples of ohmic conductors are:
Fixed resistors
Wires
Heating elements
Ohm’s Law is represented by the equation V = IR
If V and I are directly proportional, this means that the resistance R remains constant.
Ohm’s Law is relevant only at constant temperatures.
An ohmic conductor will have a current-voltage (I–V) graph that is a straight line through the origin.
Filament lamp
A filament lamp is an example of a non-ohmic conductor.
This means that the current and potential difference are not directly proportional.
This is because the resistance of the filament lamp increases as the temperature of the filament increases.
The I–V graph for a filament lamp shows the current increasing at a proportionally slower rate than the potential difference.
As the current increases, the temperature of the filament in the lamp increases.
The higher temperature causes the atoms in the metal lattice of the filament to vibrate more.
This causes an increase in resistance as it becomes more difficult for free electrons (the current) to pass through.
Resistance opposes the current, causing the current to increase at a slower rate.
Resistance and temperature
All solids are made up of vibrating atoms.
The higher the temperature, the faster these atoms vibrate.
Electric current is the flow of free electrons in a material.
The electrons collide with the vibrating atoms which impedes their flow, hence the current decreases.
So, if the current decreases, then the resistance will increase (from V = IR).
Therefore, an increase in temperature causes an increase in resistance.
Linear and non linear elements
Linear elements include:
Fixed resistors
Wires
Heating elements
Non-linear elements include:
Filament lamps
Diodes & LEDs
LDRs
Thermistors
Application of thermistors
A thermistor is a temperature sensor and is regularly used as a thermostat.
This means it automatically regulates temperature or activates a device when the temperature reaches a certain point.
Therefore, thermistors are found in:
Ovens
Refrigerators
Fire alarms
Digital thermometers
Boilers
They are commonly used to regulate and monitor the temperature in environments where it must be carefully controlled eg. food and beverage factories.
Applications of LDR
An LDR is a light sensor.
This means it automatically regulates the amount of light intensity on it or activates a device when the light intensity reaches above or below a certain point.
Therefore, LDRs are found in:
Lights that switch on when it gets dark (eg. garden lights, street lights)
Alarm clocks
Burglar alarm circuits
Light intensity meters
Security lights
The main advantage of an LDR is that these circuits are automatic therefore not needing any human time and intervention to function correctly everyday.
Practical 3 (investigating resistance)
Aim: investigate how the length of a wire at a constant temperature affects the resistance of electrical circuits.
Procedure - Set up the apparatus by connecting two crocodile clips to the thin resistance wire a distance of 10 cm apart and setting the power supply to 1.5V
Connect the wire, using the clips, to the rest of the circuit.
Record the potential difference from the voltmeter and current from the ammeter.
Move the clips in 10 cm intervals further apart.
Take new measurements from the voltmeter and ammeter for each length reading.
Continue until the crocodile clips are a length of 1 m apart.
Practical 4 (investigating I-V characteristics)
Aim: use circuit diagrams to construct appropriate circuits to investigate the I–V characteristics of a variety of circuit elements.
These include a fixed resistor at a constant temperature, a lamp and diode.
Procedure - Set up the circuit as shown with the fixed resistor.
Vary the voltage across the component by changing the resistance of the variable resistor, using a wide range of voltages (between 8-10 readings). Check the appropriate voltage reading on the voltmeter.
For each voltage, record the value of the current from the ammeter 3 times and calculate the average current.
Increase the voltage further in steps of 0.5 V and repeat steps 2 and 3.
Make sure to switch off the circuit in between readings to prevent heating of the component and wires.
Reverse the terminals of the power supply and take readings for the negative voltage (and therefore negative current).
Replace the fixed resistor with the filament lamp, then the diode, repeating the experiment for each.
