Electricity Flashcards
(74 cards)
What is current defined by ?
The rate of flow of electric charge
What is current measured in ?
Current is measured in units of amperes or amps (A)
1 amp is equivalent to a charge of 1 coulomb flowing in 1 second, or 1 A = 1 C/s
This means the size of an electric current is the amount of charge passing through a component each second
How does a current flow ?
Current flows
when a circuit is formed e.g. when a wire connects the two oppositely charged terminals of a cell
from the positive terminal to the negative terminal of a cell
How do you measure current ?
Current can be measured using an ammeter
Ammeters must be connected in series with the component being measured
What is a charge ?
The wires in an electric circuit are made of metal because it is a good conductor of electric current
In the wires, the current is a flow of negatively charged electrons
What is the conventional current?
Exam Tip
You should always consider current to be the flow of positive charge i.e. from the positive terminal to the negative terminal of a cell. This is known as conventional current.
This is in the opposite direction to electron flow, which is the flow of negatively charged electrons from the negative terminal to the positive terminal of a cell.
This is the convention we use because scientists defined conventional current before they discovered the electron
How do you calculate electric charge?
Current, charge and time are related by the equation:
charge = current × time
Q = I x t
Where:
Q = charge, measured in coulombs (C)
I = current, measured in amps (A)
t = time, measured in seconds (s)
The current, charge and time equation can be rearranged with the help of the following formula triangle:
What is voltage defined as ?
The energy transferred per unit charge passing between two points
Voltage is measured in units of volts (V)
1 volt is equivalent to the transfer of 1 joule of energy by 1 coulomb of charge, or 1 V = 1 J/C
The terminals of a cell make one end of the circuit positive and the other negative
As electrons flow through a cell, they gain energy
For example, in a 12 V cell, every coulomb of charge passing through gains 12 J of energy
As electrons flow through a circuit, they lose energy
For example, after leaving the 12 V cell, each coulomb of charge will transfer 12 J of energy to the wires and components in the circuit
How to measure voltage ?
Voltage can be measured using a voltmeter
Voltmeters must be set up in parallel with the component being measured
Exam Tip
When you are building a circuit in class, always connect the voltmeter last. Make the whole circuit first and check it works, and then connect the voltmeter so that the leads are on each side of the component you are measuring. This will save you a lot of time waiting for your teacher to troubleshoot your circuit!
You might sometimes see voltage called potential difference. This term can be useful when thinking about voltmeters as the potential difference describes a difference between two points, therefore the voltmeter has to be connected between two points in the circuit.
How do you calculate voltage ?
The equation linking the energy transferred, voltage and charge is given below:
energy transferred = charge × voltage
E = Q X V
Where:
E = energy transferred, measured in joules (J)
Q = charge moved, measured in coulombs (C)
V = voltage, measured in volts (V)
This can be rearranged using the formula triangle below:
How is resistance defined as ?
The opposition of a component to the flow of electric current through it
What is resistance measured in ?
Resistance is measured in units of ohms (Ω)
A resistance of 1 Ω is equivalent to a voltage across a component of 1 V which produces a current of 1 A through it
The resistance of a component controls the size of the current in a circuit
For a given voltage across a component:
The higher the resistance, the lower the current that can flow
The lower the resistance, the higher the current that can flow
All electrical components, including wires, have some value of resistance
Wires are often made from copper because it has a low electrical resistance
This is why it is known as a good conductor
How do you compare current and resistor ?
The current, resistance and potential difference of a component in a circuit are calculated using the equation:
voltage = current × resistance
V = I x R
Where:
V = voltage, measured in volts (V)
I = current, measured in amps (A)
R = resistance, measured in ohms (Ω)
This equation can be rearranged with the help of the following formula triangle:
How do you calculate voltage ?
Exam Tip
In exam questions, the resistance of the wires, batteries, ammeters and voltmeters are always assumed to be zero (in the case of voltmeters, they have extremely high resistances so that current does not flow through them, and this has a negligible effect on the overall resistance of the circuit)
What are the two ways to join electrical components ?
There are two ways of joining electrical components:
in series
in parallel
What Is a series circuit ?
A series circuit is a circuit that has only one loop, or one path that the electrons can take
In a series circuit, the current has the same value at any point
This is because the electrons have only one path they can take
Therefore, the number of electrons passing a fixed point per unit time is the same at all locations
This means that all components
in a series circuit have the same current
The amount of current flowing in a series circuit depends on:
the voltage of the power source
the number (and type) of components
Increasing the voltage of the power source drives more current around the circuit
So, decreasing the voltage of the power source reduces the current
Increasing the number of components in the circuit increases the total resistance
Hence less current flows through the circuit
What is a parallel circuit ?
A parallel circuit is a circuit that has two or more loops, or more than one path that electrons can take
Parallel circuits contain junctions and branches
Junctions are points where two or more wires meet to form a new branch
Branches are the sections of wire between junctions
What is a current in a parallel circuit ?
In a parallel circuit, the current has different values at different points in the circuit
This is because the current splits at a junction
Therefore, the electrons have different paths they can take
The sum of the current in the individual branches is equal to the total current before (and after) the branches
Why is a current conserved at a junction in a circuit?
At a junction, the current is always conserved
This means the amount of current flowing into the junction is equal to the amount of current flowing out of it
This is because the charge is conserved
Current does not always split equally – often there will be more current in some branches than in others
The current in each branch will only be identical if the resistance of the components along each branch is identical
Current behaves in this way because it is the flow of electrons:
Electrons, or any charge, cannot be created or destroyed
This means the total number of electrons (and hence current) going around a circuit must remain the same
When the electrons reach a junction, however, some of them will go one way and the rest will go the other
What is voltage in a series ?
In a series circuit, the total voltage of a power supply is shared between the components.
What will be the voltage for two identical components ( with equal resistance) ?
For two identical components (with equal resistance), the voltage across them will be:
the same
equal to half the total voltage of the power supply
What will be the voltage for two non identical components (with different values of resistance)?
For two non-identical components (with different values of resistance), the voltage will be:
higher across the component with the higher resistance
lower across the component with lower resistance
What amount of voltage is in a parallel circuit ?
In a parallel circuit, the total voltage across each branch is the same as the voltage of the power supply