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P18 Electrical quantities - Coggle Diagram
P18 Electrical quantities
Electric current
When two oppositely charged conductors are connected together (by a length of wire), charge will flow between the two conductors
This flow of charge is called an electric current
The greater the flow of charge, the greater the electric current
In a metal, current is caused by a flow of electrons
current= potential difference ÷ resistance
Charge, current and time
The current is the charge passing a point in a circuit every second
charge = current x time
Q x I x t
Where the symbols
Q stands for charge (measured in coulombs, C)
I stands for current (measured in amps, A)
Measuring current
Current is measured using an ammeter
Ammeters should always be connected in series with the part of the circuit you wish to measure the current through
Potential difference
The potential difference (voltage) between two points in a circuit is related to the amount of energy transferred between those points in the circuit
Potential difference is measure in volts (V)
As charge flows around a circuit, energy is transferred to or from the charge
The potential difference between two points in a circuit is the amount of energy transferred by each unit of charge passing between those two points
The unit of voltage, the volt (V), is the same as a joule per coulomb (J/C)
Potential difference can be measured using a voltmeter
The voltmeter should be connected in parallel with the part of the circuits you want to measure the potential difference of
potential difference = current x resistance
Resistance
Resistance is the opposition to current
The higher the resistance, the lower the current
The unit of resistance is the ohm (Ω)
Resistance = potential difference ÷ current
Resistance of a wire
As electrons pass through a wire, they collide with the metal ions in the wire
The ions get in the way of the electrons, resisting their flow
If the wire is longer, each electron will collide with more ions and so there will be more resistance:
The longer a wire, the greater its resistance
If the wire is thicker (greater diameter) there is more space for the electrons and so more electrons can flow:
The thicker a wire, the smaller its resistance
The resistance of a wire is proportional to its length
This means that if the length of a wire is doubled, its resistance will double
The resistance of a wire is inversely proportional to its cross-sectional area
This means that is the cross-sectional area of a wire is doubled, its resistance will halve
voltage and current relationship
As the potential difference (voltage) across a component is increased, the current in the component also increases
The precise relationship between voltage and current can be different for different types of components
A resistor is very simple:
The current is proportional to the potential difference
This is because the resistor has a constant resistance
For a filament lamp the relationship is more complicated:
The current increases at a proportionally slower rate than the potential difference
This is because:
The current causes the filament in the lamp to heat up
As the filament gets hot its resistance increases
This opposes the current, causing it to increase at a slower rate
Electrical energy
As electricity passes around a circuit, energy is transferred from the power source to the various components (which may then transfer energy to the surroundings)
As charge passes through the power supply it is given energy
As it passes through each component it loses some energy (transferring that energy to the component)
The amount of electrical energy used by a component or appliance depends upon three things:
The potential difference
The amount of time the device is used for
The current
energy transferred = current × potential difference × time
E = I × V × t
Where the unit of energy is the joule (J)
Electrical power
Power is the rate of energy transfer (the amount of energy transferred per second)
Power = Current × potential difference
P = I×V
The unit of power is the watt (W), which is the same as a joule per second (J/s)
nergy transferred = power × time = current × potential difference × time
A
conductor
is a material that allows charge (usually electrons) to flow through it.
An
insulator
is a material that has no free charges are hence does not allow the flow of charge through it.
Electromotive force
The Electromotive Force (EMF) is the name given to the Potential Difference (Voltage) of the power source in a circuit
The Electromotive Force (EMF) is measured in Volts (V)