What are the key learning points about charge, current and voltage?
The quantitative relationship between current, charge and time is given by the equation: charge = current × time.
Charge is measured in coulombs.
Current is the rate of flow of charge and is measured in amps using an ammeter.
Voltage is measured across a component using a voltmeter.
What is electrical charge and current?
An electric current is a flow of charged particles.
In metal conductors, the charged particles are Negatively charged sub-atomic particles that can move through the structure of a substance, usually a metal or graphite. A material with many free electrons is a good conductor..
The electrons are free to move from one Electrically charged particle, formed when an atom gains or loses electrons. to another and a net flow of these electrons in one direction is an electric current.
A source of energy, such as a cell or battery, is required to make the free electrons move in one direction.
Charge
Electrons are negatively charged particles and they transfer electrical energy from a cell, through conducting wires, as an electric current.
Charge is measured in coulombs, C.
The amount of negative charge an electron has is 1.6 x 10-19 C.
i.e. 1 C = \(\frac{1}{1.6 {\times}10^{-19}~C}\) = 6.25 billion billion electrons.
In other words, it takes 6,250,000,000,000,000,000 electrons to make up 1 coulomb of charge.
A coulomb of charge is just a very large group of electrons.
What is the relationship between current I and quantity of charge Q?
An electric current is a flow of charged particles.
The size of an electric current is the rate of flow of charge.
Current I = \(\frac{\text{quantity of charge Q}}{\text{time taken t}}\)
I = \(\frac{\text{Q}}{\text{t}}\)
This is often remembered as:
Quantity of charge Q = current I x time t
Q = It
Where:
Q = quantity of charge in coulombs, C
I = current in amperes, A
t = time in seconds, s
| I = \(\frac{\text{Q}}{\text{t}}\) | I = Q ÷ t |
| Q = It | Q = I x t |
| t = \(\frac{\text{Q}}{\text{I}}\) | t = Q ÷ I |
One ampere is the current that flows when one coulomb of charge passes a point in a circuit in one second.
Example Q1
A current of 1.5 A flows through a simple electrical circuit.
How many coulombs of charge flow past a point in 60 seconds?
Answer
Q = It
I = 1.5 A
t = 60 s
Q = 1.5 A x 60 s
Q = 90 C
The quantity of charge flowing past a point in 60 s is 90 C.
Question
How much charge has moved past a point in a circuit if a current of 13 A flows for 10 s?
Answer
Q = It
I = 13 A
t = 10 s
Q = 13 A x 10 s
Q = 130 C
The quantity of charge flowing past a point in the circuit in 10 s is 130 C.
Question
How much current flows when 10 C passes down a wire in 2 s?
Answer
I = \(\frac{\text{Q}}{\text{t}}\)
Q = 10 C
t = 2 s
I = \(\frac{\text{10 C}}{\text{2 s}}\)
I = 5 A
The current flowing is 5 A.
Example Q2
What is the current if 20 C of charge flows through the filament of a lamp in
5 s?
Answer
I = \(\frac{\text{Q}}{\text{t}}\)
Q = 20 C
t = 5 s
\(I = \frac {20~C}{5~s} \text = 4A\)
The current flowing through the lamp is 4 A.
Question
How much charge flows when a current of 2 A flows for 3 minutes?
Answer
Q = It
I = 2 A
t = 3 minutes = 3 x 60 = 180 s
Q = 2 A x 180 s = 360 C
The quantity of charge is 360 C.
How to measure current and voltage
Current is measured using an ammeter.
To measure the current flowing through a component in a circuit, an ammeter is always connected in series with the component.
Key points
When measuring current:
Current is measured in amperes, A.
The current flowing through a component in a circuit is measured using an ammeter.
The ammeter must be connected in series with the component.
There is a The potential difference across a cell, electrical supply or electrical component. It is measured in volts (V). (or potential difference) across an electrical component when an electric current flows through it.
Key point
Measuring voltage (sometimes called The potential difference (or voltage) of a supply is a measure of the energy given to the charge carriers in a circuit. The unit this is measured with is volts (V).)
Voltage is measured in volts, V.
The voltage across a component in a circuit is measured using a voltmeter.
The voltmeter must be connected in parallel with the component.
This shows how to connect a voltmeter for measuring voltage and an ammeter for measuring current.
An ammeter needs to measure the flow of charge, so it is in series.
This means that all the charge has to flow through it and can be counted.
A voltmeter measures voltage across a component.
This means it is in parallel so it can measure the voltage at two different points in the circuit.
Measuring voltage and current can be used to measure .
How much do you know about current, charge and voltage?
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