16.2 Current

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Flow of charge

We have been talking about moving charge. We need to be able to deal with numbers. How much charge is moving, how fast is it moving? The concept that represents this information is called current . Current allows us to quantify the movement of charge.

When we talk about current we talk about how much charge moves past a fixed point in circuit in one second. Think of charges being pushed around the circuit by the battery, there are charges in the wires but unless there is a battery they won't move. When one charge moves the charges next to it also move. They keep their spacing as if you had a tube of marbles like in this picture.

If you push one marble into the tube one must come out the other side. If you look at any point in the tube and push one marble into the tube, one marble will move past the point you are looking at. This is similar to charges in the wires of a circuit.

If one charge moves they all move and the same number move at every point in the circuit. This is due to the conservation of charge.

Current

Now that we've thought about the moving charges and visualised what is happening we need to get back to quantifying moving charge. I've already told you that we call moving charge current but we still need to define it precisely.

Current

Current is the rate at which charges moves past a fixed point in a circuit. We use the symbol I to show current and it is measured in amperes (A). One ampere is one coulomb of charge moving in one second.

$I=\frac{Q}{\Delta t}$

When current flows in a circuit we show this on a diagram by adding arrows. The arrows show the direction of flow in a circuit. By convention we say that charge flows from the positive terminal on a battery to the negative terminal. We measure current with an ammeter

Series circuits

In a series circuit, the charge has a single path from the battery, returning to the battery.

The arrows in this picture show you the direction that charge will flow in the circuit. They don't show you much charge will flow, only the direction.

Interesting fact

Benjamin Franklin made a guess about the direction of charge flow when rubbing smooth wax with rough wool.He thought that the charges flowed from the wax to the wool (i.e. from positive to negative) which was opposite to the realdirection. Due to this, electrons are said to have a negative charge and so objects which Ben Franklin called “negative” (meaning a shortage of charge) really have an excessof electrons. By the time the true direction of electron flow was discovered, the convention of “positive” and “negative” hadalready been so well accepted in the scientific world that no effort was made to change it.
A battery does not produce the same amount of current no matter what is connected to it. While the voltageproduced by a battery is constant, the amount of current supplied depends on what is in the circuit.

How does the current through the battery in a circuit with several resistors in series compare to the current in a circuit with asingle resistor (assuming all the resistors are the same)?

Experiment : current in series circuits

Aim:

To determine the effect of multiple resistors on current in a circuit

Apparatus:

• Battery
• Resistors
• Light bulb
• Wires

Method:

1. Construct the following circuits
2. Rank the three circuits in terms of the brightness of the bulb.

Conclusions:

The brightness of the bulb is an indicator of how much current is flowing. If the bulb gets brighter because of a change then more current is flowing. If the bulb gets dimmer less current is flowing. You will find that the more resistors you have the dimmer the bulb.

Parallel circuits

How does the current through the battery in a circuit with several resistors in parallel compare to the current in a circuit with asingle resistor?

Experiment : current in series circuits

Aim:

To determine the effect of multiple resistors on current in a circuit

Apparatus:

• Battery
• Resistors
• Light bulb
• Wires

Method:

1. Construct the following circuits
2. Rank the three circuits in terms of the brightness of the bulb.

Conclusions:

The brightness of the bulb is an indicator of how much current is flowing. If the bulb gets brighter because of a change then more current is flowing. If the bulb gets dimmer less current is flowing. You will find that the more resistors you have the brighter the bulb.

Why is this the case? Why do more resistors make it easier for charge to flow in the circuit? It is because they are in parallel so there are more paths for charge to take to move. You can think of it like a highway with more lanes, or the tube of marbles splitting into multiple parallel tubes. The more branches there are, the easier it is for charge to flow. You will learn more about the total resistance of parallel resistors later but always remember that more resistors in parallel mean more pathways. In series the pathways come one after the other so it does not make it easier for charge to flow.

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