Electric Fields, Field Lines and Electric Forces for SS 1

Electric Field

Performance Objectives

By the end of the lesson, students should be able to:

1. explain the meaning of electric field, and draw electric force lines generated by:
   1. isolated positive charge,
   2. isolated negative charge,
   3. two like charges placed near each other,
   4. two unlike charges placed near each other;
2. explain electric current as continuous flow of charges;
3. interpret resistance as opposition to flow of charges (electric current);
4. set up a simple electric circuit and measure electric current and voltage in the circuit;
5. calculate the electric current in a simple circuit;
6. distinguish between electrical conductors and insulators.

Meaning of Electric Field

An electric field is a region around an electric charge where another electric charge experiences an electric force.

Imagine a magnet. It has a region around it where it can attract or repel other magnetic materials. Similarly, an electric charge creates an invisible region around itself. If another charge enters this region, it will feel a push or a pull. This region of influence is called an electric field.

Electric Lines of Force

Electric lines of force, also known as electric field lines, are imaginary lines used to represent the direction and strength of an electric field.

These lines help to visualise the electric field around charges. The direction of the electric field at any point is given by the tangent to the field line at that point. The density of the field lines indicates the strength of the electric field; where lines are closer together, the field is stronger.

Properties of Electric Field Lines

Electric field lines have specific properties that help in understanding electric fields:

1. They originate from positive charges and terminate on negative charges.
2. They never intersect each other. If they did, it would mean the electric field has two directions at the point of intersection, which is impossible.
3. They are closer together where the electric field is stronger and farther apart where it is weaker.
4. They are always perpendicular to the surface of a conductor.
5. They do not pass through a conductor in electrostatic equilibrium.

Drawing Electric Field Lines

The pattern of electric field lines depends on the type and arrangement of charges.

Isolated Positive Charge

For an isolated positive charge, electric field lines radiate outwards from the charge. This indicates that a positive test charge placed near it would be repelled outwards.

Isolated Negative Charge

For an isolated negative charge, electric field lines converge inwards towards the charge. This indicates that a positive test charge placed near it would be attracted inwards.

Two Like Charges Placed Near Each Other

When two like charges (both positive or both negative) are placed near each other, their electric field lines repel each other. The lines bend away from each other, creating a region of weaker field between the charges.

Two Unlike Charges Placed Near Each Other

When two unlike charges (one positive and one negative) are placed near each other, their electric field lines originate from the positive charge and terminate on the negative charge. The lines curve from one charge to the other, indicating attraction.

Electric Current

Electric current is the continuous flow of electric charges through a conductor.

In most common circuits, these charges are electrons moving through a metal wire. When a path is provided, these charges move from one point to another, creating an electric current. The unit of electric current is the Ampere (A).

Potential Difference

Potential difference between two points in an electric field is the work done per unit positive charge in moving the charge from one point to another against the electric field.

It is also known as voltage. A potential difference is necessary to make charges flow and create an electric current. It is like a “push” or “pressure” that drives the charges. The unit of potential difference is the Volt (V).

Resistance

Resistance is the opposition to the flow of electric charges (electric current) in a material.

When charges flow through a conductor, they encounter obstacles, such as atoms and other electrons, which resist their movement. This opposition is called resistance. Materials with high resistance are called insulators, while materials with low resistance are called conductors. The unit of resistance is the Ohm (Ω).

Ohm’s Law

Ohm’s Law states that the current flowing through a metallic conductor is directly proportional to the potential difference (voltage) across its ends, provided that the temperature and other physical conditions remain constant.

Mathematically, Ohm’s Law can be expressed as:

V = IR

Where:

1. V is the potential difference (voltage) in Volts (V).
2. I is the electric current in Amperes (A).
3. R is the resistance in Ohms (Ω).

Calculating Electric Currents and Voltages

Ohm’s Law can be rearranged to calculate any of the three quantities if the other two are known:

1. To find current (I): I = V / R
2. To find voltage (V): V = IR
3. To find resistance (R): R = V / I

Worked Example 1

A simple circuit has a resistor of 10 Ω connected to a 6 V battery. Calculate the current flowing through the circuit.

Solution:

1. Given:
   1. Resistance (R) = 10 Ω
   2. Voltage (V) = 6 V
2. Formula: I = V / R
3. Calculation: I = 6 V / 10 Ω = 0.6 A
4. The current flowing through the circuit is 0.6 A.

Worked Example 2

If a current of 2 A flows through a resistor of 5 Ω, what is the potential difference across the resistor?

Solution:

1. Given:
   1. Current (I) = 2 A
   2. Resistance (R) = 5 Ω
2. Formula: V = IR
3. Calculation: V = 2 A × 5 Ω = 10 V
4. The potential difference across the resistor is 10 V.

Electric Circuit

An electric circuit is a complete, closed path through which electric current can flow.

A simple electric circuit typically consists of a power source (like a battery), a load (like a lamp or resistor), connecting wires, and a switch (key) to open or close the circuit.

Setting Up Simple Electric Circuits

Setting up a simple electric circuit involves connecting components in a way that allows current to flow. The following steps outline how to set up a basic circuit to light a lamp:

The required materials are:

1. An electric cell (battery)
2. An electric lamp (bulb)
3. Connecting wires
4. A key (switch)

The procedure involves the following steps:

1. Connect one end of a connecting wire to the positive terminal of the electric cell.
2. Connect the other end of this wire to one terminal of the key (switch).
3. Connect another wire from the other terminal of the key to one terminal of the electric lamp.
4. Connect a third wire from the other terminal of the electric lamp to the negative terminal of the electric cell.
5. Ensure all connections are firm.
6. Close the key (switch). The lamp should light up, indicating current flow.
7. Open the key to break the circuit and turn off the lamp.

Measuring Electric Current and Voltage

Ammeters and voltmeters are used to measure electric current and potential difference (voltage) in a circuit.

Measuring Electric Current with an Ammeter

An ammeter is an instrument used to measure electric current. It must be connected in series with the component through which the current is to be measured.

The procedure involves the following steps:

1. Break the circuit at the point where the current is to be measured.
2. Connect the ammeter in the gap created, ensuring the positive terminal of the ammeter is connected towards the positive terminal of the power source, and the negative terminal towards the negative terminal.
3. Close the key. The ammeter will show the reading of the current in Amperes.

Safety Tip: Ammeters have very low resistance and must never be connected in parallel across a power source, as this can cause a short circuit and damage the ammeter or power source.

Measuring Voltage with a Voltmeter

A voltmeter is an instrument used to measure the potential difference (voltage) across two points in a circuit. It must be connected in parallel across the component whose voltage is to be measured.

The procedure involves the following steps:

1. Connect the voltmeter across the terminals of the component (e.g., a lamp or resistor) whose voltage is to be measured.
2. Ensure the positive terminal of the voltmeter is connected to the side of the component closer to the positive terminal of the power source, and the negative terminal to the side closer to the negative terminal.
3. Close the key. The voltmeter will show the reading of the voltage in Volts.

Safety Tip: Voltmeters have very high resistance and draw very little current, so they can be safely connected in parallel.

Electrical Conductors and Insulators

Materials can be classified based on how easily electric charges flow through them.

Electrical Conductors

Electrical conductors are materials that allow electric charges to flow through them easily.

They have many free electrons that can move readily when a potential difference is applied. Examples of good electrical conductors include:

1. Copper (used in electrical wires)
2. Aluminium (used in overhead power lines)
3. Silver (excellent conductor, but expensive)
4. Gold (used in connectors for corrosion resistance)
5. Iron

Electrical Insulators

Electrical insulators are materials that do not allow electric charges to flow through them easily.

They have very few free electrons, and therefore, they offer very high resistance to the flow of current. Insulators are used to prevent electric current from flowing where it is not wanted, for example, to protect people from electric shock. Examples of good electrical insulators include:

1. Rubber (used to cover electrical wires)
2. Plastic (used for casing of electrical appliances)
3. Glass
4. Wood (dry)
5. Ceramics
6. Air