Electric Fields
Electric field definition: An electric field is an invisible area or region around a charged object where other charged particles can feel a push or pull. This push or pull is called an electric force, and it happens because of the presence of other electric charges nearby.
Field as force field: You can think of an electric field like a “force field” in superhero movies—it surrounds the charged object and can affect other charges that enter this space.
Field line representation: Scientists draw special lines, called electric field lines, to show what this electric field looks like. These lines show the direction a small, positive test charge would move if placed in the field.
Lines start and end: These field lines always start at positive charges and end at negative charges. That means the lines go outward from positive charges and inward toward negative charges.
Direction of force: The arrows on these lines show the direction of the force that a positive charge would feel if it were placed in the field.
Field strength and line spacing: When the field lines are drawn very close together, it means the electric field is strong in that area. If the lines are far apart, the field is weaker.
Field direction rule: The direction of the electric field is always the same as the direction a positive charge would move if placed there.
Field from positive charge: If there is just one positive charge, the field lines go straight out in all directions like rays from the sun.
Field to negative charge: If there is just one negative charge, the field lines point inward toward it from all directions.
Dipole field lines: When a positive and a negative charge are near each other (like in a dipole), the field lines curve from the positive charge to the negative charge.
Like charges repulsion: If there are two similar charges, such as two positive charges, the field lines bend away from each other because like charges repel.
Parallel plates field: If you have two flat plates, one positive and one negative, placed parallel to each other, the electric field between them is shown as straight, evenly spaced, parallel lines. This means the field is strong and uniform.
Electric Field Strength (E)
Definition of E: Electric field strength, which we write as E, tells us how powerful or strong the electric field is at a specific point in space. It shows how much force a positive charge would feel if placed at that point. To find this strength, we divide the force (F) acting on the positive charge by the amount of charge (Q). So, E = F/Q.
Force per charge formula: The formula for calculating electric field strength is E = F/Q. This means that the electric field strength is the amount of force experienced by a charge divided by the size of that charge. The unit used to measure this is newton per coulomb (N C⁻¹). It tells us how many newtons of force act on each coulomb of charge.
Voltage per distance formula: In some situations—like between two parallel metal plates—the electric field is the same strength everywhere. This is called a uniform electric field. In that case, we can also calculate electric field strength using the formula E = V/d, where V is the voltage or potential difference between the plates, and d is the distance between them. The unit here is volt per meter (V m⁻¹).
Equation relationship: The formula V = Ed comes from linking several important physics equations. First, the energy transferred to a charge is found using W = QV. Second, work done by a force is W = Fd. And from the electric field formula, E = F/Q. When we connect all these equations, we get V = Ed, showing how voltage, field strength, and distance are related.
Effects of Electric Fields on Charges
Force on charged object: If you place something that has an electric charge inside an electric field, it will experience a push or pull (a force). If the object has a positive charge, the force will go in the same direction as the electric field. But if the object has a negative charge, the force will go in the opposite direction.
Motion under force: If the charged object is able to move, it won’t just sit still. The force from the electric field will make it accelerate, which means it starts moving faster and faster in the direction of the force.
Polarisation of neutral objects: Even things that have no overall charge (called neutral objects) can be affected by an electric field. Inside the object, the positive and negative parts shift just a little. One side of the object becomes slightly more positive, and the other side becomes slightly more negative. This is called polarisation, and it can cause the object to be attracted to charged things.
Electric Current
Current definition: Electric current is the rate at which electric charge flows through a material, like a wire. You can think of it like how water flows through a pipe, except this time it’s electric charges (not water) that are moving.
Charge flow basis: The flow of current is made by tiny charged particles. In metals, the particles that move and carry the charge are called electrons.
Current formula: The formula used to calculate current is I = Q/t, where I is the current, Q is the total amount of charge, and t is the time it takes. This means the current tells us how much charge passes through a point in the circuit every second. We can also rearrange the formula as Q = It if we need to find the total charge.
Current unit: The unit used to measure electric current is the Ampere (A). If one coulomb of charge flows through the wire every second, the current is one ampere. So, 1 A = 1 C/s.
Conventional current direction: In diagrams and explanations, we usually say that electric current flows from the positive terminal of a battery to the negative terminal. This is called the conventional direction of current.
Actual electron flow: But in reality, the electrons, which are the actual charge carriers in a wire, move in the opposite direction—from the negative terminal to the positive terminal.
Charge carriers: The particles that carry electric charge are called electrons. Each electron has a very tiny amount of negative charge, about 1.6 × 10⁻¹⁹ coulombs.
Total charge equation: If you want to find out how much total charge is flowing, and you know how many electrons are involved, you can use the formula Q = ne, where n is the number of electrons and e is the charge on one electron.
Potential Difference
Voltage definition: Potential difference, which we also call voltage, is a measure of how much energy is needed to move one unit of charge from one point to another inside an electric field. It shows how much “push” each charge gets.
Voltage formula: The formula to calculate voltage is V = W/Q, where V is the voltage, W is the energy used (or work done), and Q is the charge. It can also be rearranged to W = QV if we want to find the energy used.
Voltage unit: Voltage is measured in volts (V). One volt means that one joule of energy is needed to move one coulomb of charge. So, 1 V = 1 J/C.
Measuring voltage: To measure voltage in a circuit, we use an instrument called a voltmeter. This device must be connected in parallel with the part of the circuit we want to measure, not in series.
Energy per charge: Voltage tells us how much energy each unit of charge is given. If the voltage is higher, then each charge has more energy to move through the circuit and do things like power a bulb or motor.
Key Relationships
Interconnection of concepts: All these ideas—electric fields, current, and voltage—are closely connected. An electric field pushes charges and makes them move. That movement of charges is called electric current, and it needs energy to happen. The amount of energy each charge gets is described by voltage.