AQA A-level Physics Introduction to electricity
This page covers the following topics:
Current is the rate at which electrons or electric charge flow through a point in an electric circuit. Current is measured in amperes (A) and is calculated using the following formula: Current = Charge/time. The current in a circuit can be measured using an ammeter placed in series with the component through which the current is desired. The current at any point in a loop will be the same.
An open circuit is one in which there is a break such that no current can flow. A closed circuit is a complete one in which current can flow. A short circuit is one in which an incorrect connection or a damaged part will cause the current in the circuit to take a shorter route to the one intended. This may lead to a decrease in resistance in the circuit and thus an increase in current. This high current flow may damage the circuit, which is why short circuits are dangerous.
Charge is what an object possesses if it feels a force when placed in an electrical field. Electricity is negatively charged particles called electrons, which carry energy through circuits. Charge is measured in Coulombs, C, and it can be calculated using the following formula: Charge = Current × time.
Resistance is the opposition to the current flow in a circuit and is measured in Ohms (Ω). As current flows through a material, the electric charge will collide with the ions in the metal material it is flowing through, which makes it more difficult for it to flow and so resistance in the circuit exists. Resistance in a circuit is measured by measuring the current and potential difference in it and calculating the resistance based on these values. The resistance in a wire is directly proportional to the length of the wire. As the length of a wire increases, the electric current will collide with more ions during its route and thus there will be a greater resistance. Further, the resistance in a wire is inversely proportional to the thickness of the wire. As the thickness of the wire decreases, there is less space for charge to flow, thus creating a greater resistance to the current.
Potential difference, also known as voltage, is the amount of energy transferred by each unit of charge passing between two points in a circuit and is measured in volts (V). It can be calculated using the following formula: Work done = Charge × Voltage. As charge flows through a circuit, it does work, and thus potential difference can be described as the work done or the energy gained in a circuit per unit charge. The potential difference across a component of a circuit can be measured by placing a voltmeter in parallel with it.
Ohm's Law states that the current flowing through a component in a circuit is directly proportional to the voltage across it, given that it is at a constant temperature. This is also given as the formula: V = IR, where V is voltage, I is current and R is resistance.
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