Introduction to electricity for OCR GCSE Physics
This page covers the following topics:
1. Circuit symbols and diagrams
3. Charge flow
5. Potential difference
6. Ohm's law
The given circuit symbols can be used to draw basic circuit diagrams.
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.
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.
A circuit is set up such that 2.6 A of current flow through it. How many coulombs of charge flow through a point in the circuit in 2 minutes?
2 minutes = 120 seconds. Using the formula for charge, charge = 2.6 A × 120 s = 312 C.
State the equipment present in the given circuit.
The equipment present is a cell, a resistor and a lamp.
The current through a lamp in a circuit is 3.2 A. Calculate how much charge flows through the lamp in 1.5 minutes.
1.5 minutes = 90 seconds. Using the formula for current, charge = 3.2 A × 90 s = 288 C.
Which ammeter position will have a greater value for the given circuit?
The current at any point in a loop will be the same, therefore both ammeters will show the same value.
What is the condition required for Ohm's Law to hold?
The temperature across the component in the circuit must be constant.
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