Current and Potential Difference (GCSE Physics)
Current and Potential Difference
Potential Difference
Cells and Batteries
As we said above, cells or batteries are required to move charge around the circuit. They are like ‘electron pumps’. In a circuit you can have cells / batteries and components:
- Cells or batteries – The cells or batteries transfer energy to the charge as it passes in the circuit. The energy given to the charges as they pass is measured in volts (V).
- Components – in contrast, components take energy away from charge as it passes. For example, a buzzer uses energy to produce sound.
Voltage
- Voltage is the energy transferred per coulomb of charge. We can define voltage (potential difference) as the energy transferred per coulomb of charge that passes. It is measured between two points in a circuit. It can also be referred to as the electromotive force (EMF).
- Voltage means different things for cells/batteries and components. For cells or batteries, the voltage tells us the amount of energy given to the charges in the circuit. For components, the voltage tells us the amount of energy taken away from the charges in the circuit.
Voltmeters and Ammeters
Voltmeter
Voltages are measured using voltmeters.
Remember, a voltage for a component or cell is the energy transferred per coulomb. The voltmeter therefore measures the energy before and after the component to see the difference. So the voltmeter measures voltage (potential difference) across two points.
This diagram shows 1 coulomb of charge moving around a circuit.
- At the cell, the charge gets given 6J of energy, so the cell has a voltage of 6V (6J per coulomb of charge that passes). The voltmeter measures the difference in energy from before (0J) to after (6J) to work out the voltage.
- At the lamp, the charge loses 4J of energy, so the lamp has a voltage of 4V. The voltmeter measures the difference in energy from before (6J) to after (2J),
- At the motor, the charge loses 2J of energy, so the lamp has a voltage of 2V. The voltmeter measures the difference in energy from before (2J) to after (0J).
The voltage of the cell is always split amongst the components – none is left over.
We previously looked at how to measure current using an ammeter. This table compares the use of a voltmeter to an ammeter.
Calculating Potential Difference
We can calculate potential difference using the following equation.
Where:
- potential difference, V, in volts, V
- current, I, in amperes, A (amp is acceptable for ampere)
- resistance, R, in ohms, Ω
Question: A series circuit has a lamp, an ammeter, a 4Ω resistor and an 8V cell. Assuming the lamp and ammeter are resistance-free, what is the current reading on the ammeter?
1. Write out the equation.
In this instance, we need to rearrange the equation to find the current.
2. Substitute in the numbers.
In physics, current refers to the flow of electrical charge through a conductor. It is measured in units of Amperes (A).
Current and voltage are related through Ohm’s Law, which states that the current through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the resistance between them.
Potential difference, also known as voltage, is the measure of energy per unit of charge that is available to move electrical charge from one point to another. It is measured in units of Volts (V).
Current refers to the flow of electrical charge through a conductor, while potential difference refers to the energy available to move electrical charge from one point to another. In other words, current is the flow of charge, while potential difference is the energy that drives the flow of charge.
A conductor is a material that allows electrical charge to flow through it easily. Examples of conductors include metals such as copper and aluminum.
An insulator is a material that does not allow electrical charge to flow through it easily. Examples of insulators include rubber and plastic.
A circuit is a closed loop of conductors and other components that allow electrical charge to flow through it.
Resistance is the measure of a material’s ability to resist the flow of electrical charge. It is measured in units of Ohms (Ω).
The temperature of a material affects its resistance, with an increase in temperature leading to an increase in resistance. This relationship is known as the temperature coefficient of resistance.
An electric cell, also known as a battery, is a device that converts chemical energy into electrical energy. It consists of two electrodes, separated by an electrolyte, that produce a potential difference when connected in a circuit.
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