physics notes

Introduction


Recent decades have seen a huge expansion in consumer electronics. While the functions of new devices differ widely, their power still largely comes from either batteries or mains power supplies.

In the simple circuits met so far, we have pictured power supplies as being able to provide as much current as the components require. In reality, this is not the case.

The

energy

A system has energy when it has the capacity to do work. The scientific unit of energy is the joule.energy provided by any

power

The power of system is a measurement of the rate at which energy is transferred from one form to another. The scientific unit of power is the watt.power supply, such as a battery, to each

coulomb

The coulomb is the unit of charge. One coulomb is approximately equivalent to the charge carried by 6.25 × 10¹⁸ electrons.coulomb of charge leaving a battery or

power

The power of system is a measurement of the rate at which energy is transferred from one form to another. The scientific unit of power is the watt.power supply is called the

electromotive force

The electromotive force (often abbreviated to EMF) of a battery is a measurement of its capacity to provide energy to the charges.electromotive force or

EMF

EMF is the common abbreviation for the electromotive force – a measurement of a battery's capacity to provide energy to the charges.EMF. The electromotive force is measured in units of joules per

coulomb

The coulomb is the unit of charge. One coulomb is approximately equivalent to the charge carried by 6.25 × 10¹⁸ electrons.coulomb (JC⁻¹) or volts (V).


Click on the figure below to interact with the model.




Turn on the circuit in Fig.1 by clicking on the switch. Then connect the voltmeters across the buzzer and lamp by dragging them into the spaces in the circuit. Note the values for the p.d.s across the two components.



In Fig.1, the buzzer and the lamp were attached to the terminals of the batteries. They could just as easily have been connected to the output terminals of a power supply. When any set of components is connected across the terminals of a battery or a power supply like this, they are collectively called the external circuit. This is to distinguish such components from the internal circuit within the power supply. The

resistance

The opposition to the flow of current provided by a circuit is called resistance. Resistance is measured in units called Ohms.resistance of the

external circuit

All the components connected across the terminals of a battery or power supply are collectively known as the external circuit.external circuit is often referred to as the load resistance.


We can explain the 'lost volts' in Fig.2 by the fact that the LED and the lamp are not the only sources of resistance in the circuit. In fact, the battery also has a small internal resistance.

Any battery or power supply with an

internal resistance

All batteries or power supplies have internal resistance. This resistance has the effect of reducing the output p.d. as the current supplied increases.internal resistance can be pictured as shown in Fig.3. Here, the internal resistance of 2 Ω is shown as a resistor in series with the power source.




Internal resistance is a feature of all practical power supplies. Different designs of battery will have different internal resistances, and the manufacturers will often take steps to minimize the internal resistance.


The 12 V output from a car battery will have the same output

voltage

The voltage across a component is the electrical energy transferred by 1 coulomb of charge passing through the component.voltage as a 12 V domestic battery. However, the car battery needs to have a much lower internal resistance and the design needed to achieve this results in the car battery being larger and heavier.

The voltmeter in the Fig.4 below shows the 'lost volts', even though in practice this is a quantity that could not be measured experimentally.


Click on the figure below to interact with the model.




Set the variable resistor in the circuit of Fig.4 to 10 Ω. Close the switch and note the values of the p.d.s across the internal resistance and the load resistor.