Electric Current, Potential Difference and Resistance 8.3.2 Electrical Energy in the Home

Electric Charge The unit of electric charge is the Coulomb (abbreviated C). Ordinary matter is made up of atoms which have positively charged nuclei and negatively charged electrons surrounding them. Charge is quantized as a multiple of the electron or proton charge: The influence of charges is characterized in terms of the forces between them (Coulomb's law) and the electric field and voltage produced by them.

Coulombs Electrons are very small. You can have a million electrons flowing around a circuit without noticing, they won't light a lamp. In physics, we take a very, very large number of electrons as 1 unit of charge - called a Coulomb. Charge is given the symbol Q. 1 Coulomb = 6.2 x 1018 electrons. (This is 6.2 million million million electrons). Such a large number of electrons can do useful things(like light a lamp).

A charge in an electric field experiences an electric force. Work is done by the electric field if the electric force acting on the charge causes it to move from one point to another. These two points differ in their electric potential. The magnitude of the work done on the charge by the electric field is a measure of the difference in potential.

Lines of equipotential Try this Quick Quiz! Lines of equipotential - +

Let’s go to the video! Introductory Video – Voltage and Current

Electrical Potential Difference The electric potential difference (V) is the work done per unit charge as a charge is moved between two points in an electric field. V = W q Where W = work in Joules q = charge in Coulombs The volt (V) is the unit used to measure electric potential difference.

Your Turn! Do question 13 p117 of your text.

There are Volts and there are Volts Students frequently confuse the symbol V used to express the unit (volt) with the term used in the expression for electric potential difference. It should be emphasized that sometimes the same symbol can be used in different ways to express different things.

Voltage Voltage is electric potential energy per unit charge, measured in joules per coulomb ( = volts). Energy is measured in Joules. The power supply (the cell or battery) gives an amount of energy to each Coulomb going around an electric circuit. 1 Volt = 1 Joule per Coulomb. The word "per" means "divided by", so  Voltage = Energy ÷ Charge. This can be rearranged to give Energy = Voltage x Charge.   W (Energy) = V x q. A 6 Volt cell gives 6 Joules of energy to each Coulomb going around the circuit. Voltage (which is also called potential difference, or p.d.) is measured using a voltmeter.

Measuring Voltage Voltage (which is also called potential difference, or p.d.) is measured using a voltmeter. The voltmeter, shown as a circle with the letter V inside, is always connected in parallel with the component. (The voltmeter is said to be connected across the component, where the word "across" means "in parallel with"). The circuit on the left would show the voltage of the cell.

The circuit on the right shows the voltmeter connected across a lamp The circuit on the right shows the voltmeter connected across a lamp. This will tell you how many Joules of energy are being converted from electrical energy into light energy (+heat)for each Coulomb which passes through it. A reading of 6 Volts tells you that 6 Joules of energy are being converted for each Coulomb passing through the lamp. A reading of 10 Volts tells you that 10 Joules of energy are being converted for each Coulomb passing through the lamp.

Electromotive Force (EMF) When a voltage is generated by a battery, or by the magnetic force according to Faraday's Law (next year), this generated voltage has been traditionally called an "electromotive force" or emf. The emf represents energy per unit charge (voltage) which has been made available by the generating mechanism and is not a "force". The term emf is retained for historical reasons.

Voltage and Emf

Think of Coulombs as though they are buses, taking a large number of electrons (like passengers)from one side of the cell, through all the components in the circuit, and back to the other side of the cell. This is called direct current.

Electric Current Electric current is the rate of charge (Q or q) flow past a given point in an electric circuit under the influence of an electric field. It is measured in Coulombs/second which is named Amperes.

The "rate of flow of coulombs" (called "current") around an electric circuit is measured in amps. 1 Amp = 1 Coulomb per second. The word "per" means "divided by", so  current = charge ÷ time Current, which is given the symbol I, is shown using an ammeter. I = Q/t

Measuring Current The ammeter, shown as a circle with the letter A inside, is always connected in series with a component. If the ammeter reads 1 Amp, then the current (I) = 1 Amp at that point in the circuit. I = 1 Amp = 1 Coulomb per second. If the ammeter reads 6 Amps, then I = 6 Amps = 6 Coulombs per second.

I = Q/t Charge, as we saw earlier is given the symbol Q and is measured in Coulombs. So current = charge ÷ time. I = Q ÷ t This can be rearranged to give Q = I x t, or, charge = current x time

Conventional Electric Current Although it is electrons which are the mobile charge carriers which are responsible for electric current in conductors such as wires, it has long been the convention to take the direction of electric current as if it were the positive charges which are moving.

Let’s go to the Video Voltage - Current