Electric Current Electricity Lesson 1

Learning Objectives To establish what you already understand about electricity. To know what is meant by an electric current. To know how to calculate the charge flow in a circuit. To be able to define the coulomb.

The Plan... To check what you remember from GCSE. Build some circuits to check/change your ideas. Discuss what is meant by electric current. Practice some calculations.

Electricity Random Fact Electrons only move through a wire at a speed of about 1mm/sec.

Electric Current The electric current is the rate of flow of charge in a wire or component.  unit is the ampere (A) Due to the passage of charge particles referred to as charge carriers. In metals  the charge carriers are electrons. In liquids & gases  the charge carriers are ions.

The Coulomb The unit of charge is the coulomb (C), which is defined as the charge flow in one second when the current is one ampere. The symbol for charge is Q. The symbol for the unit, coulomb is C. The charge on an electron is e=1.6 × C

Charge Flow For a current I, the charge flow ΔQ in a time Δt is given by:- The symbol Δ is delta, a Greek capital letter Δ, meaning “change in”.

Current For a current I, the charge flow ΔQ in a time Δt is given by:-

Question If the charge on one electron is e=1.6 × C, how many electrons are needed to make up 1 C of charge?

Answer If the charge on one electron is e=1.6 × C, how many electrons are needed to make up 1 C of charge?

Possible Trap There are some important multipliers for current: 1 microamp (1 μA) = 1 × A 1 milliamp (1 mA) = 1 × A You must use current in amps, charge in coulombs and time in seconds for calculations. Watch out for this!

Worked Example What is the charge passing a point if a current of 10 pA flows for 1 year?

Learning Objectives To establish what you already understand about electricity. To know what is meant by an electric current. To know how to calculate the charge flow in a circuit. To be able to define the coulomb.

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Calculating the number of electrons Knowing that the charge on an electron is –1.6 ´ 10–19 C, you can calculate the number of electrons in a 'spoonful' of charge. A typical spoonful of negative charge is –2 nC. So the number of electrons is:

Discussion: Defining current, the coulomb Current is defined as rate of change of charge. This can be done graphically. Current is the gradient of a graph of charge transferred against time. I = dQ/dt. The idea of the gradient can be introduced by asking how the charge transferred by the shuttling ball increases with time - it will go up in a series of steps but, given a large number of transfers, these will approximate to a constant slope. The average current is equal to its gradient. The equation I = Q/t (familiar from pre-16 science lessons) is useful but stress that this refers to an average current I and care must be taken when I is changing. A current of one amp is equivalent to a flow of one coulomb per second. The coulomb defined as the charge passed by a current of 1 A in 1 s, i.e. 1 C = 1 A s.

Introductory questions on charge and current Convert 25 mA to A 2.Convert 0.50 A to mA 3.A torch bulb passes a current of 120 mA. (a)How many coulombs of charge flow through the lamp in 1 minute? (b)How many coulombs of charge flow through the lamp in 1 hour? (c)How many electrons leave the negative terminal of the cell each second? 4.A car battery is rated as 36 A h. In principle this means it could pass a current of 1 A for 36 h before it runs down. How much charge passes through the battery if it is completely run down? 5.An electron beam in a beam tube carries a current of 125  A. (a)What charge is delivered to the screen of the tube every second? (b)How many electrons hit the screen each second?

Circuit rules Current rules Current rules At any junction in a circuit the total current leaving the junction is equal to the total current entering the junction (Kirchhoff’s current Law) The current entering a component is the same as the current leaving the component (from KS 3 and 4) The current passing through 2 or more components in series is the same through each component. (from KS 3 and 4)

Kirchhoff’s current law The current entering any junction is equal to the current leaving that junction. i1 + i4 = i2 + i3

Conclusions The current is the charge per second : I = dQ/dt. At any junction in a circuit the total current leaving the junction is equal to the total current entering the junction (Kirchhoff’s current Law) The current entering a component is the same as the current leaving the component The current passing through 2 or more components in series is the same through each component.