Introduction to Autumn Term Miss O’Donnell

What questions are we looking at today? What is an electric current? How can we calculate the charge flow in a circuit? What are charge carriers?

Current flow conditions What conditions must be satisfied to make a current pass round a circuit? Circuit must be complete There must be a source of potential difference What is Ohm’s Law? How does this relate to electric current? How is the current carried? What carries the current?

Show current through a conductor using VPLab9 Explaining Electricity in Electricity DC The convention is to show the direction of current in a circuit is from positive to negative Why is this?

Units and Equations Unit of current is: Unit of charge is: Ampere (A) Unit of charge is: Coulomb (C): this is equal to the charge flow in one second when the current is 1 ampere Symbol for charge is Q For a current I, the charge flow ∆Q = Ix∆t Why do we use the ∆ sign?

Current and charge analogies Lesson 2 Current and charge analogies

Actual Analogy Electrons flowing in a circuit. Balls flowing through a hollow pipe. The cell pushes the electrons in the wire. An engine pushes the balls in the pipe. When the electrons go through the bulb or a resistor, they cause the wires to heat up. When the balls have to be squeezed through a thin section of the pipe, they will make the pipe hot. When the cell runs out the current stops flowing. When the engine runs out of fuel it will stop pushing the balls. The flow stops. Measuring the current is counting the number of Coulombs passing a certain point in the circuit each second. Watching the number of balls passing a certain point of the pipe each second The current is the same everywhere in the circuit. The no. of balls entering the narrow strip is equal to no. of balls exiting it. The speed of the balls is constant throughout.

Actual Analogy Electrons flowing in a circuit. Balls flowing through a hollow pipe. The cell pushes the electrons in the wire. An engine pushes the balls in the pipe. When the electrons go through the bulb or a resistor, they cause the wires to heat up. When the balls have to be squeezed through a thin section of the pipe, they will make the pipe hot. When the cell runs out the current stops flowing. When the engine runs out of fuel it will stop pushing the balls. The flow stops. Measuring the current is counting the number of Coulombs passing a certain point in the circuit each second. Watching the number of balls passing a certain point of the pipe each second The current is the same everywhere in the circuit. The no. of balls entering the narrow strip is equal to no. of balls exiting it. The speed of the balls is constant throughout.

Actual Analogy Electrons flowing in a circuit. Balls flowing through a hollow pipe. The cell pushes the electrons in the wire. An engine pushes the balls in the pipe. When the electrons go through the bulb or a resistor, they cause the wires to heat up. When the balls have to be squeezed through a thin section of the pipe, they will make the pipe hot. When the cell runs out the current stops flowing. When the engine runs out of fuel it will stop pushing the balls. The flow stops. Measuring the current is counting the number of Coulombs passing a certain point in the circuit each second. Watching the number of balls passing a certain point of the pipe each second The current is the same everywhere in the circuit. The no. of balls entering the narrow strip is equal to no. of balls exiting it. The speed of the balls is constant throughout.

Actual Analogy Electrons flowing in a circuit. Balls flowing through a hollow pipe. The cell pushes the electrons in the wire. An engine pushes the balls in the pipe. When the electrons go through the bulb or a resistor, they cause the wires to heat up. When the balls have to be squeezed through a thin section of the pipe, they will make the pipe hot. When the cell runs out the current stops flowing. When the engine runs out of fuel it will stop pushing the balls. The flow stops. Measuring the current is counting the number of Coulombs passing a certain point in the circuit each second. Watching the number of balls passing a certain point of the pipe each second The current is the same everywhere in the circuit. The no. of balls entering the narrow strip is equal to no. of balls exiting it. The speed of the balls is constant throughout.

Actual Analogy Electrons flowing in a circuit. Balls flowing through a hollow pipe. The cell pushes the electrons in the wire. An engine pushes the balls in the pipe. When the electrons go through the bulb or a resistor, they cause the wires to heat up. When the balls have to be squeezed through a thin section of the pipe, they will make the pipe hot. When the cell runs out the current stops flowing. When the engine runs out of fuel it will stop pushing the balls. The flow stops. Measuring the current is counting the number of Coulombs passing a certain point in the circuit each second. Watching the number of balls passing a certain point of the pipe each second The current is the same everywhere in the circuit. The no. of balls entering the narrow strip is equal to no. of balls exiting it. The speed of the balls is constant throughout.

Actual Analogy Electrons flowing in a circuit. Balls flowing through a hollow pipe. The cell pushes the electrons in the wire. An engine pushes the balls in the pipe. When the electrons go through the bulb or a resistor, they cause the wires to heat up. When the balls have to be squeezed through a thin section of the pipe, they will make the pipe hot. When the cell runs out the current stops flowing. When the engine runs out of fuel it will stop pushing the balls. The flow stops. Measuring the current is counting the number of Coulombs passing a certain point in the circuit each second. Watching the number of balls passing a certain point of the pipe each second The current is the same everywhere in the circuit. The no. of balls entering the narrow strip is equal to no. of balls exiting it. The speed of the balls is constant throughout.

Adding another cell will increase the current. Adding another motor will speed up the balls in the pipe. Adding a resistor will cause the current in the whole circuit to be reduced. Current is still the same everywhere. The current in P is equal to the current in Q. Adding an obstruction in the pipe will make the balls go slower. The rate of flow of the balls at is the same as the rate at Q.

Adding another cell will increase the current. Adding another motor will speed up the balls in the pipe. Adding a resistor will cause the current in the whole circuit to be reduced. Current is still the same everywhere. The current in P is equal to the current in Q. Adding an obstruction in the pipe will make the balls go slower. The rate of flow of the balls at is the same as the rate at Q. P Q

The sum of the currents entering a point is equal to the sum of the currents leaving that point. The number of balls entering any junction is the same as the number of balls exiting that junction.

Rearrange the formula and answer the questions The charge flow for a current of 1A in 10 seconds The charge flow for a current of 3A in 10 seconds The charge flow for a current of 5A in 200 seconds Calculate the average current in a wire through which a charge of 15C passes in 5s Calculate the average current in a wire through which a charge of 15C passes in 100s ANSWERS: 10C 30C 1000C 3A 0.15A

What is the charge on an electron? Each electron has a charge of 1.6 x 10-19C That means that a current of 1A along a wire is due to 6.25 x 1018 electrons passing along the wire each second Question: Calculate the number of electrons passing a point in the wire in 10 mins when the current is: 1.0μA 5.0A Answer: 3.8 x 1015 1.9 x 1022

Objectives Can we answer these questions? A steady current of 0.25A passes through a torch bulb for 6 minutes. Calculate the charge which flows through the bulb in this time. In an electron beam experiment, the beam current is 1.2mA. Calculate a) the charge flowing along the beam each minute b) the number of electrons that pass along the beam each minute ΔQ = 0.25 × 6 × 60 = 90 C 72mC, 4.5x10 to the 17

Quiz

Current and charge quiz 1. Calculate the charge passing through a lamp in three minutes when a steady current of 0.4 A is flowing. I = Q t Q = I t Q = 0.4 x 3 x 60 = 72 Coulomb

Current and charge quiz 2. Calculate the number of electrons flowing through a resistor when a current of 2.3A flows for 5 minutes I = Q t Q = I x t Q = 2.3 x 5 x 60 = 690 Coulomb no. of electrons = 690 /1.6x10-19 = 4.31x1021

Current and charge quiz 3. What is the current in a circuit if 2.5x1020 electrons pass a given point every 8 seconds Charge (C) = no. of electrons x charge of one electrons Charge (C) = 2.5x1020 x 1.6x10-19 = 40 Coulombs I = Q t Current = 40/8 = 5 Amps

Current and charge quiz 4. How long does it take for a current of 0.3A to supply a charge of 48C? I = Q t t = Q/I t = 48/0.3 = 160 seconds

Current and charge quiz 5. How many electrons pass a point when a current of 0.4A flows for 900 seconds? I = Q t Q = I x t = 0.4 x 900 = 360 Coulomb no. of electrons = total charge / charge of one electron no. of electrons = 360 / 1.6 x 10-19 = 2.25 x 1021

Current and charge quiz 6. A torch bulb passes a current of 120 mA. How many coulombs of charge flow through the lamp in 1 minute? Q = I x t = 120x10-3 x 60 = 7.2 Coulomb

Current and charge quiz 7. 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? Q = I x t = 1 x 36 x 60 x 60 = 129600 Coulomb

8. An electron beam in a beam tube carries a current of 125 A. Current and charge quiz 8. 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? (a) Q = I x t = 125 x 10-6 x 1 = 125 x 10-6 Coulomb (b) no. of electrons = 125 x 10-6 / 1.6x10-19 = 7.8 x 1014

summary Charge is a s a fundamental property of some types of particles (e.g. protons and electrons). There is a law of force: like charges repel, unlike charges attract. These are the forces that push charges around electric circuits. An electron has a charge of -1.6x10-19 Coulombs A proton has a charge of + 1.6x10-19 Coulombs Charge carriers are any charged particles that can move. e.g. electrons, protons or ions. When charge carriers move, they transfer energy from the source e.g. the cell to the component e.g. the bulb. One Coulomb is the charge of 6.2x1018 electrons Current is the rate of flow of charge. One Amp flows when one Coulomb of charge passes a certain point in a circuit in one second.

4 – Electric current Current and charge What is an electric current? How can we calculate the charge flow in a circuit? What are charge carriers? 3.1.3 Electrical conduction More about charge carriers How science works - Rechargeable batteries How science works – Physics and the human genome project 46- 47 Key diagram – Convention for current