DC Circuits Circuits can be very simple……

Or complex …………

Key idea: Electric Current. Electric current is a flow of electrons, (usually in a metal wire.)

- + Electric Field

The electrons are in an electric field, so they experience a force that pushes them along the wire. - + Electric Field Force

For a current to flow you need two things: 1) A source of electrical energy

For a current to flow you need two things: 1) A source of electrical energy

For a current to flow you need two things: 1) A source of electrical energy

2) …… AND a closed circuit

The drift velocity of electron in a wire is a few mm per sec. Why does the lamp turn on as soon as the switch is closed?

Electric Current The size of the current flowing is the amount of charge passing a point in one second.

In a torch, this is about six million million million electrons in one second. This is called One Amp of current Remember six million million million electrons is called One Coulomb of charge.

So One Amp is One Coulomb flowing every second

Key idea: Voltage: This is the amount of energy gained by each coulomb of charge in a battery (or other source) Or The amount of energy lost by each coulomb of charge in a lamp (or other user)

What happens when the voltage is increased? Electrons have more energy and they travel faster What happens when the resistance is increased? Electrons have same energy but they travel slower

Ohms Law

What does the graph show? Which resistor has highest resistance? V I

This is an ohmic resistor. The resistance is constant. V I

Non Ohmic Resistors Some things don’t have constant resistance

current voltage R=….

nb, the resistance is NOT the slope of the graph in this case. Because the resistance increases with temperature, increases as the current increases

Diode Diode is forward biased, resistance is LOW I

Diode Diode is reverse biased, resistance is HIGH

current voltage

Series and Parallel Circuits Current in series circuit is the same at any point. Voltage gained in battery = Voltage lost in lamp 2 Amps 6 Volts

Series Circuit 6V0V 3V Current is now only 1 Amp. Resistance is double) Voltage gained in battery = Voltage lost in both lamps 1 Amp Current is the same at any point. (not same as before) 3V

Current through each lamp is 2 Amp Voltage gained in a loop = Voltage lost in the loop 2Amp Current from battery splits, and is shared by the lamps 4Amp Voltage gained in battery = Voltage lost in each lamp

examples

Find the size of the current (I) 3 A I

Find the size of the voltage (V) V 4 V 3 V

7 V

1 A

Find the size of the resistance 6Ω6Ω15Ω

21Ω

What happens to the total resistance if another resistor is added in series 6Ω6Ω15Ω 6Ω6Ω 6Ω6Ω

increases

Find the size of the current (I) 10 V V 4 V 6Ω6Ω I

1A

Find the size of the current (I) I 3A 6A

9A

Find the size of the current (I) I 2A 6A

4A

Find the size of the voltage (V) 4 V V

Find the size of the resistance 6Ω6Ω 3Ω3Ω

2Ω

What happens to the total resistance if another resistor is added in parallel

decreases

Find the size of the current (I) 12 V 6Ω6Ω I

2A

Find the size of the current (I) 6Ω6Ω 3A 12Ω I

4.5A

Find the size of the total resistance 6Ω6Ω 12Ω 10Ω

14Ω

Find the size of the voltage across R 6Ω6Ω 12Ω R 4 V 5 V

1 V

Find the size of the voltage across R 6Ω6Ω R 7 V 5 V

2 V

Find the size of the voltage across the 6Ω 6Ω6Ω 3A 12Ω 10Ω

18 V

Find the size of the voltage across the 10Ω 6Ω6Ω 3A 12Ω 10Ω

45 V

Calculate the current in each resistor Calculate the unknown resistance 4 V 16 Ω 8 Ω 12 V

0.5A 1A R = 4/1.5

Now for a couple of longer ones…………………

Calculate the current in each resistor 50 Ω 100 Ω 300 Ω 12 V 200 Ω

Calculate the resistance of the parallel bit Calculate the total resistance. Calculate the total current (same as I 50 ) Calculate the currents in the parallel bit 50 Ω 100 Ω 300 Ω 12 V 200 Ω SOLUTION

Calculate the power output of the 200Ω resistor with the switch closed Explain what happens to the current in the 150 Ω resistor if the switch is opened 150 Ω 100 Ω 200 Ω 240 V

150 Ω 100 Ω 200 Ω 240 V Calculate the resistance of the parallel bit Calculate the total resistance. Calculate the total current (same as I 150 ) Calculate the currents in the parallel bit Use P = I 2 R SOLUTION

Explain what happens to the current in the 150 Ω resistor if the switch is opened A parallel resistor is removed. Total resistance increases Total current decreases (I= V/R battery voltage unchanged) Current through 150 Ω resistor decreases 150 Ω 100 Ω 200 Ω 240 V SOLUTION