Circuits

Q = charge (coulombs/C)
I = current (amps/A)
t = time (s)

Current (I) = rate of flow of charge

- If an object becomes charged by storing excess electrons, the flow of the object can discharge it, producing a current.
- Electrons will flow from a spark. The spark allows excess electrons to flow between charged clouds and the Earth.
- Massive scale example: in a thunderstorm

Questions:

1. An electron has a charge of 1.6 × 10^-19. How many electrons would a cloud need to store for a IC charge to flow through?

2. a) A boy rubs his trainers against nylon carpet that stores a -0.4C charge. He then touches a metal door handle and experiences a shock lasting 0.05s. He calculates a current that flows during shock:

0.4 ÷ 0.5 = 0.08A

b) Why does he feel a shock?

The metal on the door handle is a conductor. The electrons built up flow as the boy onto the metal into the earth discharging by the metal.

3. A Van de Graaf generator is capable of storing 2 mC, which can be discharged in 0.1 s. What amount of current is required to achieve this?

Conventional Current

- Conventional current is the flow of a positive charge to a negative charge, which is the reverse of electron flow.
- Measured in amps (A).

Symbols

Conservation of Current

- A junction in a circuit doesn't use up the current.
- Whatever amount flows into a junction must flow out.

Examples

Current

Control Circuit

Current Rules

- Series: In the loop, the current is the same everywhere.
- Parallel: Across the loops, the current splits up.

Advantages - A single switch to control the current remains constant.

Voltage

Voltage (V): The energy transported by a charge.
E = QV
Energy = Charge Voltage
E = I t V

Energy = J, charge = C, and voltage = V (volts).
1 V = 1J/C

- Potential difference: Voltage across a compound (how much energy is transferred from one end to the other)
- Always place a voltmeter in parallel across components or a circuit.

Questions

1. A 9V cell can deliver 0.3C of charge; what amount of chemical energy does it store?

 9 × 0.3 = 2.7J

2. A computer contains a rechargeable 12V, 3A battery. How much energy does it supply if it is used for 2 hours?

E = I + V
E = 12 × 3 × (2 × 60 × 60) = 259200J

3. An 18J/s, 2mA USB can is attached to the laptop.
a) How much charge does the battery lose in 30 seconds? 30 × 18 = 540J
b) How much energy is converted from the battery in 30s? 0.002 × 30 = 0.060
c) What voltage does the battery deliver to the *fan* for it to work? 540/0.06 = 9000V

Parallel Circuit Advantages:
- One appliance breaks; all others remain powered.
- Each loop gets full voltage.
→ Bulbs are brighter.

Circuits Voltage

Control

Series

1)

2)

Parallel 3)

Summary

- In a series circuit, voltage splits between components.
- In a parallel circuit, the voltage remains constant across all components.

Circuit Q's Current + Voltage

a) What voltage across lamps is the switch open?
A: 6V; B: 6V; C: 0V
b) If the switch is closed and the lamp C is on, the voltage is 4V. What voltage will A and B have?
A: 8V B: = 4V (Hint: Follow the path of the circuit one loop at a time.)

Resistance

- As more collisions take place, the metal atoms vibrate more as they have more kinetic energy.
- Therefore, as the atoms gain more KE, the temperature increases.
- As the wire gets hotter, the number of collisions between the electrons and atoms increases.
- Therefore, the resistance increases.

More resistance in a) → more collisions

Ohm's Law

- The resistance of a component is measured in ohms .
- Voltage (V), current (I), and resistance (R) are linked in the equation:
V = IR

- Ohm's law only applies to the component that is kept at a fixed temp.
- In an ohmic conductor the current is directly proportional to the voltage, it has constant resistance.
- The resistance changes as the voltage and current changes.

Example Experiment

Method: Change voltage by adjusting power supply in increments. Record voltage and current readings in a table. Then draw a graph with voltage on the X axis and current on the Y axis. Add line of best fit.

Result: The filament lamp doesn't obey Ohm's law as the line doesn't pass through the origin and is curved; it's not directly proportional.
This could be attributed to the components having heated up. This is not true of all components. 
However, it increases the resistance as the temperature increases.

NOTE: Resistor circuit 1 pg 11/12 of booklet obeys Ohm's Law.

Questions

1. A 24V battery powers a lamp, and 1.5A flows. What's the resistance?

2. An LED has 20 mA flowing through a resistance of 150$\Omega$. What's the voltage?

Experiment Resistance of a Wire

Aim: To investigate the connection between the length of a piece of wire and its resistance.

Equipment

- 1m wire fixed to a metre rule.
- Variable voltage d.c. power supply (0-5V)
- dc ammeter and voltmeter
- 2 crocodile clips and 5 leads

Safety

- Don't allow the power supply.
- Don't allow wire to overheat. Switch off between readings.

Method

1. Adjust crocodile clips so they're 90 cm apart.
2. Turn on the power supply and adjust it so that the voltmeter reads 2V.
3. Enter V + I into a table.
4. Change the distance between the crocodile clips to 80 cm.
5. Don't do repeats—wire will overheat.
6. Change the length in 10-cm intervals, and record V and I each time in the table.
7. Plot a graph with resistance on the y-axis and length of wire on the x-axis to aid in addressing the experiment's goal.

Conclusion

- The length of the wire + the resistance of the wire are directly proportional.
- The current increased when the length of the wire decreased. Charge can flow faster through shorter distances.
- When using a wire with half the original cross-sectional area, the resistance doubles, leading to more collisions in a smaller CSA.
- The type of wire you use in this experiment must remain constant to prevent it from changing temperature and increasing resistance.

Non-Ohmic Conductors

An ohmic conductor, such as a resistor, operates at a constant temperature.

- In a non-ohmic conductor, resistance changes as voltage and current change.
- Filament lamps, diodes, LDRs, and thermistors are all non-ohmic conductors.
E.g., a filament lamp

Diodes

- Diodes only let current flow in one way → when voltage is positive. 
- The graph is not directly proportional → diodes aren't ohmic conductors.
- Threshold voltage: The minimum voltage required to cause current to flow.

- Until the threshold voltage, resistance is high. After the threshold, voltage resistance decreases.
- Diodes only work when they're forward biased (no current flows when reversed biased).
- Diodes require a specific voltage to turn on.

- LEDs don't heat up like filament lamps, making them more efficient.
- However, you cannot dim LEDs.

Non-ohmic Thermistor

The temperature alters the resistance of a thermistor.

Thermistors are semiconductors (not made of metal).

Conclusion

- As temperature increases, the resistance of the thermistor decreases.

Uses

- A thermistor can be used for temperature sensors—e.g., fire alarms, handling food (ovens), cold storage (bridges), digital thermometers, and monitoring heating.

Non-ohmic LDRs

They have less resistance in light, resulting in a higher current.

Uses

- Automatic streetlights
- Alarm clocks
- Light sensors

Conclusion

- As light intensity increases, resistance decreases.
- There's a maximum resistance. The resistance increases until the point where the light intensity decreases.