electric circuit - resistance

Here’s a set of revision notes based on the electric circuits worksheet, focusing on key concepts and formulas that could come up in a Year 10 test:

### 1. Key Formulas:

- Ohm’s Law:

\( V = I \times R \)

Voltage (V) = Current (I) × Resistance (R)

- Power Formula:

\( P = I \times V \)

Power (P) = Current (I) × Voltage (V)

- Energy Transferred:

\( E = Q \times V \)

Energy (E) = Charge (Q) × Voltage (V)

### 2. Resistors:

- Resistors in Series:

- Total resistance = \( R_1 + R_2 + \dots \)

- The same current flows through each resistor.

- Voltage divides across resistors.

- Resistors in Parallel:

- Total resistance is less than the smallest resistor.

- The voltage is the same across each resistor.

- Current divides between the resistors.

### 3. Components:

- Voltmeter: Measures voltage across a component. It must be connected in parallel to the component.

- Ammeter: Measures current through a component. It must be connected in series in the circuit.

### 4. Light Dependent Resistor (LDR):

- Resistance decreases as light intensity increases.

- When the LDR is in a circuit:

- More light → lower resistance → higher current.

- Less light → higher resistance → lower current.

### 5. Thermistor:

- Resistance decreases as temperature increases.

- Used to control current based on temperature changes.

### 6. Direct Current (DC):

- A current that flows in one direction only (e.g., from a battery).

### 7. Important Relationships:

- Voltage across components in series: The sum of individual voltages equals the total voltage of the power supply.

- Voltage across components in parallel: Each component gets the full voltage of the power supply.

### 8. Practical Applications:

- Thermistor in Circuits:

Used in temperature sensing. As the temperature increases, the resistance of the thermistor decreases, allowing more current to flow.

- LDR in Circuits:

Used in light-sensitive circuits (like streetlights). In the dark, the LDR’s resistance increases, decreasing current flow, which can trigger a lamp to switch on.

### 9. Current and Resistance:

- Increasing the number of resistors in series increases the total resistance, which reduces the current.

- Decreasing resistance (e.g., with LDRs or thermistors) increases the current.

### 10. Example Calculations:

- Ohm's Law Example:

For a resistor of 19 Ω connected to a 1.6 V cell:

\( I = \frac{V}{R} = \frac{1.6}{19} \approx 0.084 \, A \)

- Power Example:

For a toaster with a power rating of 2.8 kW connected to 230 V mains:

\( I = \frac{P}{V} = \frac{2800}{230} \approx 12.17 \, A \)

### 11. Understanding Circuit Behavior:

- When light decreases in an LDR circuit, resistance increases, so the voltage across the LDR increases while the current decreases.

- When a thermistor cools down, resistance increases, causing the current to decrease.

### 12. Multiple Choice Concepts:

- Current: Defined as the rate of flow of charge.

- Voltage: Defined as the energy transferred per unit charge.

These notes cover the essential concepts, relationships, and calculations that will help you revise effectively for a Y10 test on electric circuits.