# OCR - P3: Electric circuits

 P3.4 What determines the rate of energy transfer in a circuit? Background to the topic What you should be able to do: The energy transferred when electric charge flows through a component (or device), depends on the amount of charge that passes and the potential difference across the component. The power rating (in watts, W) of an electrical device is a measure of the rate at which an electrical power supply transfers energy to the device and/or its surroundings. The greater the potential difference, the faster the charges move through the circuit, and the more energy each charge transfers. The National Grid uses transformers to step down the current for power transmission. The power output from a transformer cannot be greater than the power input, therefore if the current increases, the potential difference must decrease. Transmitting power with a lower current through the cables results in less power being dissipated during transmission. 1. Describe the energy transfers that take place when a system is changed by work done when a current flows through a component 2. Explain, with reference to examples, how the power transfer in any circuit device is related to the energy transferred from the power supply to the device and its surroundings over a given time: power (W) = energy (J) ÷ time (s) 3. Recall and use the relationship between the potential difference across the component and the total charge to calculate the energy transferred in an electric circuit when a current flows through a component: energy transferred (work done) (J) = charge (C) × potential difference (V) 4. Recall and apply the relationships between power transferred in any circuit device, the potential difference across it, the current through it, and its resistance: power (W) = potential difference (V) × current (A) power (W) = (current (A))2 × resistance (Ω) 5. Use the idea of conservation of energy to show that when a transformer steps up the voltage, the output current must decrease and vice versa a) select and use the equation: potential difference across primary coil × current in primary coil = potential difference across secondary coil × current in secondary coil