Your hospital is installing a new MRI scanner using a 3.5 -H superconducting solenoid carrying \(1.8 \mathrm{kA} .\) Copper is embedded in the coils to carry the current in the event of a quench (see Example 27.9)\(.\) As safety officer, you're to specify (a) the maximum resistance that will limit power dissipation to \(100 \mathrm{kW}\) immediately after a loss of superconductivity and (b) the time it will take the power to drop to \(50 \mathrm{kW}\). What specs do you give?

Power dissipation in electrical circuits refers to the process by which an electric device converts electrical energy into other forms of energy, usually heat, which is not useful for the device's intended function. In the context of a superconducting solenoid, such as one used in an MRI scanner, power dissipation becomes critical when the system enters a 'quench'—a phase transition from the superconducting to the resistive state, which leads to the generation of heat.

During a quench, it is crucial to limit the power dissipation to prevent damage to the system. This is done by incorporating materials like copper that can safely carry the current after the loss of superconductivity. To calculate the maximum resistance that will limit power dissipation to a specific value, the formula \( P = I^2R \) is used. Where \( P \) is the power dissipated in watts (W), \( I \) is the current in amperes (A), and \( R \) is the resistance in ohms (Ω). It's essential for safety officers to calculate these parameters accurately to ensure the MRI scanner operates within safe limits.

Electrical resistance is a fundamental concept that quantifies how strongly a material opposes the flow of electric current. It is a critical factor in determining the power dissipation of an electrical circuit. When a superconducting solenoid transitions from a superconducting to a normal resistive state, the resistance suddenly becomes non-zero, and it starts dissipating power.

To safeguard the system, it is necessary to specify the maximum resistance that can exist without exceeding safety limits of power dissipation. For the given MRI application, using the rearrangement \( R = P/I^2 \) of the power dissipation formula, we input the desired power limit and the known current to find the appropriate resistance value. This process ensures that even in the case of a quench, the electric currents do not lead to excessive thermal energy release, which could damage the scanner or pose a safety risk.

Exponential decay is a mathematical concept that describes the process of reducing an amount by a consistent percentage rate over a period of time. In physics and engineering, this concept often applies to the decrease in power levels over time in systems like the solenoid in an MRI machine after a quench occurs.

To model this situation, the formula \( P = P_0 e^{-t/\tau} \) is used, where \( P_0 \) is the initial power, \( P \) is the final power, \( t \) is time, and \( \tau \) is the time constant, a parameter indicating how quickly the power decreases. For the superconducting solenoid, knowing the initial inductance and the resistance allows us to calculate \( \tau \) as \( L/R \), and from there, we can predict when the power will have halved. Understanding the exponential decay of power is indispensable for ensuring that the solenoid's energy dissipation returns to safe levels within an acceptable timeframe after a quench.