IB Physics SL D.3.6 Force Between Parallel Wires

Practise calculating force per unit length between long parallel currents, identifying attraction or repulsion and connecting each wire's magnetic field to the force on the other.

Exam points

calculate F/L = μ₀I₁I₂/(2πr) using perpendicular separation between the wires
state that currents in the same direction attract and opposite directions repel
derive the interaction by finding one wire's field and applying BIL to the other

[Maximum number: 3]

An electrically heated pad is designed to keep a pet warm.

The pad is heated using a resistor that is placed inside the pad. The dimensions of the resistor are shown on the diagram. The resistor has a resistance of $4.2 \Omega$ and a total length of 1.25 m .

Figure for Question D.3.6—Force between parallel wires question 1 — IB Physics SL

When there is a current in the resistor, the temperature in the pad changes from a room temperature of $20^{\circ} \mathrm{C}$ to its operating temperature at $35^{\circ} \mathrm{C}$ .

(a)

When there is a current in the resistor, magnetic forces act between the resistor strips.

For the part of the resistor labelled RS,

[ 3 ]

(i)

outline the magnetic force acting on it due to the current in PQ .

[ 1 ]

(ii)

state and explain the net magnetic force acting on it due to the currents in PQ and TU .

[ 2 ]

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D.3.6—Force between parallel wires question 1

Question (a)

Question (i)

Question (ii)