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Faculty of Science Technology and Engineering Department of Physics Senior Laboratory Current balance Objectives When a steady electric current flows perpendicularly across a uniform magnetic field it experiences a force. This experiment aims to investigate this effect, and to determine the direction of the force relative to the current and magnetic field. You will design and perform a series of experiments to show how the magnitude of the force depends upon the current and the length of the conductor that is in the field. Task You are provided with a current balance apparatus (Figure 1), power supply and a magnet. This current balance consists of five loops of conducting wire supported on a pivoted aluminium frame. Current may be made to flow in one or up to five of the loops at a time in either direction. If the end of the loop is situated in a perpendicular magnetic field, when the current is switched on, the magnetic force on the current will unbalance the apparatus. By moving the sliding weights to rebalance it, the magnitude of this magnetic force may be measured. A scale is etched on one arm of the balance, so that the distance moved by the slider can be measured. The circuitry of the balance cannot cope currents greater than 5 Amps, so please do not exceed this level of current. Figure 1: Schematic diagram of current balance apparatus and circuitry. Start by familiarising yourself with the apparatus. Use the two sliding weights to balance the apparatus, then apply a magnetic field to either end of the loop. Pass a current through just one of the conducting loops and observe the direction of the resulting magnetic force, relative to the direction of the current and the applied field. Change the magnitude and direction of the current, observe qualitatively the effect this has on the magnetic force. Having familiarised yourself with the apparatus, you should design and perform a series of quantitative experiments aiming to: (1) determine how the size of the magnetic force is dependant on the size of the current flowing in the conductor. (2) determine how the size of the force is dependant on the length of the conductor which is in the field. (3) measure the value (in Tesla) of the field of the magnet provided. For each of these, the balance should be set up with the magnet positioned at the end of the arm that has the distance scale, and orientated so that the magnetic force will be directed upwards when a current is passed through the conductor. The sliding weight on this arm should be positioned at the zero-mark. The weight on the opposite arm should be adjusted to balance the apparatus in the absence of a current. When a current is applied, you should re-balance the apparatus by moving the weight on the scaled arm outwards, while keeping the opposite weight fixed in position. The distance moved by the weight is directly proportional to the force applied by the magnetic field to the end of the balance. In your report, make sure you discuss why this is the case. Use the position of the sliding weight to quantify the magnetic force as a function of the current applied to the conductor, and of the number of conducting loops through which the current flows. For tasks (1) and (2) you can use the position of the sliding weight as a measure of the force. Look up the relationship that relates the force to the applied field, current and length of conductor in the field. Is this consistent with your data? To complete task (3) you need to determine the magnitude (in Newtons) of the magnetic force from the measurement of the position of the sliding weight. To do this, what other information do you need to know? When you have determined a value for the field, you can measure the field directly using the laboratory’s Gaussmeter for comparison.

## Faculty of Science Technology and Engineering Department of Physics Senior Laboratory Current balance Objectives When a steady electric current flows perpendicularly across a uniform magnetic field it experiences a force. This experiment aims to investigate this effect, and to determine the direction of the force relative to the current and magnetic field. You will design and perform a series of experiments to show how the magnitude of the force depends upon the current and the length of the conductor that is in the field. Task You are provided with a current balance apparatus (Figure 1), power supply and a magnet. This current balance consists of five loops of conducting wire supported on a pivoted aluminium frame. Current may be made to flow in one or up to five of the loops at a time in either direction. If the end of the loop is situated in a perpendicular magnetic field, when the current is switched on, the magnetic force on the current will unbalance the apparatus. By moving the sliding weights to rebalance it, the magnitude of this magnetic force may be measured. A scale is etched on one arm of the balance, so that the distance moved by the slider can be measured. The circuitry of the balance cannot cope currents greater than 5 Amps, so please do not exceed this level of current. Figure 1: Schematic diagram of current balance apparatus and circuitry. Start by familiarising yourself with the apparatus. Use the two sliding weights to balance the apparatus, then apply a magnetic field to either end of the loop. Pass a current through just one of the conducting loops and observe the direction of the resulting magnetic force, relative to the direction of the current and the applied field. Change the magnitude and direction of the current, observe qualitatively the effect this has on the magnetic force. Having familiarised yourself with the apparatus, you should design and perform a series of quantitative experiments aiming to: (1) determine how the size of the magnetic force is dependant on the size of the current flowing in the conductor. (2) determine how the size of the force is dependant on the length of the conductor which is in the field. (3) measure the value (in Tesla) of the field of the magnet provided. For each of these, the balance should be set up with the magnet positioned at the end of the arm that has the distance scale, and orientated so that the magnetic force will be directed upwards when a current is passed through the conductor. The sliding weight on this arm should be positioned at the zero-mark. The weight on the opposite arm should be adjusted to balance the apparatus in the absence of a current. When a current is applied, you should re-balance the apparatus by moving the weight on the scaled arm outwards, while keeping the opposite weight fixed in position. The distance moved by the weight is directly proportional to the force applied by the magnetic field to the end of the balance. In your report, make sure you discuss why this is the case. Use the position of the sliding weight to quantify the magnetic force as a function of the current applied to the conductor, and of the number of conducting loops through which the current flows. For tasks (1) and (2) you can use the position of the sliding weight as a measure of the force. Look up the relationship that relates the force to the applied field, current and length of conductor in the field. Is this consistent with your data? To complete task (3) you need to determine the magnitude (in Newtons) of the magnetic force from the measurement of the position of the sliding weight. To do this, what other information do you need to know? When you have determined a value for the field, you can measure the field directly using the laboratory’s Gaussmeter for comparison.

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