Electricity and Magnetic Effects of Electric Current
Electric Current is the flow of electric charge through a conductor. Electric current (I) is defined as the rate of flow of charge (Q) through a point in a circuit per unit time (t). I = Q / t.
- SI unit of Electric current (I) is Ampere (A) One ampere equals one Coulomb per second.
Potential Difference (V) between two points is the work done (W) in moving a unit charge (Q) between those points: V = W / Q.
- SI unit of potential difference (V) is volt (V). One volt equals one joule per coulomb.
- The instrument used to measure potential difference is a Voltmeter, which is always connected in parallel.
Ohm’s Law establishes the relationship between the Potential Difference (V) across a conductor and the Current (I) flowing through it. it’s observed that the ratio V/I remains constant, which implies a straight-line graph. This constant ratio is called Resistance (R). The mathematical forms of Ohm’s Law: V ∝ R (V = IR)
- SI unit of resistance is ohm (Ω), defined as 1 volt per 1 ampere. A device called a Rheostat is used to vary resistance and hence control the current.
Factors on Which Resistance Depends
- Longer wires have more resistance. (length)
- Thicker wires have less resistance. (cross-sectional area)
- Combining both R = ρ (L/A). Here, ρ is resistivity, a material-specific constant. Its unit is Ω·m.
Materials are classified as Resistivity:
- Conductors: Silver, Copper, Aluminium, etc. (very low resistivity)
- Alloys: Nichrome, Constantan, etc. (higher resistivity but stable at high temperature)
- Insulators: Glass, Ebonite, etc. (very high resistivity)
Resistors in Series and Parallel
In series, resistors are connected end-to-end and the same current flows through each. the potential difference across a series of three resistors using a voltmeter and confirming that the total voltage (V) across the combination is equal to the sum of the voltages across individual resistors: V = V₁ + V₂ + V₃, This is followed by the application of Ohm’s Law for resistors in series, showing that the total resistance (Rₛ) in a series is: Rₛ = R₁ + R₂ + R₃
In parallel, resistors are connected across the same two points, offering multiple paths for current and potential difference across each resistor is the same.
parallel circuits and current distribution across resistors. A key observation is that the total current II in a parallel circuit is the sum of the individual branch currents: I = I1 + I2 + I3
Using Ohm’s Law I=V/R, the concept of equivalent resistance in parallel circuits is derived as: 1/Rp = 1/R1 + 1/ R2 + 1/ R3
Difference between series and parallel circuits is highlighted. Series circuits carry the same current throughout but are impractical when devices need different currents. A break in one component stops the entire circuit. In contrast, parallel circuits are more reliable for domestic usage as the current divides and each component functions independently.
Heating effect of electric current, When current flows through a resistor, energy is used and converted into heat. This is utilized in devices like heaters and irons. The heat produced is given by: H = V I t
Electric power is measured in watts, and commercial consumption is measured in kilowatt-hours (kWh).
When electric current flows through a conductor, it generates a magnetic field. Magnetic field lines are imaginary lines; they emerge from the north pole and merge at the south pole. The strength of the magnetic field is higher where the lines are close together.
Right-Hand Thumb Rule : Thumb in direction of current, curled fingers show magnetic field lines. If current flows east to west, then: Magnetic field below the wire: clockwise. Magnetic field above the wire: anti-clockwise.
Magnetic Field due to a Circular Loop When the wire is bent into a loop, the magnetic field lines form concentric circles at every point. At the center of the loop, the field lines appear as straight lines. All sections of the loop contribute to the magnetic field in the same direction at the center.
magnetic field due to a current in a circular coil and solenoid. When current passes through a circular coil, it produces a magnetic field similar to that of a bar magnet. If the coil has multiple turns, the magnetic field becomes stronger. A solenoid, which is a coil of many circular turns wrapped closely in a cylinder, produces a uniform magnetic field inside it. This uniform field is similar to that of a bar magnet, with one end acting as the north pole and the other as the south. This magnetic field can be used to magnetize materials like soft iron, forming an electromagnet.
force on a current-carrying conductor in a magnetic field. An experiment shows that a current-carrying conductor placed in a magnetic field experiences a force, and this force changes direction when the direction of current or magnetic field is reversed.
Fleming’s Left-Hand Rule, helps to determine the direction of motion/force when current and magnetic field directions are known. This principle is applied in electric motors, generators, and loudspeakers.
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