Electrical Resistance
Electrical Resistance
Resistance (R)
Any device in a circuit which converts electrical energy into some other form impedes the current. The
device which converts electrical energy to heat energy is termed a resistor and its ability to impede
current is termed resistance. So resistance can be defined as the opposition to current caused by a
resistor.
The value of resistance of a conductor depends upon four things;
1. The material from which the conductor is made,
2 The length of the conductor (in the direction of the current path),
3 The thickness (cross-sectional area of the conductor),
4 The temperature at which it operates (in general, the higher its operating temperature the greater its
resistance).
Unit of Resistance - Ohm (Ω)
Resistance is measured in ohms (symbol Ω)). All materials at normal room temperature have electrical
resistance. If the material is classed as a conductor its resistance is probably much less than 1000
ohms. If the material is classed as an insulator its resistance is probably well in excess of 20,000,000
ohms.
The current in a circuit can be changed by changing the value of the supply e.m.f. (voltage) or the
value of the resistance
If the supply e.m.f is increased but the resistance remains the same the current will increase.
If the supply e.m.f remains the same but the resistance is increased the current will decrease.
NOTE: It is not possible to change the resistance by changing the e.m.f or the current. The only way to
change the resistance is by physically altering the resistor (different material, longer wires, or by
changing the conductor area).
Energy and Heat
Whenever there is current in a circuit, energy will be converted from electrical energy to some other
form. If resistance is present in the circuit the electrical energy will be converted to heat energy. A
lamp will get hot when in use because its filament has resistance and besides converting electrical
energy to light it will also convert some to heat.
Overload
The amount of current in a circuit increases as the load increases; the term ‘load' refers to the total
demand made by all of the electrical devices in the circuit.
Adding devices, such as irons, heaters, washing machines, etc to a circuit increases the load and hence
the current. Increasing the load so that it is over the specified maximum for the circuit causes
excessive current to flow (known as overload current).
Resistance (R)
Any device in a circuit which converts electrical energy into some other form impedes the current. The
device which converts electrical energy to heat energy is termed a resistor and its ability to impede
current is termed resistance. So resistance can be defined as the opposition to current caused by a
resistor.
The value of resistance of a conductor depends upon four things;
1. The material from which the conductor is made,
2 The length of the conductor (in the direction of the current path),
3 The thickness (cross-sectional area of the conductor),
4 The temperature at which it operates (in general, the higher its operating temperature the greater its
resistance).
Unit of Resistance - Ohm (Ω)
Resistance is measured in ohms (symbol Ω)). All materials at normal room temperature have electrical
resistance. If the material is classed as a conductor its resistance is probably much less than 1000
ohms. If the material is classed as an insulator its resistance is probably well in excess of 20,000,000
ohms.
The current in a circuit can be changed by changing the value of the supply e.m.f. (voltage) or the
value of the resistance
If the supply e.m.f is increased but the resistance remains the same the current will increase.
If the supply e.m.f remains the same but the resistance is increased the current will decrease.
NOTE: It is not possible to change the resistance by changing the e.m.f or the current. The only way to
change the resistance is by physically altering the resistor (different material, longer wires, or by
changing the conductor area).
Energy and Heat
Whenever there is current in a circuit, energy will be converted from electrical energy to some other
form. If resistance is present in the circuit the electrical energy will be converted to heat energy. A
lamp will get hot when in use because its filament has resistance and besides converting electrical
energy to light it will also convert some to heat.
Overload
The amount of current in a circuit increases as the load increases; the term ‘load' refers to the total
demand made by all of the electrical devices in the circuit.
Adding devices, such as irons, heaters, washing machines, etc to a circuit increases the load and hence
the current. Increasing the load so that it is over the specified maximum for the circuit causes
excessive current to flow (known as overload current).
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