Revision Notes for Class 12 Physics
Chapter 3 – Current Electricity
3.1 Introduction
1. In earlier chapters, charges were considered at rest; in this chapter, we focus on charges in
motion, which constitute an electric current.
2. Electric current occurs naturally in phenomena like lightning, where charges flow from
clouds to the earth.
3. Steady currents are found in everyday devices like torches and clocks.
4. This chapter covers the basic laws governing steady electric currents in conductors.
5. Steady electric current is similar to the smooth flow of water in a river, unlike the erratic
flow of lightning.
6. Understanding steady electric current is essential for comprehending the functioning of
various electrical devices.
3.2 Electric Current
1. Electric current (I) is defined as the rate at which charge flows through a cross-sectional
area.
q
2. For steady current, I = , where q is the net charge and t is the time interval.
t
3. Current can flow in both directions and if the current is negative, it indicates a flow in the
opposite direction.
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, Q
4. For non-steady currents, the instantaneous current is defined as I = lim , where Q is
t →0 t
the charge flowing in time t .
5. The SI unit of electric current is the ampere (A).
6. Currents in household devices are typically of the order of a few amperes, while lightning
carries tens of thousands of amperes.
3.3 Electric Currents in Conductors
1. Charges experience a force when an electric field is applied, leading to motion and the
creation of an electric current.
2. In some materials, like metals, electrons are free to move and contribute to electric
current.
3. Conductors, such as metals, allow current to flow because of free electrons that move in
the presence of an electric field.
4. When no electric field is applied, the thermal motion of electrons causes random
movement, with no net current.
www.vedantu.com 2
Chapter 3 – Current Electricity
3.1 Introduction
1. In earlier chapters, charges were considered at rest; in this chapter, we focus on charges in
motion, which constitute an electric current.
2. Electric current occurs naturally in phenomena like lightning, where charges flow from
clouds to the earth.
3. Steady currents are found in everyday devices like torches and clocks.
4. This chapter covers the basic laws governing steady electric currents in conductors.
5. Steady electric current is similar to the smooth flow of water in a river, unlike the erratic
flow of lightning.
6. Understanding steady electric current is essential for comprehending the functioning of
various electrical devices.
3.2 Electric Current
1. Electric current (I) is defined as the rate at which charge flows through a cross-sectional
area.
q
2. For steady current, I = , where q is the net charge and t is the time interval.
t
3. Current can flow in both directions and if the current is negative, it indicates a flow in the
opposite direction.
www.vedantu.com 1
, Q
4. For non-steady currents, the instantaneous current is defined as I = lim , where Q is
t →0 t
the charge flowing in time t .
5. The SI unit of electric current is the ampere (A).
6. Currents in household devices are typically of the order of a few amperes, while lightning
carries tens of thousands of amperes.
3.3 Electric Currents in Conductors
1. Charges experience a force when an electric field is applied, leading to motion and the
creation of an electric current.
2. In some materials, like metals, electrons are free to move and contribute to electric
current.
3. Conductors, such as metals, allow current to flow because of free electrons that move in
the presence of an electric field.
4. When no electric field is applied, the thermal motion of electrons causes random
movement, with no net current.
www.vedantu.com 2
