§161 Coulomb’ s force law for pointlike charges 5. The principle of superposition The total electrical force on given charge is the vector sum of the electrical forces caused by the other charges, calculated as if each acted alone Fw=F1+F2+… Q Q3°F3F2 816.2 The electric field of static charge and electric field lines 1. The definition of electric field of static charge F at point electric field E qt The electric field at a point in space is the force per Coulomb RE at that point(N/C), direct the direction of force exerted on a positive test charge Electric field Test charge is small enough, does not alter the distribution of the charges creating the field 6
6 §16.1 Coulomb’s force law for pointlike charges 5. The principle of superposition The total electrical force on given charge is the vector sum of the electrical forces caused by the other charges, calculated as if each acted alone. L r r r Ftotal = F1 + F2 + q Q1 Q3 Q2 + + + − F1 r F3 r F2 r §16.2 The electric field of static charge and electric field lines 1. The definition of electric field of static charge test q F E r r electric field = Test charge is small enough, does not alter the distribution of the charges creating the field. The electric field at a point in space is the force per Coulomb at that point(N/C), direct the direction of force exerted on a positive test charge
816.2 The electric field of static charge and electric field lines Notice OThe electric field depends on the shape and the distribution of the charges @the gravitational force is always parrallel to the gravitational field in the gravitational case. but the electrical force can be either parallel or antiparallel to the electric field The force on a positive charge +q is parallel to the electric field. the force on a negative charge-g is antiparallel to the electric field F=gE 816.2 The electric field of static charge and electric field lines 2. The electric field of pointlike charge distribution Test F charge F 4兀E021 Q F10 For +O E q za q
7 Notice: 1The electric field depends on the shape and the distribution of the charges. 2the gravitational force is always parrallel to the gravitational field in the gravitational case, but the electrical force can be either parallel or antiparallel to the electric field. The force on a positive charge +q is parallel to the electric field, the force on a negative charge -q is antiparallel to the electric field. F qE r r = §16.2 The electric field of static charge and electric field lines §16.2 The electric field of static charge and electric field lines 2. The electric field of pointlike charge distribution Test charge r r F q r Q 2 21 0 21 ˆ 4 1 r r qQ F πε = r r r Q q F E ˆ 4 1 2 πε 0 = = r r r r Q q F E ˆ 4 1 2 πε 0 = = − r r For +Q For -Q
816.2 The electric field of static charge and electric field lines 3. The principle of superposition The total electric field at the point when all he pointlike charges are present simultaneously at their fixed position is the vector sum of the individual fields E F F2 +—2+…orE r4兀E0r E1+E2 4. Electric field lines A useful alternative geometric representation of electric field 816.2 The electric field of static charge and electric field lines The properties of the electric field lines: OThe electric field lines begin on positive charges and end on negative charges; The direction of the electric field at any point is tangent to the field line passing through that point and in the direction indicated by arrows on the field line 2The electric field is strong where field lines are close together and weak where they are far apart. The number of lines passing through square meter oriented perpendicular to the lines is proportional to the magnitude of the electric field 8
8 §16.2 The electric field of static charge and electric field lines 3. The principle of superposition The total electric field at the point when all the pointlike charges are present simultaneously at their fixed position is the vector sum of the individual fields. L r r L r r r r = + + = = + + 1 2 1 2 E E q F q F q F E 4. Electric field lines A useful alternative geometric representation of electric field. i i i i r r Q E ˆ 4 2 0 = ∑ πε r or 1The electric field lines begin on positive charges and end on negative charges; The direction of the electric field at any point is tangent to the field line passing through that point and in the direction indicated by arrows on the field line. The properties of the electric field lines: §16.2 The electric field of static charge and electric field lines 2The electric field is strong where field lines are close together and weak where they are far apart. The number of lines passing through a square meter oriented perpendicular to the lines is proportional to the magnitude of the electric field
816.2 The electric field of static charge and electric field lines 3 Field lines are never cross. The field at any point has a unique direction Examples: E1 816.2 The electric field of static charge and electric field lines
9 §16.2 The electric field of static charge and electric field lines Examples: 3Field lines are never cross. The field at any point has a unique direction. §16.2 The electric field of static charge and electric field lines
16. 3 The distribution of electric field and calculation 1. The electric field of the electric dipole l2 l2 F Electric dipole moment p=gd 816.3 The distribution of electric field and calculation @along the axis of the dipole y +oe a d/2 E-E+E-gI 4m6 -k (Z z十 2zd E 2 p z > 4z 10
10 1. The electric field of the electric dipole §16.3 The distribution of electric field and calculation − + d r p Q d r r = − Q + Q Electric dipole moment p Q d r r = k d z d z Q E E E ˆ] ) 2 ( 1 ) 2 ( 1 [ 4 2 2 0 + − − = + + − = πε r r r k d z Q zd ˆ ) 4 ( 2 4 2 2 0 2 − = πε 3 4 0 2 z p E πε r r = z >> d 1along the axis of the dipole §16.3 The distribution of electric field and calculation − Q + Q d 2 z E+ r E− r o A z y